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| <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN" |
| "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []> |
| |
| <!-- ****************************************************** --> |
| <!-- Header --> |
| <!-- ****************************************************** --> |
| <book id="Writing-an-ALSA-Driver"> |
| <bookinfo> |
| <title>Writing an ALSA Driver</title> |
| <author> |
| <firstname>Takashi</firstname> |
| <surname>Iwai</surname> |
| <affiliation> |
| <address> |
| <email>tiwai@suse.de</email> |
| </address> |
| </affiliation> |
| </author> |
| |
| <date>Oct 15, 2007</date> |
| <edition>0.3.7</edition> |
| |
| <abstract> |
| <para> |
| This document describes how to write an ALSA (Advanced Linux |
| Sound Architecture) driver. |
| </para> |
| </abstract> |
| |
| <legalnotice> |
| <para> |
| Copyright (c) 2002-2005 Takashi Iwai <email>tiwai@suse.de</email> |
| </para> |
| |
| <para> |
| This document is free; you can redistribute it and/or modify it |
| under the terms of the GNU General Public License as published by |
| the Free Software Foundation; either version 2 of the License, or |
| (at your option) any later version. |
| </para> |
| |
| <para> |
| This document is distributed in the hope that it will be useful, |
| but <emphasis>WITHOUT ANY WARRANTY</emphasis>; without even the |
| implied warranty of <emphasis>MERCHANTABILITY or FITNESS FOR A |
| PARTICULAR PURPOSE</emphasis>. See the GNU General Public License |
| for more details. |
| </para> |
| |
| <para> |
| You should have received a copy of the GNU General Public |
| License along with this program; if not, write to the Free |
| Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, |
| MA 02111-1307 USA |
| </para> |
| </legalnotice> |
| |
| </bookinfo> |
| |
| <!-- ****************************************************** --> |
| <!-- Preface --> |
| <!-- ****************************************************** --> |
| <preface id="preface"> |
| <title>Preface</title> |
| <para> |
| This document describes how to write an |
| <ulink url="http://www.alsa-project.org/"><citetitle> |
| ALSA (Advanced Linux Sound Architecture)</citetitle></ulink> |
| driver. The document focuses mainly on PCI soundcards. |
| In the case of other device types, the API might |
| be different, too. However, at least the ALSA kernel API is |
| consistent, and therefore it would be still a bit help for |
| writing them. |
| </para> |
| |
| <para> |
| This document targets people who already have enough |
| C language skills and have basic linux kernel programming |
| knowledge. This document doesn't explain the general |
| topic of linux kernel coding and doesn't cover low-level |
| driver implementation details. It only describes |
| the standard way to write a PCI sound driver on ALSA. |
| </para> |
| |
| <para> |
| If you are already familiar with the older ALSA ver.0.5.x API, you |
| can check the drivers such as <filename>sound/pci/es1938.c</filename> or |
| <filename>sound/pci/maestro3.c</filename> which have also almost the same |
| code-base in the ALSA 0.5.x tree, so you can compare the differences. |
| </para> |
| |
| <para> |
| This document is still a draft version. Any feedback and |
| corrections, please!! |
| </para> |
| </preface> |
| |
| |
| <!-- ****************************************************** --> |
| <!-- File Tree Structure --> |
| <!-- ****************************************************** --> |
| <chapter id="file-tree"> |
| <title>File Tree Structure</title> |
| |
| <section id="file-tree-general"> |
| <title>General</title> |
| <para> |
| The ALSA drivers are provided in two ways. |
| </para> |
| |
| <para> |
| One is the trees provided as a tarball or via cvs from the |
| ALSA's ftp site, and another is the 2.6 (or later) Linux kernel |
| tree. To synchronize both, the ALSA driver tree is split into |
| two different trees: alsa-kernel and alsa-driver. The former |
| contains purely the source code for the Linux 2.6 (or later) |
| tree. This tree is designed only for compilation on 2.6 or |
| later environment. The latter, alsa-driver, contains many subtle |
| files for compiling ALSA drivers outside of the Linux kernel tree, |
| wrapper functions for older 2.2 and 2.4 kernels, to adapt the latest kernel API, |
| and additional drivers which are still in development or in |
| tests. The drivers in alsa-driver tree will be moved to |
| alsa-kernel (and eventually to the 2.6 kernel tree) when they are |
| finished and confirmed to work fine. |
| </para> |
| |
| <para> |
| The file tree structure of ALSA driver is depicted below. Both |
| alsa-kernel and alsa-driver have almost the same file |
| structure, except for <quote>core</quote> directory. It's |
| named as <quote>acore</quote> in alsa-driver tree. |
| |
| <example> |
| <title>ALSA File Tree Structure</title> |
| <literallayout> |
| sound |
| /core |
| /oss |
| /seq |
| /oss |
| /instr |
| /ioctl32 |
| /include |
| /drivers |
| /mpu401 |
| /opl3 |
| /i2c |
| /l3 |
| /synth |
| /emux |
| /pci |
| /(cards) |
| /isa |
| /(cards) |
| /arm |
| /ppc |
| /sparc |
| /usb |
| /pcmcia /(cards) |
| /oss |
| </literallayout> |
| </example> |
| </para> |
| </section> |
| |
| <section id="file-tree-core-directory"> |
| <title>core directory</title> |
| <para> |
| This directory contains the middle layer which is the heart |
| of ALSA drivers. In this directory, the native ALSA modules are |
| stored. The sub-directories contain different modules and are |
| dependent upon the kernel config. |
| </para> |
| |
| <section id="file-tree-core-directory-oss"> |
| <title>core/oss</title> |
| |
| <para> |
| The codes for PCM and mixer OSS emulation modules are stored |
| in this directory. The rawmidi OSS emulation is included in |
| the ALSA rawmidi code since it's quite small. The sequencer |
| code is stored in <filename>core/seq/oss</filename> directory (see |
| <link linkend="file-tree-core-directory-seq-oss"><citetitle> |
| below</citetitle></link>). |
| </para> |
| </section> |
| |
| <section id="file-tree-core-directory-ioctl32"> |
| <title>core/ioctl32</title> |
| |
| <para> |
| This directory contains the 32bit-ioctl wrappers for 64bit |
| architectures such like x86-64, ppc64 and sparc64. For 32bit |
| and alpha architectures, these are not compiled. |
| </para> |
| </section> |
| |
| <section id="file-tree-core-directory-seq"> |
| <title>core/seq</title> |
| <para> |
| This directory and its sub-directories are for the ALSA |
| sequencer. This directory contains the sequencer core and |
| primary sequencer modules such like snd-seq-midi, |
| snd-seq-virmidi, etc. They are compiled only when |
| <constant>CONFIG_SND_SEQUENCER</constant> is set in the kernel |
| config. |
| </para> |
| </section> |
| |
| <section id="file-tree-core-directory-seq-oss"> |
| <title>core/seq/oss</title> |
| <para> |
| This contains the OSS sequencer emulation codes. |
| </para> |
| </section> |
| |
| <section id="file-tree-core-directory-deq-instr"> |
| <title>core/seq/instr</title> |
| <para> |
| This directory contains the modules for the sequencer |
| instrument layer. |
| </para> |
| </section> |
| </section> |
| |
| <section id="file-tree-include-directory"> |
| <title>include directory</title> |
| <para> |
| This is the place for the public header files of ALSA drivers, |
| which are to be exported to user-space, or included by |
| several files at different directories. Basically, the private |
| header files should not be placed in this directory, but you may |
| still find files there, due to historical reasons :) |
| </para> |
| </section> |
| |
| <section id="file-tree-drivers-directory"> |
| <title>drivers directory</title> |
| <para> |
| This directory contains code shared among different drivers |
| on different architectures. They are hence supposed not to be |
| architecture-specific. |
| For example, the dummy pcm driver and the serial MIDI |
| driver are found in this directory. In the sub-directories, |
| there is code for components which are independent from |
| bus and cpu architectures. |
| </para> |
| |
| <section id="file-tree-drivers-directory-mpu401"> |
| <title>drivers/mpu401</title> |
| <para> |
| The MPU401 and MPU401-UART modules are stored here. |
| </para> |
| </section> |
| |
| <section id="file-tree-drivers-directory-opl3"> |
| <title>drivers/opl3 and opl4</title> |
| <para> |
| The OPL3 and OPL4 FM-synth stuff is found here. |
| </para> |
| </section> |
| </section> |
| |
| <section id="file-tree-i2c-directory"> |
| <title>i2c directory</title> |
| <para> |
| This contains the ALSA i2c components. |
| </para> |
| |
| <para> |
| Although there is a standard i2c layer on Linux, ALSA has its |
| own i2c code for some cards, because the soundcard needs only a |
| simple operation and the standard i2c API is too complicated for |
| such a purpose. |
| </para> |
| |
| <section id="file-tree-i2c-directory-l3"> |
| <title>i2c/l3</title> |
| <para> |
| This is a sub-directory for ARM L3 i2c. |
| </para> |
| </section> |
| </section> |
| |
| <section id="file-tree-synth-directory"> |
| <title>synth directory</title> |
| <para> |
| This contains the synth middle-level modules. |
| </para> |
| |
| <para> |
| So far, there is only Emu8000/Emu10k1 synth driver under |
| the <filename>synth/emux</filename> sub-directory. |
| </para> |
| </section> |
| |
| <section id="file-tree-pci-directory"> |
| <title>pci directory</title> |
| <para> |
| This directory and its sub-directories hold the top-level card modules |
| for PCI soundcards and the code specific to the PCI BUS. |
| </para> |
| |
| <para> |
| The drivers compiled from a single file are stored directly |
| in the pci directory, while the drivers with several source files are |
| stored on their own sub-directory (e.g. emu10k1, ice1712). |
| </para> |
| </section> |
| |
| <section id="file-tree-isa-directory"> |
| <title>isa directory</title> |
| <para> |
| This directory and its sub-directories hold the top-level card modules |
| for ISA soundcards. |
| </para> |
| </section> |
| |
| <section id="file-tree-arm-ppc-sparc-directories"> |
| <title>arm, ppc, and sparc directories</title> |
| <para> |
| They are used for top-level card modules which are |
| specific to one of these architectures. |
| </para> |
| </section> |
| |
| <section id="file-tree-usb-directory"> |
| <title>usb directory</title> |
| <para> |
| This directory contains the USB-audio driver. In the latest version, the |
| USB MIDI driver is integrated in the usb-audio driver. |
| </para> |
| </section> |
| |
| <section id="file-tree-pcmcia-directory"> |
| <title>pcmcia directory</title> |
| <para> |
| The PCMCIA, especially PCCard drivers will go here. CardBus |
| drivers will be in the pci directory, because their API is identical |
| to that of standard PCI cards. |
| </para> |
| </section> |
| |
| <section id="file-tree-oss-directory"> |
| <title>oss directory</title> |
| <para> |
| The OSS/Lite source files are stored here in Linux 2.6 (or |
| later) tree. In the ALSA driver tarball, this directory is empty, |
| of course :) |
| </para> |
| </section> |
| </chapter> |
| |
| |
| <!-- ****************************************************** --> |
| <!-- Basic Flow for PCI Drivers --> |
| <!-- ****************************************************** --> |
| <chapter id="basic-flow"> |
| <title>Basic Flow for PCI Drivers</title> |
| |
| <section id="basic-flow-outline"> |
| <title>Outline</title> |
| <para> |
| The minimum flow for PCI soundcards is as follows: |
| |
| <itemizedlist> |
| <listitem><para>define the PCI ID table (see the section |
| <link linkend="pci-resource-entries"><citetitle>PCI Entries |
| </citetitle></link>).</para></listitem> |
| <listitem><para>create <function>probe()</function> callback.</para></listitem> |
| <listitem><para>create <function>remove()</function> callback.</para></listitem> |
| <listitem><para>create a <structname>pci_driver</structname> structure |
| containing the three pointers above.</para></listitem> |
| <listitem><para>create an <function>init()</function> function just calling |
| the <function>pci_register_driver()</function> to register the pci_driver table |
| defined above.</para></listitem> |
| <listitem><para>create an <function>exit()</function> function to call |
| the <function>pci_unregister_driver()</function> function.</para></listitem> |
| </itemizedlist> |
| </para> |
| </section> |
| |
| <section id="basic-flow-example"> |
| <title>Full Code Example</title> |
| <para> |
| The code example is shown below. Some parts are kept |
| unimplemented at this moment but will be filled in the |
| next sections. The numbers in the comment lines of the |
| <function>snd_mychip_probe()</function> function |
| refer to details explained in the following section. |
| |
| <example> |
| <title>Basic Flow for PCI Drivers - Example</title> |
| <programlisting> |
| <![CDATA[ |
| #include <linux/init.h> |
| #include <linux/pci.h> |
| #include <linux/slab.h> |
| #include <sound/core.h> |
| #include <sound/initval.h> |
| |
| /* module parameters (see "Module Parameters") */ |
| /* SNDRV_CARDS: maximum number of cards supported by this module */ |
| static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX; |
| static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR; |
| static bool enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP; |
| |
| /* definition of the chip-specific record */ |
| struct mychip { |
| struct snd_card *card; |
| /* the rest of the implementation will be in section |
| * "PCI Resource Management" |
| */ |
| }; |
| |
| /* chip-specific destructor |
| * (see "PCI Resource Management") |
| */ |
| static int snd_mychip_free(struct mychip *chip) |
| { |
| .... /* will be implemented later... */ |
| } |
| |
| /* component-destructor |
| * (see "Management of Cards and Components") |
| */ |
| static int snd_mychip_dev_free(struct snd_device *device) |
| { |
| return snd_mychip_free(device->device_data); |
| } |
| |
| /* chip-specific constructor |
| * (see "Management of Cards and Components") |
| */ |
| static int snd_mychip_create(struct snd_card *card, |
| struct pci_dev *pci, |
| struct mychip **rchip) |
| { |
| struct mychip *chip; |
| int err; |
| static struct snd_device_ops ops = { |
| .dev_free = snd_mychip_dev_free, |
| }; |
| |
| *rchip = NULL; |
| |
| /* check PCI availability here |
| * (see "PCI Resource Management") |
| */ |
| .... |
| |
| /* allocate a chip-specific data with zero filled */ |
| chip = kzalloc(sizeof(*chip), GFP_KERNEL); |
| if (chip == NULL) |
| return -ENOMEM; |
| |
| chip->card = card; |
| |
| /* rest of initialization here; will be implemented |
| * later, see "PCI Resource Management" |
| */ |
| .... |
| |
| err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops); |
| if (err < 0) { |
| snd_mychip_free(chip); |
| return err; |
| } |
| |
| snd_card_set_dev(card, &pci->dev); |
| |
| *rchip = chip; |
| return 0; |
| } |
| |
| /* constructor -- see "Constructor" sub-section */ |
| static int snd_mychip_probe(struct pci_dev *pci, |
| const struct pci_device_id *pci_id) |
| { |
| static int dev; |
| struct snd_card *card; |
| struct mychip *chip; |
| int err; |
| |
| /* (1) */ |
| if (dev >= SNDRV_CARDS) |
| return -ENODEV; |
| if (!enable[dev]) { |
| dev++; |
| return -ENOENT; |
| } |
| |
| /* (2) */ |
| err = snd_card_create(index[dev], id[dev], THIS_MODULE, 0, &card); |
| if (err < 0) |
| return err; |
| |
| /* (3) */ |
| err = snd_mychip_create(card, pci, &chip); |
| if (err < 0) { |
| snd_card_free(card); |
| return err; |
| } |
| |
| /* (4) */ |
| strcpy(card->driver, "My Chip"); |
| strcpy(card->shortname, "My Own Chip 123"); |
| sprintf(card->longname, "%s at 0x%lx irq %i", |
| card->shortname, chip->ioport, chip->irq); |
| |
| /* (5) */ |
| .... /* implemented later */ |
| |
| /* (6) */ |
| err = snd_card_register(card); |
| if (err < 0) { |
| snd_card_free(card); |
| return err; |
| } |
| |
| /* (7) */ |
| pci_set_drvdata(pci, card); |
| dev++; |
| return 0; |
| } |
| |
| /* destructor -- see the "Destructor" sub-section */ |
| static void snd_mychip_remove(struct pci_dev *pci) |
| { |
| snd_card_free(pci_get_drvdata(pci)); |
| pci_set_drvdata(pci, NULL); |
| } |
| ]]> |
| </programlisting> |
| </example> |
| </para> |
| </section> |
| |
| <section id="basic-flow-constructor"> |
| <title>Constructor</title> |
| <para> |
| The real constructor of PCI drivers is the <function>probe</function> callback. |
| The <function>probe</function> callback and other component-constructors which are called |
| from the <function>probe</function> callback cannot be used with |
| the <parameter>__init</parameter> prefix |
| because any PCI device could be a hotplug device. |
| </para> |
| |
| <para> |
| In the <function>probe</function> callback, the following scheme is often used. |
| </para> |
| |
| <section id="basic-flow-constructor-device-index"> |
| <title>1) Check and increment the device index.</title> |
| <para> |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static int dev; |
| .... |
| if (dev >= SNDRV_CARDS) |
| return -ENODEV; |
| if (!enable[dev]) { |
| dev++; |
| return -ENOENT; |
| } |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| where enable[dev] is the module option. |
| </para> |
| |
| <para> |
| Each time the <function>probe</function> callback is called, check the |
| availability of the device. If not available, simply increment |
| the device index and returns. dev will be incremented also |
| later (<link |
| linkend="basic-flow-constructor-set-pci"><citetitle>step |
| 7</citetitle></link>). |
| </para> |
| </section> |
| |
| <section id="basic-flow-constructor-create-card"> |
| <title>2) Create a card instance</title> |
| <para> |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| struct snd_card *card; |
| int err; |
| .... |
| err = snd_card_create(index[dev], id[dev], THIS_MODULE, 0, &card); |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| The details will be explained in the section |
| <link linkend="card-management-card-instance"><citetitle> |
| Management of Cards and Components</citetitle></link>. |
| </para> |
| </section> |
| |
| <section id="basic-flow-constructor-create-main"> |
| <title>3) Create a main component</title> |
| <para> |
| In this part, the PCI resources are allocated. |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| struct mychip *chip; |
| .... |
| err = snd_mychip_create(card, pci, &chip); |
| if (err < 0) { |
| snd_card_free(card); |
| return err; |
| } |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| The details will be explained in the section <link |
| linkend="pci-resource"><citetitle>PCI Resource |
| Management</citetitle></link>. |
| </para> |
| </section> |
| |
| <section id="basic-flow-constructor-main-component"> |
| <title>4) Set the driver ID and name strings.</title> |
| <para> |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| strcpy(card->driver, "My Chip"); |
| strcpy(card->shortname, "My Own Chip 123"); |
| sprintf(card->longname, "%s at 0x%lx irq %i", |
| card->shortname, chip->ioport, chip->irq); |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| The driver field holds the minimal ID string of the |
| chip. This is used by alsa-lib's configurator, so keep it |
| simple but unique. |
| Even the same driver can have different driver IDs to |
| distinguish the functionality of each chip type. |
| </para> |
| |
| <para> |
| The shortname field is a string shown as more verbose |
| name. The longname field contains the information |
| shown in <filename>/proc/asound/cards</filename>. |
| </para> |
| </section> |
| |
| <section id="basic-flow-constructor-create-other"> |
| <title>5) Create other components, such as mixer, MIDI, etc.</title> |
| <para> |
| Here you define the basic components such as |
| <link linkend="pcm-interface"><citetitle>PCM</citetitle></link>, |
| mixer (e.g. <link linkend="api-ac97"><citetitle>AC97</citetitle></link>), |
| MIDI (e.g. <link linkend="midi-interface"><citetitle>MPU-401</citetitle></link>), |
| and other interfaces. |
| Also, if you want a <link linkend="proc-interface"><citetitle>proc |
| file</citetitle></link>, define it here, too. |
| </para> |
| </section> |
| |
| <section id="basic-flow-constructor-register-card"> |
| <title>6) Register the card instance.</title> |
| <para> |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| err = snd_card_register(card); |
| if (err < 0) { |
| snd_card_free(card); |
| return err; |
| } |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| Will be explained in the section <link |
| linkend="card-management-registration"><citetitle>Management |
| of Cards and Components</citetitle></link>, too. |
| </para> |
| </section> |
| |
| <section id="basic-flow-constructor-set-pci"> |
| <title>7) Set the PCI driver data and return zero.</title> |
| <para> |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| pci_set_drvdata(pci, card); |
| dev++; |
| return 0; |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| In the above, the card record is stored. This pointer is |
| used in the remove callback and power-management |
| callbacks, too. |
| </para> |
| </section> |
| </section> |
| |
| <section id="basic-flow-destructor"> |
| <title>Destructor</title> |
| <para> |
| The destructor, remove callback, simply releases the card |
| instance. Then the ALSA middle layer will release all the |
| attached components automatically. |
| </para> |
| |
| <para> |
| It would be typically like the following: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static void snd_mychip_remove(struct pci_dev *pci) |
| { |
| snd_card_free(pci_get_drvdata(pci)); |
| pci_set_drvdata(pci, NULL); |
| } |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| The above code assumes that the card pointer is set to the PCI |
| driver data. |
| </para> |
| </section> |
| |
| <section id="basic-flow-header-files"> |
| <title>Header Files</title> |
| <para> |
| For the above example, at least the following include files |
| are necessary. |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| #include <linux/init.h> |
| #include <linux/pci.h> |
| #include <linux/slab.h> |
| #include <sound/core.h> |
| #include <sound/initval.h> |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| where the last one is necessary only when module options are |
| defined in the source file. If the code is split into several |
| files, the files without module options don't need them. |
| </para> |
| |
| <para> |
| In addition to these headers, you'll need |
| <filename><linux/interrupt.h></filename> for interrupt |
| handling, and <filename><asm/io.h></filename> for I/O |
| access. If you use the <function>mdelay()</function> or |
| <function>udelay()</function> functions, you'll need to include |
| <filename><linux/delay.h></filename> too. |
| </para> |
| |
| <para> |
| The ALSA interfaces like the PCM and control APIs are defined in other |
| <filename><sound/xxx.h></filename> header files. |
| They have to be included after |
| <filename><sound/core.h></filename>. |
| </para> |
| |
| </section> |
| </chapter> |
| |
| |
| <!-- ****************************************************** --> |
| <!-- Management of Cards and Components --> |
| <!-- ****************************************************** --> |
| <chapter id="card-management"> |
| <title>Management of Cards and Components</title> |
| |
| <section id="card-management-card-instance"> |
| <title>Card Instance</title> |
| <para> |
| For each soundcard, a <quote>card</quote> record must be allocated. |
| </para> |
| |
| <para> |
| A card record is the headquarters of the soundcard. It manages |
| the whole list of devices (components) on the soundcard, such as |
| PCM, mixers, MIDI, synthesizer, and so on. Also, the card |
| record holds the ID and the name strings of the card, manages |
| the root of proc files, and controls the power-management states |
| and hotplug disconnections. The component list on the card |
| record is used to manage the correct release of resources at |
| destruction. |
| </para> |
| |
| <para> |
| As mentioned above, to create a card instance, call |
| <function>snd_card_create()</function>. |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| struct snd_card *card; |
| int err; |
| err = snd_card_create(index, id, module, extra_size, &card); |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| The function takes five arguments, the card-index number, the |
| id string, the module pointer (usually |
| <constant>THIS_MODULE</constant>), |
| the size of extra-data space, and the pointer to return the |
| card instance. The extra_size argument is used to |
| allocate card->private_data for the |
| chip-specific data. Note that these data |
| are allocated by <function>snd_card_create()</function>. |
| </para> |
| </section> |
| |
| <section id="card-management-component"> |
| <title>Components</title> |
| <para> |
| After the card is created, you can attach the components |
| (devices) to the card instance. In an ALSA driver, a component is |
| represented as a struct <structname>snd_device</structname> object. |
| A component can be a PCM instance, a control interface, a raw |
| MIDI interface, etc. Each such instance has one component |
| entry. |
| </para> |
| |
| <para> |
| A component can be created via |
| <function>snd_device_new()</function> function. |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| snd_device_new(card, SNDRV_DEV_XXX, chip, &ops); |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| This takes the card pointer, the device-level |
| (<constant>SNDRV_DEV_XXX</constant>), the data pointer, and the |
| callback pointers (<parameter>&ops</parameter>). The |
| device-level defines the type of components and the order of |
| registration and de-registration. For most components, the |
| device-level is already defined. For a user-defined component, |
| you can use <constant>SNDRV_DEV_LOWLEVEL</constant>. |
| </para> |
| |
| <para> |
| This function itself doesn't allocate the data space. The data |
| must be allocated manually beforehand, and its pointer is passed |
| as the argument. This pointer (<parameter>chip</parameter> in the |
| above example) is used as the identifier for the instance. |
| </para> |
| |
| <para> |
| Each pre-defined ALSA component such as ac97 and pcm calls |
| <function>snd_device_new()</function> inside its |
| constructor. The destructor for each component is defined in the |
| callback pointers. Hence, you don't need to take care of |
| calling a destructor for such a component. |
| </para> |
| |
| <para> |
| If you wish to create your own component, you need to |
| set the destructor function to the dev_free callback in |
| the <parameter>ops</parameter>, so that it can be released |
| automatically via <function>snd_card_free()</function>. |
| The next example will show an implementation of chip-specific |
| data. |
| </para> |
| </section> |
| |
| <section id="card-management-chip-specific"> |
| <title>Chip-Specific Data</title> |
| <para> |
| Chip-specific information, e.g. the I/O port address, its |
| resource pointer, or the irq number, is stored in the |
| chip-specific record. |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| struct mychip { |
| .... |
| }; |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| In general, there are two ways of allocating the chip record. |
| </para> |
| |
| <section id="card-management-chip-specific-snd-card-new"> |
| <title>1. Allocating via <function>snd_card_create()</function>.</title> |
| <para> |
| As mentioned above, you can pass the extra-data-length |
| to the 4th argument of <function>snd_card_create()</function>, i.e. |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| err = snd_card_create(index[dev], id[dev], THIS_MODULE, |
| sizeof(struct mychip), &card); |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| struct <structname>mychip</structname> is the type of the chip record. |
| </para> |
| |
| <para> |
| In return, the allocated record can be accessed as |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| struct mychip *chip = card->private_data; |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| With this method, you don't have to allocate twice. |
| The record is released together with the card instance. |
| </para> |
| </section> |
| |
| <section id="card-management-chip-specific-allocate-extra"> |
| <title>2. Allocating an extra device.</title> |
| |
| <para> |
| After allocating a card instance via |
| <function>snd_card_create()</function> (with |
| <constant>0</constant> on the 4th arg), call |
| <function>kzalloc()</function>. |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| struct snd_card *card; |
| struct mychip *chip; |
| err = snd_card_create(index[dev], id[dev], THIS_MODULE, 0, &card); |
| ..... |
| chip = kzalloc(sizeof(*chip), GFP_KERNEL); |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| The chip record should have the field to hold the card |
| pointer at least, |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| struct mychip { |
| struct snd_card *card; |
| .... |
| }; |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| Then, set the card pointer in the returned chip instance. |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| chip->card = card; |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| Next, initialize the fields, and register this chip |
| record as a low-level device with a specified |
| <parameter>ops</parameter>, |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static struct snd_device_ops ops = { |
| .dev_free = snd_mychip_dev_free, |
| }; |
| .... |
| snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops); |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| <function>snd_mychip_dev_free()</function> is the |
| device-destructor function, which will call the real |
| destructor. |
| </para> |
| |
| <para> |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static int snd_mychip_dev_free(struct snd_device *device) |
| { |
| return snd_mychip_free(device->device_data); |
| } |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| where <function>snd_mychip_free()</function> is the real destructor. |
| </para> |
| </section> |
| </section> |
| |
| <section id="card-management-registration"> |
| <title>Registration and Release</title> |
| <para> |
| After all components are assigned, register the card instance |
| by calling <function>snd_card_register()</function>. Access |
| to the device files is enabled at this point. That is, before |
| <function>snd_card_register()</function> is called, the |
| components are safely inaccessible from external side. If this |
| call fails, exit the probe function after releasing the card via |
| <function>snd_card_free()</function>. |
| </para> |
| |
| <para> |
| For releasing the card instance, you can call simply |
| <function>snd_card_free()</function>. As mentioned earlier, all |
| components are released automatically by this call. |
| </para> |
| |
| <para> |
| For a device which allows hotplugging, you can use |
| <function>snd_card_free_when_closed</function>. This one will |
| postpone the destruction until all devices are closed. |
| </para> |
| |
| </section> |
| |
| </chapter> |
| |
| |
| <!-- ****************************************************** --> |
| <!-- PCI Resource Management --> |
| <!-- ****************************************************** --> |
| <chapter id="pci-resource"> |
| <title>PCI Resource Management</title> |
| |
| <section id="pci-resource-example"> |
| <title>Full Code Example</title> |
| <para> |
| In this section, we'll complete the chip-specific constructor, |
| destructor and PCI entries. Example code is shown first, |
| below. |
| |
| <example> |
| <title>PCI Resource Management Example</title> |
| <programlisting> |
| <![CDATA[ |
| struct mychip { |
| struct snd_card *card; |
| struct pci_dev *pci; |
| |
| unsigned long port; |
| int irq; |
| }; |
| |
| static int snd_mychip_free(struct mychip *chip) |
| { |
| /* disable hardware here if any */ |
| .... /* (not implemented in this document) */ |
| |
| /* release the irq */ |
| if (chip->irq >= 0) |
| free_irq(chip->irq, chip); |
| /* release the I/O ports & memory */ |
| pci_release_regions(chip->pci); |
| /* disable the PCI entry */ |
| pci_disable_device(chip->pci); |
| /* release the data */ |
| kfree(chip); |
| return 0; |
| } |
| |
| /* chip-specific constructor */ |
| static int snd_mychip_create(struct snd_card *card, |
| struct pci_dev *pci, |
| struct mychip **rchip) |
| { |
| struct mychip *chip; |
| int err; |
| static struct snd_device_ops ops = { |
| .dev_free = snd_mychip_dev_free, |
| }; |
| |
| *rchip = NULL; |
| |
| /* initialize the PCI entry */ |
| err = pci_enable_device(pci); |
| if (err < 0) |
| return err; |
| /* check PCI availability (28bit DMA) */ |
| if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 || |
| pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) { |
| printk(KERN_ERR "error to set 28bit mask DMA\n"); |
| pci_disable_device(pci); |
| return -ENXIO; |
| } |
| |
| chip = kzalloc(sizeof(*chip), GFP_KERNEL); |
| if (chip == NULL) { |
| pci_disable_device(pci); |
| return -ENOMEM; |
| } |
| |
| /* initialize the stuff */ |
| chip->card = card; |
| chip->pci = pci; |
| chip->irq = -1; |
| |
| /* (1) PCI resource allocation */ |
| err = pci_request_regions(pci, "My Chip"); |
| if (err < 0) { |
| kfree(chip); |
| pci_disable_device(pci); |
| return err; |
| } |
| chip->port = pci_resource_start(pci, 0); |
| if (request_irq(pci->irq, snd_mychip_interrupt, |
| IRQF_SHARED, KBUILD_MODNAME, chip)) { |
| printk(KERN_ERR "cannot grab irq %d\n", pci->irq); |
| snd_mychip_free(chip); |
| return -EBUSY; |
| } |
| chip->irq = pci->irq; |
| |
| /* (2) initialization of the chip hardware */ |
| .... /* (not implemented in this document) */ |
| |
| err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops); |
| if (err < 0) { |
| snd_mychip_free(chip); |
| return err; |
| } |
| |
| snd_card_set_dev(card, &pci->dev); |
| |
| *rchip = chip; |
| return 0; |
| } |
| |
| /* PCI IDs */ |
| static struct pci_device_id snd_mychip_ids[] = { |
| { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR, |
| PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, }, |
| .... |
| { 0, } |
| }; |
| MODULE_DEVICE_TABLE(pci, snd_mychip_ids); |
| |
| /* pci_driver definition */ |
| static struct pci_driver driver = { |
| .name = KBUILD_MODNAME, |
| .id_table = snd_mychip_ids, |
| .probe = snd_mychip_probe, |
| .remove = snd_mychip_remove, |
| }; |
| |
| /* module initialization */ |
| static int __init alsa_card_mychip_init(void) |
| { |
| return pci_register_driver(&driver); |
| } |
| |
| /* module clean up */ |
| static void __exit alsa_card_mychip_exit(void) |
| { |
| pci_unregister_driver(&driver); |
| } |
| |
| module_init(alsa_card_mychip_init) |
| module_exit(alsa_card_mychip_exit) |
| |
| EXPORT_NO_SYMBOLS; /* for old kernels only */ |
| ]]> |
| </programlisting> |
| </example> |
| </para> |
| </section> |
| |
| <section id="pci-resource-some-haftas"> |
| <title>Some Hafta's</title> |
| <para> |
| The allocation of PCI resources is done in the |
| <function>probe()</function> function, and usually an extra |
| <function>xxx_create()</function> function is written for this |
| purpose. |
| </para> |
| |
| <para> |
| In the case of PCI devices, you first have to call |
| the <function>pci_enable_device()</function> function before |
| allocating resources. Also, you need to set the proper PCI DMA |
| mask to limit the accessed I/O range. In some cases, you might |
| need to call <function>pci_set_master()</function> function, |
| too. |
| </para> |
| |
| <para> |
| Suppose the 28bit mask, and the code to be added would be like: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| err = pci_enable_device(pci); |
| if (err < 0) |
| return err; |
| if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 || |
| pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) { |
| printk(KERN_ERR "error to set 28bit mask DMA\n"); |
| pci_disable_device(pci); |
| return -ENXIO; |
| } |
| |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| </section> |
| |
| <section id="pci-resource-resource-allocation"> |
| <title>Resource Allocation</title> |
| <para> |
| The allocation of I/O ports and irqs is done via standard kernel |
| functions. Unlike ALSA ver.0.5.x., there are no helpers for |
| that. And these resources must be released in the destructor |
| function (see below). Also, on ALSA 0.9.x, you don't need to |
| allocate (pseudo-)DMA for PCI like in ALSA 0.5.x. |
| </para> |
| |
| <para> |
| Now assume that the PCI device has an I/O port with 8 bytes |
| and an interrupt. Then struct <structname>mychip</structname> will have the |
| following fields: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| struct mychip { |
| struct snd_card *card; |
| |
| unsigned long port; |
| int irq; |
| }; |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| For an I/O port (and also a memory region), you need to have |
| the resource pointer for the standard resource management. For |
| an irq, you have to keep only the irq number (integer). But you |
| need to initialize this number as -1 before actual allocation, |
| since irq 0 is valid. The port address and its resource pointer |
| can be initialized as null by |
| <function>kzalloc()</function> automatically, so you |
| don't have to take care of resetting them. |
| </para> |
| |
| <para> |
| The allocation of an I/O port is done like this: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| err = pci_request_regions(pci, "My Chip"); |
| if (err < 0) { |
| kfree(chip); |
| pci_disable_device(pci); |
| return err; |
| } |
| chip->port = pci_resource_start(pci, 0); |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| <!-- obsolete --> |
| It will reserve the I/O port region of 8 bytes of the given |
| PCI device. The returned value, chip->res_port, is allocated |
| via <function>kmalloc()</function> by |
| <function>request_region()</function>. The pointer must be |
| released via <function>kfree()</function>, but there is a |
| problem with this. This issue will be explained later. |
| </para> |
| |
| <para> |
| The allocation of an interrupt source is done like this: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| if (request_irq(pci->irq, snd_mychip_interrupt, |
| IRQF_SHARED, KBUILD_MODNAME, chip)) { |
| printk(KERN_ERR "cannot grab irq %d\n", pci->irq); |
| snd_mychip_free(chip); |
| return -EBUSY; |
| } |
| chip->irq = pci->irq; |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| where <function>snd_mychip_interrupt()</function> is the |
| interrupt handler defined <link |
| linkend="pcm-interface-interrupt-handler"><citetitle>later</citetitle></link>. |
| Note that chip->irq should be defined |
| only when <function>request_irq()</function> succeeded. |
| </para> |
| |
| <para> |
| On the PCI bus, interrupts can be shared. Thus, |
| <constant>IRQF_SHARED</constant> is used as the interrupt flag of |
| <function>request_irq()</function>. |
| </para> |
| |
| <para> |
| The last argument of <function>request_irq()</function> is the |
| data pointer passed to the interrupt handler. Usually, the |
| chip-specific record is used for that, but you can use what you |
| like, too. |
| </para> |
| |
| <para> |
| I won't give details about the interrupt handler at this |
| point, but at least its appearance can be explained now. The |
| interrupt handler looks usually like the following: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id) |
| { |
| struct mychip *chip = dev_id; |
| .... |
| return IRQ_HANDLED; |
| } |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| Now let's write the corresponding destructor for the resources |
| above. The role of destructor is simple: disable the hardware |
| (if already activated) and release the resources. So far, we |
| have no hardware part, so the disabling code is not written here. |
| </para> |
| |
| <para> |
| To release the resources, the <quote>check-and-release</quote> |
| method is a safer way. For the interrupt, do like this: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| if (chip->irq >= 0) |
| free_irq(chip->irq, chip); |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| Since the irq number can start from 0, you should initialize |
| chip->irq with a negative value (e.g. -1), so that you can |
| check the validity of the irq number as above. |
| </para> |
| |
| <para> |
| When you requested I/O ports or memory regions via |
| <function>pci_request_region()</function> or |
| <function>pci_request_regions()</function> like in this example, |
| release the resource(s) using the corresponding function, |
| <function>pci_release_region()</function> or |
| <function>pci_release_regions()</function>. |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| pci_release_regions(chip->pci); |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| When you requested manually via <function>request_region()</function> |
| or <function>request_mem_region</function>, you can release it via |
| <function>release_resource()</function>. Suppose that you keep |
| the resource pointer returned from <function>request_region()</function> |
| in chip->res_port, the release procedure looks like: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| release_and_free_resource(chip->res_port); |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| Don't forget to call <function>pci_disable_device()</function> |
| before the end. |
| </para> |
| |
| <para> |
| And finally, release the chip-specific record. |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| kfree(chip); |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| We didn't implement the hardware disabling part in the above. |
| If you need to do this, please note that the destructor may be |
| called even before the initialization of the chip is completed. |
| It would be better to have a flag to skip hardware disabling |
| if the hardware was not initialized yet. |
| </para> |
| |
| <para> |
| When the chip-data is assigned to the card using |
| <function>snd_device_new()</function> with |
| <constant>SNDRV_DEV_LOWLELVEL</constant> , its destructor is |
| called at the last. That is, it is assured that all other |
| components like PCMs and controls have already been released. |
| You don't have to stop PCMs, etc. explicitly, but just |
| call low-level hardware stopping. |
| </para> |
| |
| <para> |
| The management of a memory-mapped region is almost as same as |
| the management of an I/O port. You'll need three fields like |
| the following: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| struct mychip { |
| .... |
| unsigned long iobase_phys; |
| void __iomem *iobase_virt; |
| }; |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| and the allocation would be like below: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| if ((err = pci_request_regions(pci, "My Chip")) < 0) { |
| kfree(chip); |
| return err; |
| } |
| chip->iobase_phys = pci_resource_start(pci, 0); |
| chip->iobase_virt = ioremap_nocache(chip->iobase_phys, |
| pci_resource_len(pci, 0)); |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| and the corresponding destructor would be: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static int snd_mychip_free(struct mychip *chip) |
| { |
| .... |
| if (chip->iobase_virt) |
| iounmap(chip->iobase_virt); |
| .... |
| pci_release_regions(chip->pci); |
| .... |
| } |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| </section> |
| |
| <section id="pci-resource-device-struct"> |
| <title>Registration of Device Struct</title> |
| <para> |
| At some point, typically after calling <function>snd_device_new()</function>, |
| you need to register the struct <structname>device</structname> of the chip |
| you're handling for udev and co. ALSA provides a macro for compatibility with |
| older kernels. Simply call like the following: |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| snd_card_set_dev(card, &pci->dev); |
| ]]> |
| </programlisting> |
| </informalexample> |
| so that it stores the PCI's device pointer to the card. This will be |
| referred by ALSA core functions later when the devices are registered. |
| </para> |
| <para> |
| In the case of non-PCI, pass the proper device struct pointer of the BUS |
| instead. (In the case of legacy ISA without PnP, you don't have to do |
| anything.) |
| </para> |
| </section> |
| |
| <section id="pci-resource-entries"> |
| <title>PCI Entries</title> |
| <para> |
| So far, so good. Let's finish the missing PCI |
| stuff. At first, we need a |
| <structname>pci_device_id</structname> table for this |
| chipset. It's a table of PCI vendor/device ID number, and some |
| masks. |
| </para> |
| |
| <para> |
| For example, |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static struct pci_device_id snd_mychip_ids[] = { |
| { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR, |
| PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, }, |
| .... |
| { 0, } |
| }; |
| MODULE_DEVICE_TABLE(pci, snd_mychip_ids); |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| The first and second fields of |
| the <structname>pci_device_id</structname> structure are the vendor and |
| device IDs. If you have no reason to filter the matching |
| devices, you can leave the remaining fields as above. The last |
| field of the <structname>pci_device_id</structname> struct contains |
| private data for this entry. You can specify any value here, for |
| example, to define specific operations for supported device IDs. |
| Such an example is found in the intel8x0 driver. |
| </para> |
| |
| <para> |
| The last entry of this list is the terminator. You must |
| specify this all-zero entry. |
| </para> |
| |
| <para> |
| Then, prepare the <structname>pci_driver</structname> record: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static struct pci_driver driver = { |
| .name = KBUILD_MODNAME, |
| .id_table = snd_mychip_ids, |
| .probe = snd_mychip_probe, |
| .remove = snd_mychip_remove, |
| }; |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| The <structfield>probe</structfield> and |
| <structfield>remove</structfield> functions have already |
| been defined in the previous sections. |
| The <structfield>name</structfield> |
| field is the name string of this device. Note that you must not |
| use a slash <quote>/</quote> in this string. |
| </para> |
| |
| <para> |
| And at last, the module entries: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static int __init alsa_card_mychip_init(void) |
| { |
| return pci_register_driver(&driver); |
| } |
| |
| static void __exit alsa_card_mychip_exit(void) |
| { |
| pci_unregister_driver(&driver); |
| } |
| |
| module_init(alsa_card_mychip_init) |
| module_exit(alsa_card_mychip_exit) |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| Note that these module entries are tagged with |
| <parameter>__init</parameter> and |
| <parameter>__exit</parameter> prefixes. |
| </para> |
| |
| <para> |
| Oh, one thing was forgotten. If you have no exported symbols, |
| you need to declare it in 2.2 or 2.4 kernels (it's not necessary in 2.6 kernels). |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| EXPORT_NO_SYMBOLS; |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| That's all! |
| </para> |
| </section> |
| </chapter> |
| |
| |
| <!-- ****************************************************** --> |
| <!-- PCM Interface --> |
| <!-- ****************************************************** --> |
| <chapter id="pcm-interface"> |
| <title>PCM Interface</title> |
| |
| <section id="pcm-interface-general"> |
| <title>General</title> |
| <para> |
| The PCM middle layer of ALSA is quite powerful and it is only |
| necessary for each driver to implement the low-level functions |
| to access its hardware. |
| </para> |
| |
| <para> |
| For accessing to the PCM layer, you need to include |
| <filename><sound/pcm.h></filename> first. In addition, |
| <filename><sound/pcm_params.h></filename> might be needed |
| if you access to some functions related with hw_param. |
| </para> |
| |
| <para> |
| Each card device can have up to four pcm instances. A pcm |
| instance corresponds to a pcm device file. The limitation of |
| number of instances comes only from the available bit size of |
| the Linux's device numbers. Once when 64bit device number is |
| used, we'll have more pcm instances available. |
| </para> |
| |
| <para> |
| A pcm instance consists of pcm playback and capture streams, |
| and each pcm stream consists of one or more pcm substreams. Some |
| soundcards support multiple playback functions. For example, |
| emu10k1 has a PCM playback of 32 stereo substreams. In this case, at |
| each open, a free substream is (usually) automatically chosen |
| and opened. Meanwhile, when only one substream exists and it was |
| already opened, the successful open will either block |
| or error with <constant>EAGAIN</constant> according to the |
| file open mode. But you don't have to care about such details in your |
| driver. The PCM middle layer will take care of such work. |
| </para> |
| </section> |
| |
| <section id="pcm-interface-example"> |
| <title>Full Code Example</title> |
| <para> |
| The example code below does not include any hardware access |
| routines but shows only the skeleton, how to build up the PCM |
| interfaces. |
| |
| <example> |
| <title>PCM Example Code</title> |
| <programlisting> |
| <![CDATA[ |
| #include <sound/pcm.h> |
| .... |
| |
| /* hardware definition */ |
| static struct snd_pcm_hardware snd_mychip_playback_hw = { |
| .info = (SNDRV_PCM_INFO_MMAP | |
| SNDRV_PCM_INFO_INTERLEAVED | |
| SNDRV_PCM_INFO_BLOCK_TRANSFER | |
| SNDRV_PCM_INFO_MMAP_VALID), |
| .formats = SNDRV_PCM_FMTBIT_S16_LE, |
| .rates = SNDRV_PCM_RATE_8000_48000, |
| .rate_min = 8000, |
| .rate_max = 48000, |
| .channels_min = 2, |
| .channels_max = 2, |
| .buffer_bytes_max = 32768, |
| .period_bytes_min = 4096, |
| .period_bytes_max = 32768, |
| .periods_min = 1, |
| .periods_max = 1024, |
| }; |
| |
| /* hardware definition */ |
| static struct snd_pcm_hardware snd_mychip_capture_hw = { |
| .info = (SNDRV_PCM_INFO_MMAP | |
| SNDRV_PCM_INFO_INTERLEAVED | |
| SNDRV_PCM_INFO_BLOCK_TRANSFER | |
| SNDRV_PCM_INFO_MMAP_VALID), |
| .formats = SNDRV_PCM_FMTBIT_S16_LE, |
| .rates = SNDRV_PCM_RATE_8000_48000, |
| .rate_min = 8000, |
| .rate_max = 48000, |
| .channels_min = 2, |
| .channels_max = 2, |
| .buffer_bytes_max = 32768, |
| .period_bytes_min = 4096, |
| .period_bytes_max = 32768, |
| .periods_min = 1, |
| .periods_max = 1024, |
| }; |
| |
| /* open callback */ |
| static int snd_mychip_playback_open(struct snd_pcm_substream *substream) |
| { |
| struct mychip *chip = snd_pcm_substream_chip(substream); |
| struct snd_pcm_runtime *runtime = substream->runtime; |
| |
| runtime->hw = snd_mychip_playback_hw; |
| /* more hardware-initialization will be done here */ |
| .... |
| return 0; |
| } |
| |
| /* close callback */ |
| static int snd_mychip_playback_close(struct snd_pcm_substream *substream) |
| { |
| struct mychip *chip = snd_pcm_substream_chip(substream); |
| /* the hardware-specific codes will be here */ |
| .... |
| return 0; |
| |
| } |
| |
| /* open callback */ |
| static int snd_mychip_capture_open(struct snd_pcm_substream *substream) |
| { |
| struct mychip *chip = snd_pcm_substream_chip(substream); |
| struct snd_pcm_runtime *runtime = substream->runtime; |
| |
| runtime->hw = snd_mychip_capture_hw; |
| /* more hardware-initialization will be done here */ |
| .... |
| return 0; |
| } |
| |
| /* close callback */ |
| static int snd_mychip_capture_close(struct snd_pcm_substream *substream) |
| { |
| struct mychip *chip = snd_pcm_substream_chip(substream); |
| /* the hardware-specific codes will be here */ |
| .... |
| return 0; |
| |
| } |
| |
| /* hw_params callback */ |
| static int snd_mychip_pcm_hw_params(struct snd_pcm_substream *substream, |
| struct snd_pcm_hw_params *hw_params) |
| { |
| return snd_pcm_lib_malloc_pages(substream, |
| params_buffer_bytes(hw_params)); |
| } |
| |
| /* hw_free callback */ |
| static int snd_mychip_pcm_hw_free(struct snd_pcm_substream *substream) |
| { |
| return snd_pcm_lib_free_pages(substream); |
| } |
| |
| /* prepare callback */ |
| static int snd_mychip_pcm_prepare(struct snd_pcm_substream *substream) |
| { |
| struct mychip *chip = snd_pcm_substream_chip(substream); |
| struct snd_pcm_runtime *runtime = substream->runtime; |
| |
| /* set up the hardware with the current configuration |
| * for example... |
| */ |
| mychip_set_sample_format(chip, runtime->format); |
| mychip_set_sample_rate(chip, runtime->rate); |
| mychip_set_channels(chip, runtime->channels); |
| mychip_set_dma_setup(chip, runtime->dma_addr, |
| chip->buffer_size, |
| chip->period_size); |
| return 0; |
| } |
| |
| /* trigger callback */ |
| static int snd_mychip_pcm_trigger(struct snd_pcm_substream *substream, |
| int cmd) |
| { |
| switch (cmd) { |
| case SNDRV_PCM_TRIGGER_START: |
| /* do something to start the PCM engine */ |
| .... |
| break; |
| case SNDRV_PCM_TRIGGER_STOP: |
| /* do something to stop the PCM engine */ |
| .... |
| break; |
| default: |
| return -EINVAL; |
| } |
| } |
| |
| /* pointer callback */ |
| static snd_pcm_uframes_t |
| snd_mychip_pcm_pointer(struct snd_pcm_substream *substream) |
| { |
| struct mychip *chip = snd_pcm_substream_chip(substream); |
| unsigned int current_ptr; |
| |
| /* get the current hardware pointer */ |
| current_ptr = mychip_get_hw_pointer(chip); |
| return current_ptr; |
| } |
| |
| /* operators */ |
| static struct snd_pcm_ops snd_mychip_playback_ops = { |
| .open = snd_mychip_playback_open, |
| .close = snd_mychip_playback_close, |
| .ioctl = snd_pcm_lib_ioctl, |
| .hw_params = snd_mychip_pcm_hw_params, |
| .hw_free = snd_mychip_pcm_hw_free, |
| .prepare = snd_mychip_pcm_prepare, |
| .trigger = snd_mychip_pcm_trigger, |
| .pointer = snd_mychip_pcm_pointer, |
| }; |
| |
| /* operators */ |
| static struct snd_pcm_ops snd_mychip_capture_ops = { |
| .open = snd_mychip_capture_open, |
| .close = snd_mychip_capture_close, |
| .ioctl = snd_pcm_lib_ioctl, |
| .hw_params = snd_mychip_pcm_hw_params, |
| .hw_free = snd_mychip_pcm_hw_free, |
| .prepare = snd_mychip_pcm_prepare, |
| .trigger = snd_mychip_pcm_trigger, |
| .pointer = snd_mychip_pcm_pointer, |
| }; |
| |
| /* |
| * definitions of capture are omitted here... |
| */ |
| |
| /* create a pcm device */ |
| static int snd_mychip_new_pcm(struct mychip *chip) |
| { |
| struct snd_pcm *pcm; |
| int err; |
| |
| err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm); |
| if (err < 0) |
| return err; |
| pcm->private_data = chip; |
| strcpy(pcm->name, "My Chip"); |
| chip->pcm = pcm; |
| /* set operators */ |
| snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK, |
| &snd_mychip_playback_ops); |
| snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE, |
| &snd_mychip_capture_ops); |
| /* pre-allocation of buffers */ |
| /* NOTE: this may fail */ |
| snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV, |
| snd_dma_pci_data(chip->pci), |
| 64*1024, 64*1024); |
| return 0; |
| } |
| ]]> |
| </programlisting> |
| </example> |
| </para> |
| </section> |
| |
| <section id="pcm-interface-constructor"> |
| <title>Constructor</title> |
| <para> |
| A pcm instance is allocated by the <function>snd_pcm_new()</function> |
| function. It would be better to create a constructor for pcm, |
| namely, |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static int snd_mychip_new_pcm(struct mychip *chip) |
| { |
| struct snd_pcm *pcm; |
| int err; |
| |
| err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm); |
| if (err < 0) |
| return err; |
| pcm->private_data = chip; |
| strcpy(pcm->name, "My Chip"); |
| chip->pcm = pcm; |
| .... |
| return 0; |
| } |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| The <function>snd_pcm_new()</function> function takes four |
| arguments. The first argument is the card pointer to which this |
| pcm is assigned, and the second is the ID string. |
| </para> |
| |
| <para> |
| The third argument (<parameter>index</parameter>, 0 in the |
| above) is the index of this new pcm. It begins from zero. If |
| you create more than one pcm instances, specify the |
| different numbers in this argument. For example, |
| <parameter>index</parameter> = 1 for the second PCM device. |
| </para> |
| |
| <para> |
| The fourth and fifth arguments are the number of substreams |
| for playback and capture, respectively. Here 1 is used for |
| both arguments. When no playback or capture substreams are available, |
| pass 0 to the corresponding argument. |
| </para> |
| |
| <para> |
| If a chip supports multiple playbacks or captures, you can |
| specify more numbers, but they must be handled properly in |
| open/close, etc. callbacks. When you need to know which |
| substream you are referring to, then it can be obtained from |
| struct <structname>snd_pcm_substream</structname> data passed to each callback |
| as follows: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| struct snd_pcm_substream *substream; |
| int index = substream->number; |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| After the pcm is created, you need to set operators for each |
| pcm stream. |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK, |
| &snd_mychip_playback_ops); |
| snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE, |
| &snd_mychip_capture_ops); |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| The operators are defined typically like this: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static struct snd_pcm_ops snd_mychip_playback_ops = { |
| .open = snd_mychip_pcm_open, |
| .close = snd_mychip_pcm_close, |
| .ioctl = snd_pcm_lib_ioctl, |
| .hw_params = snd_mychip_pcm_hw_params, |
| .hw_free = snd_mychip_pcm_hw_free, |
| .prepare = snd_mychip_pcm_prepare, |
| .trigger = snd_mychip_pcm_trigger, |
| .pointer = snd_mychip_pcm_pointer, |
| }; |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| All the callbacks are described in the |
| <link linkend="pcm-interface-operators"><citetitle> |
| Operators</citetitle></link> subsection. |
| </para> |
| |
| <para> |
| After setting the operators, you probably will want to |
| pre-allocate the buffer. For the pre-allocation, simply call |
| the following: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV, |
| snd_dma_pci_data(chip->pci), |
| 64*1024, 64*1024); |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| It will allocate a buffer up to 64kB as default. |
| Buffer management details will be described in the later section <link |
| linkend="buffer-and-memory"><citetitle>Buffer and Memory |
| Management</citetitle></link>. |
| </para> |
| |
| <para> |
| Additionally, you can set some extra information for this pcm |
| in pcm->info_flags. |
| The available values are defined as |
| <constant>SNDRV_PCM_INFO_XXX</constant> in |
| <filename><sound/asound.h></filename>, which is used for |
| the hardware definition (described later). When your soundchip |
| supports only half-duplex, specify like this: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| pcm->info_flags = SNDRV_PCM_INFO_HALF_DUPLEX; |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| </section> |
| |
| <section id="pcm-interface-destructor"> |
| <title>... And the Destructor?</title> |
| <para> |
| The destructor for a pcm instance is not always |
| necessary. Since the pcm device will be released by the middle |
| layer code automatically, you don't have to call the destructor |
| explicitly. |
| </para> |
| |
| <para> |
| The destructor would be necessary if you created |
| special records internally and needed to release them. In such a |
| case, set the destructor function to |
| pcm->private_free: |
| |
| <example> |
| <title>PCM Instance with a Destructor</title> |
| <programlisting> |
| <![CDATA[ |
| static void mychip_pcm_free(struct snd_pcm *pcm) |
| { |
| struct mychip *chip = snd_pcm_chip(pcm); |
| /* free your own data */ |
| kfree(chip->my_private_pcm_data); |
| /* do what you like else */ |
| .... |
| } |
| |
| static int snd_mychip_new_pcm(struct mychip *chip) |
| { |
| struct snd_pcm *pcm; |
| .... |
| /* allocate your own data */ |
| chip->my_private_pcm_data = kmalloc(...); |
| /* set the destructor */ |
| pcm->private_data = chip; |
| pcm->private_free = mychip_pcm_free; |
| .... |
| } |
| ]]> |
| </programlisting> |
| </example> |
| </para> |
| </section> |
| |
| <section id="pcm-interface-runtime"> |
| <title>Runtime Pointer - The Chest of PCM Information</title> |
| <para> |
| When the PCM substream is opened, a PCM runtime instance is |
| allocated and assigned to the substream. This pointer is |
| accessible via <constant>substream->runtime</constant>. |
| This runtime pointer holds most information you need |
| to control the PCM: the copy of hw_params and sw_params configurations, the buffer |
| pointers, mmap records, spinlocks, etc. |
| </para> |
| |
| <para> |
| The definition of runtime instance is found in |
| <filename><sound/pcm.h></filename>. Here are |
| the contents of this file: |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| struct _snd_pcm_runtime { |
| /* -- Status -- */ |
| struct snd_pcm_substream *trigger_master; |
| snd_timestamp_t trigger_tstamp; /* trigger timestamp */ |
| int overrange; |
| snd_pcm_uframes_t avail_max; |
| snd_pcm_uframes_t hw_ptr_base; /* Position at buffer restart */ |
| snd_pcm_uframes_t hw_ptr_interrupt; /* Position at interrupt time*/ |
| |
| /* -- HW params -- */ |
| snd_pcm_access_t access; /* access mode */ |
| snd_pcm_format_t format; /* SNDRV_PCM_FORMAT_* */ |
| snd_pcm_subformat_t subformat; /* subformat */ |
| unsigned int rate; /* rate in Hz */ |
| unsigned int channels; /* channels */ |
| snd_pcm_uframes_t period_size; /* period size */ |
| unsigned int periods; /* periods */ |
| snd_pcm_uframes_t buffer_size; /* buffer size */ |
| unsigned int tick_time; /* tick time */ |
| snd_pcm_uframes_t min_align; /* Min alignment for the format */ |
| size_t byte_align; |
| unsigned int frame_bits; |
| unsigned int sample_bits; |
| unsigned int info; |
| unsigned int rate_num; |
| unsigned int rate_den; |
| |
| /* -- SW params -- */ |
| struct timespec tstamp_mode; /* mmap timestamp is updated */ |
| unsigned int period_step; |
| unsigned int sleep_min; /* min ticks to sleep */ |
| snd_pcm_uframes_t start_threshold; |
| snd_pcm_uframes_t stop_threshold; |
| snd_pcm_uframes_t silence_threshold; /* Silence filling happens when |
| noise is nearest than this */ |
| snd_pcm_uframes_t silence_size; /* Silence filling size */ |
| snd_pcm_uframes_t boundary; /* pointers wrap point */ |
| |
| snd_pcm_uframes_t silenced_start; |
| snd_pcm_uframes_t silenced_size; |
| |
| snd_pcm_sync_id_t sync; /* hardware synchronization ID */ |
| |
| /* -- mmap -- */ |
| volatile struct snd_pcm_mmap_status *status; |
| volatile struct snd_pcm_mmap_control *control; |
| atomic_t mmap_count; |
| |
| /* -- locking / scheduling -- */ |
| spinlock_t lock; |
| wait_queue_head_t sleep; |
| struct timer_list tick_timer; |
| struct fasync_struct *fasync; |
| |
| /* -- private section -- */ |
| void *private_data; |
| void (*private_free)(struct snd_pcm_runtime *runtime); |
| |
| /* -- hardware description -- */ |
| struct snd_pcm_hardware hw; |
| struct snd_pcm_hw_constraints hw_constraints; |
| |
| /* -- interrupt callbacks -- */ |
| void (*transfer_ack_begin)(struct snd_pcm_substream *substream); |
| void (*transfer_ack_end)(struct snd_pcm_substream *substream); |
| |
| /* -- timer -- */ |
| unsigned int timer_resolution; /* timer resolution */ |
| |
| /* -- DMA -- */ |
| unsigned char *dma_area; /* DMA area */ |
| dma_addr_t dma_addr; /* physical bus address (not accessible from main CPU) */ |
| size_t dma_bytes; /* size of DMA area */ |
| |
| struct snd_dma_buffer *dma_buffer_p; /* allocated buffer */ |
| |
| #if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE) |
| /* -- OSS things -- */ |
| struct snd_pcm_oss_runtime oss; |
| #endif |
| }; |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| For the operators (callbacks) of each sound driver, most of |
| these records are supposed to be read-only. Only the PCM |
| middle-layer changes / updates them. The exceptions are |
| the hardware description (hw), interrupt callbacks |
| (transfer_ack_xxx), DMA buffer information, and the private |
| data. Besides, if you use the standard buffer allocation |
| method via <function>snd_pcm_lib_malloc_pages()</function>, |
| you don't need to set the DMA buffer information by yourself. |
| </para> |
| |
| <para> |
| In the sections below, important records are explained. |
| </para> |
| |
| <section id="pcm-interface-runtime-hw"> |
| <title>Hardware Description</title> |
| <para> |
| The hardware descriptor (struct <structname>snd_pcm_hardware</structname>) |
| contains the definitions of the fundamental hardware |
| configuration. Above all, you'll need to define this in |
| <link linkend="pcm-interface-operators-open-callback"><citetitle> |
| the open callback</citetitle></link>. |
| Note that the runtime instance holds the copy of the |
| descriptor, not the pointer to the existing descriptor. That |
| is, in the open callback, you can modify the copied descriptor |
| (<constant>runtime->hw</constant>) as you need. For example, if the maximum |
| number of channels is 1 only on some chip models, you can |
| still use the same hardware descriptor and change the |
| channels_max later: |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| struct snd_pcm_runtime *runtime = substream->runtime; |
| ... |
| runtime->hw = snd_mychip_playback_hw; /* common definition */ |
| if (chip->model == VERY_OLD_ONE) |
| runtime->hw.channels_max = 1; |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| Typically, you'll have a hardware descriptor as below: |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static struct snd_pcm_hardware snd_mychip_playback_hw = { |
| .info = (SNDRV_PCM_INFO_MMAP | |
| SNDRV_PCM_INFO_INTERLEAVED | |
| SNDRV_PCM_INFO_BLOCK_TRANSFER | |
| SNDRV_PCM_INFO_MMAP_VALID), |
| .formats = SNDRV_PCM_FMTBIT_S16_LE, |
| .rates = SNDRV_PCM_RATE_8000_48000, |
| .rate_min = 8000, |
| .rate_max = 48000, |
| .channels_min = 2, |
| .channels_max = 2, |
| .buffer_bytes_max = 32768, |
| .period_bytes_min = 4096, |
| .period_bytes_max = 32768, |
| .periods_min = 1, |
| .periods_max = 1024, |
| }; |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| <itemizedlist> |
| <listitem><para> |
| The <structfield>info</structfield> field contains the type and |
| capabilities of this pcm. The bit flags are defined in |
| <filename><sound/asound.h></filename> as |
| <constant>SNDRV_PCM_INFO_XXX</constant>. Here, at least, you |
| have to specify whether the mmap is supported and which |
| interleaved format is supported. |
| When the hardware supports mmap, add the |
| <constant>SNDRV_PCM_INFO_MMAP</constant> flag here. When the |
| hardware supports the interleaved or the non-interleaved |
| formats, <constant>SNDRV_PCM_INFO_INTERLEAVED</constant> or |
| <constant>SNDRV_PCM_INFO_NONINTERLEAVED</constant> flag must |
| be set, respectively. If both are supported, you can set both, |
| too. |
| </para> |
| |
| <para> |
| In the above example, <constant>MMAP_VALID</constant> and |
| <constant>BLOCK_TRANSFER</constant> are specified for the OSS mmap |
| mode. Usually both are set. Of course, |
| <constant>MMAP_VALID</constant> is set only if the mmap is |
| really supported. |
| </para> |
| |
| <para> |
| The other possible flags are |
| <constant>SNDRV_PCM_INFO_PAUSE</constant> and |
| <constant>SNDRV_PCM_INFO_RESUME</constant>. The |
| <constant>PAUSE</constant> bit means that the pcm supports the |
| <quote>pause</quote> operation, while the |
| <constant>RESUME</constant> bit means that the pcm supports |
| the full <quote>suspend/resume</quote> operation. |
| If the <constant>PAUSE</constant> flag is set, |
| the <structfield>trigger</structfield> callback below |
| must handle the corresponding (pause push/release) commands. |
| The suspend/resume trigger commands can be defined even without |
| the <constant>RESUME</constant> flag. See <link |
| linkend="power-management"><citetitle> |
| Power Management</citetitle></link> section for details. |
| </para> |
| |
| <para> |
| When the PCM substreams can be synchronized (typically, |
| synchronized start/stop of a playback and a capture streams), |
| you can give <constant>SNDRV_PCM_INFO_SYNC_START</constant>, |
| too. In this case, you'll need to check the linked-list of |
| PCM substreams in the trigger callback. This will be |
| described in the later section. |
| </para> |
| </listitem> |
| |
| <listitem> |
| <para> |
| <structfield>formats</structfield> field contains the bit-flags |
| of supported formats (<constant>SNDRV_PCM_FMTBIT_XXX</constant>). |
| If the hardware supports more than one format, give all or'ed |
| bits. In the example above, the signed 16bit little-endian |
| format is specified. |
| </para> |
| </listitem> |
| |
| <listitem> |
| <para> |
| <structfield>rates</structfield> field contains the bit-flags of |
| supported rates (<constant>SNDRV_PCM_RATE_XXX</constant>). |
| When the chip supports continuous rates, pass |
| <constant>CONTINUOUS</constant> bit additionally. |
| The pre-defined rate bits are provided only for typical |
| rates. If your chip supports unconventional rates, you need to add |
| the <constant>KNOT</constant> bit and set up the hardware |
| constraint manually (explained later). |
| </para> |
| </listitem> |
| |
| <listitem> |
| <para> |
| <structfield>rate_min</structfield> and |
| <structfield>rate_max</structfield> define the minimum and |
| maximum sample rate. This should correspond somehow to |
| <structfield>rates</structfield> bits. |
| </para> |
| </listitem> |
| |
| <listitem> |
| <para> |
| <structfield>channel_min</structfield> and |
| <structfield>channel_max</structfield> |
| define, as you might already expected, the minimum and maximum |
| number of channels. |
| </para> |
| </listitem> |
| |
| <listitem> |
| <para> |
| <structfield>buffer_bytes_max</structfield> defines the |
| maximum buffer size in bytes. There is no |
| <structfield>buffer_bytes_min</structfield> field, since |
| it can be calculated from the minimum period size and the |
| minimum number of periods. |
| Meanwhile, <structfield>period_bytes_min</structfield> and |
| define the minimum and maximum size of the period in bytes. |
| <structfield>periods_max</structfield> and |
| <structfield>periods_min</structfield> define the maximum and |
| minimum number of periods in the buffer. |
| </para> |
| |
| <para> |
| The <quote>period</quote> is a term that corresponds to |
| a fragment in the OSS world. The period defines the size at |
| which a PCM interrupt is generated. This size strongly |
| depends on the hardware. |
| Generally, the smaller period size will give you more |
| interrupts, that is, more controls. |
| In the case of capture, this size defines the input latency. |
| On the other hand, the whole buffer size defines the |
| output latency for the playback direction. |
| </para> |
| </listitem> |
| |
| <listitem> |
| <para> |
| There is also a field <structfield>fifo_size</structfield>. |
| This specifies the size of the hardware FIFO, but currently it |
| is neither used in the driver nor in the alsa-lib. So, you |
| can ignore this field. |
| </para> |
| </listitem> |
| </itemizedlist> |
| </para> |
| </section> |
| |
| <section id="pcm-interface-runtime-config"> |
| <title>PCM Configurations</title> |
| <para> |
| Ok, let's go back again to the PCM runtime records. |
| The most frequently referred records in the runtime instance are |
| the PCM configurations. |
| The PCM configurations are stored in the runtime instance |
| after the application sends <type>hw_params</type> data via |
| alsa-lib. There are many fields copied from hw_params and |
| sw_params structs. For example, |
| <structfield>format</structfield> holds the format type |
| chosen by the application. This field contains the enum value |
| <constant>SNDRV_PCM_FORMAT_XXX</constant>. |
| </para> |
| |
| <para> |
| One thing to be noted is that the configured buffer and period |
| sizes are stored in <quote>frames</quote> in the runtime. |
| In the ALSA world, 1 frame = channels * samples-size. |
| For conversion between frames and bytes, you can use the |
| <function>frames_to_bytes()</function> and |
| <function>bytes_to_frames()</function> helper functions. |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| period_bytes = frames_to_bytes(runtime, runtime->period_size); |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| Also, many software parameters (sw_params) are |
| stored in frames, too. Please check the type of the field. |
| <type>snd_pcm_uframes_t</type> is for the frames as unsigned |
| integer while <type>snd_pcm_sframes_t</type> is for the frames |
| as signed integer. |
| </para> |
| </section> |
| |
| <section id="pcm-interface-runtime-dma"> |
| <title>DMA Buffer Information</title> |
| <para> |
| The DMA buffer is defined by the following four fields, |
| <structfield>dma_area</structfield>, |
| <structfield>dma_addr</structfield>, |
| <structfield>dma_bytes</structfield> and |
| <structfield>dma_private</structfield>. |
| The <structfield>dma_area</structfield> holds the buffer |
| pointer (the logical address). You can call |
| <function>memcpy</function> from/to |
| this pointer. Meanwhile, <structfield>dma_addr</structfield> |
| holds the physical address of the buffer. This field is |
| specified only when the buffer is a linear buffer. |
| <structfield>dma_bytes</structfield> holds the size of buffer |
| in bytes. <structfield>dma_private</structfield> is used for |
| the ALSA DMA allocator. |
| </para> |
| |
| <para> |
| If you use a standard ALSA function, |
| <function>snd_pcm_lib_malloc_pages()</function>, for |
| allocating the buffer, these fields are set by the ALSA middle |
| layer, and you should <emphasis>not</emphasis> change them by |
| yourself. You can read them but not write them. |
| On the other hand, if you want to allocate the buffer by |
| yourself, you'll need to manage it in hw_params callback. |
| At least, <structfield>dma_bytes</structfield> is mandatory. |
| <structfield>dma_area</structfield> is necessary when the |
| buffer is mmapped. If your driver doesn't support mmap, this |
| field is not necessary. <structfield>dma_addr</structfield> |
| is also optional. You can use |
| <structfield>dma_private</structfield> as you like, too. |
| </para> |
| </section> |
| |
| <section id="pcm-interface-runtime-status"> |
| <title>Running Status</title> |
| <para> |
| The running status can be referred via <constant>runtime->status</constant>. |
| This is the pointer to the struct <structname>snd_pcm_mmap_status</structname> |
| record. For example, you can get the current DMA hardware |
| pointer via <constant>runtime->status->hw_ptr</constant>. |
| </para> |
| |
| <para> |
| The DMA application pointer can be referred via |
| <constant>runtime->control</constant>, which points to the |
| struct <structname>snd_pcm_mmap_control</structname> record. |
| However, accessing directly to this value is not recommended. |
| </para> |
| </section> |
| |
| <section id="pcm-interface-runtime-private"> |
| <title>Private Data</title> |
| <para> |
| You can allocate a record for the substream and store it in |
| <constant>runtime->private_data</constant>. Usually, this |
| is done in |
| <link linkend="pcm-interface-operators-open-callback"><citetitle> |
| the open callback</citetitle></link>. |
| Don't mix this with <constant>pcm->private_data</constant>. |
| The <constant>pcm->private_data</constant> usually points to the |
| chip instance assigned statically at the creation of PCM, while the |
| <constant>runtime->private_data</constant> points to a dynamic |
| data structure created at the PCM open callback. |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static int snd_xxx_open(struct snd_pcm_substream *substream) |
| { |
| struct my_pcm_data *data; |
| .... |
| data = kmalloc(sizeof(*data), GFP_KERNEL); |
| substream->runtime->private_data = data; |
| .... |
| } |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| The allocated object must be released in |
| <link linkend="pcm-interface-operators-open-callback"><citetitle> |
| the close callback</citetitle></link>. |
| </para> |
| </section> |
| |
| <section id="pcm-interface-runtime-intr"> |
| <title>Interrupt Callbacks</title> |
| <para> |
| The field <structfield>transfer_ack_begin</structfield> and |
| <structfield>transfer_ack_end</structfield> are called at |
| the beginning and at the end of |
| <function>snd_pcm_period_elapsed()</function>, respectively. |
| </para> |
| </section> |
| |
| </section> |
| |
| <section id="pcm-interface-operators"> |
| <title>Operators</title> |
| <para> |
| OK, now let me give details about each pcm callback |
| (<parameter>ops</parameter>). In general, every callback must |
| return 0 if successful, or a negative error number |
| such as <constant>-EINVAL</constant>. To choose an appropriate |
| error number, it is advised to check what value other parts of |
| the kernel return when the same kind of request fails. |
| </para> |
| |
| <para> |
| The callback function takes at least the argument with |
| <structname>snd_pcm_substream</structname> pointer. To retrieve |
| the chip record from the given substream instance, you can use the |
| following macro. |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| int xxx() { |
| struct mychip *chip = snd_pcm_substream_chip(substream); |
| .... |
| } |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| The macro reads <constant>substream->private_data</constant>, |
| which is a copy of <constant>pcm->private_data</constant>. |
| You can override the former if you need to assign different data |
| records per PCM substream. For example, the cmi8330 driver assigns |
| different private_data for playback and capture directions, |
| because it uses two different codecs (SB- and AD-compatible) for |
| different directions. |
| </para> |
| |
| <section id="pcm-interface-operators-open-callback"> |
| <title>open callback</title> |
| <para> |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static int snd_xxx_open(struct snd_pcm_substream *substream); |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| This is called when a pcm substream is opened. |
| </para> |
| |
| <para> |
| At least, here you have to initialize the runtime->hw |
| record. Typically, this is done by like this: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static int snd_xxx_open(struct snd_pcm_substream *substream) |
| { |
| struct mychip *chip = snd_pcm_substream_chip(substream); |
| struct snd_pcm_runtime *runtime = substream->runtime; |
| |
| runtime->hw = snd_mychip_playback_hw; |
| return 0; |
| } |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| where <parameter>snd_mychip_playback_hw</parameter> is the |
| pre-defined hardware description. |
| </para> |
| |
| <para> |
| You can allocate a private data in this callback, as described |
| in <link linkend="pcm-interface-runtime-private"><citetitle> |
| Private Data</citetitle></link> section. |
| </para> |
| |
| <para> |
| If the hardware configuration needs more constraints, set the |
| hardware constraints here, too. |
| See <link linkend="pcm-interface-constraints"><citetitle> |
| Constraints</citetitle></link> for more details. |
| </para> |
| </section> |
| |
| <section id="pcm-interface-operators-close-callback"> |
| <title>close callback</title> |
| <para> |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static int snd_xxx_close(struct snd_pcm_substream *substream); |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| Obviously, this is called when a pcm substream is closed. |
| </para> |
| |
| <para> |
| Any private instance for a pcm substream allocated in the |
| open callback will be released here. |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static int snd_xxx_close(struct snd_pcm_substream *substream) |
| { |
| .... |
| kfree(substream->runtime->private_data); |
| .... |
| } |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| </section> |
| |
| <section id="pcm-interface-operators-ioctl-callback"> |
| <title>ioctl callback</title> |
| <para> |
| This is used for any special call to pcm ioctls. But |
| usually you can pass a generic ioctl callback, |
| <function>snd_pcm_lib_ioctl</function>. |
| </para> |
| </section> |
| |
| <section id="pcm-interface-operators-hw-params-callback"> |
| <title>hw_params callback</title> |
| <para> |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static int snd_xxx_hw_params(struct snd_pcm_substream *substream, |
| struct snd_pcm_hw_params *hw_params); |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| This is called when the hardware parameter |
| (<structfield>hw_params</structfield>) is set |
| up by the application, |
| that is, once when the buffer size, the period size, the |
| format, etc. are defined for the pcm substream. |
| </para> |
| |
| <para> |
| Many hardware setups should be done in this callback, |
| including the allocation of buffers. |
| </para> |
| |
| <para> |
| Parameters to be initialized are retrieved by |
| <function>params_xxx()</function> macros. To allocate |
| buffer, you can call a helper function, |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(hw_params)); |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| <function>snd_pcm_lib_malloc_pages()</function> is available |
| only when the DMA buffers have been pre-allocated. |
| See the section <link |
| linkend="buffer-and-memory-buffer-types"><citetitle> |
| Buffer Types</citetitle></link> for more details. |
| </para> |
| |
| <para> |
| Note that this and <structfield>prepare</structfield> callbacks |
| may be called multiple times per initialization. |
| For example, the OSS emulation may |
| call these callbacks at each change via its ioctl. |
| </para> |
| |
| <para> |
| Thus, you need to be careful not to allocate the same buffers |
| many times, which will lead to memory leaks! Calling the |
| helper function above many times is OK. It will release the |
| previous buffer automatically when it was already allocated. |
| </para> |
| |
| <para> |
| Another note is that this callback is non-atomic |
| (schedulable). This is important, because the |
| <structfield>trigger</structfield> callback |
| is atomic (non-schedulable). That is, mutexes or any |
| schedule-related functions are not available in |
| <structfield>trigger</structfield> callback. |
| Please see the subsection |
| <link linkend="pcm-interface-atomicity"><citetitle> |
| Atomicity</citetitle></link> for details. |
| </para> |
| </section> |
| |
| <section id="pcm-interface-operators-hw-free-callback"> |
| <title>hw_free callback</title> |
| <para> |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static int snd_xxx_hw_free(struct snd_pcm_substream *substream); |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| This is called to release the resources allocated via |
| <structfield>hw_params</structfield>. For example, releasing the |
| buffer via |
| <function>snd_pcm_lib_malloc_pages()</function> is done by |
| calling the following: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| snd_pcm_lib_free_pages(substream); |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| This function is always called before the close callback is called. |
| Also, the callback may be called multiple times, too. |
| Keep track whether the resource was already released. |
| </para> |
| </section> |
| |
| <section id="pcm-interface-operators-prepare-callback"> |
| <title>prepare callback</title> |
| <para> |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static int snd_xxx_prepare(struct snd_pcm_substream *substream); |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| This callback is called when the pcm is |
| <quote>prepared</quote>. You can set the format type, sample |
| rate, etc. here. The difference from |
| <structfield>hw_params</structfield> is that the |
| <structfield>prepare</structfield> callback will be called each |
| time |
| <function>snd_pcm_prepare()</function> is called, i.e. when |
| recovering after underruns, etc. |
| </para> |
| |
| <para> |
| Note that this callback is now non-atomic. |
| You can use schedule-related functions safely in this callback. |
| </para> |
| |
| <para> |
| In this and the following callbacks, you can refer to the |
| values via the runtime record, |
| substream->runtime. |
| For example, to get the current |
| rate, format or channels, access to |
| runtime->rate, |
| runtime->format or |
| runtime->channels, respectively. |
| The physical address of the allocated buffer is set to |
| runtime->dma_area. The buffer and period sizes are |
| in runtime->buffer_size and runtime->period_size, |
| respectively. |
| </para> |
| |
| <para> |
| Be careful that this callback will be called many times at |
| each setup, too. |
| </para> |
| </section> |
| |
| <section id="pcm-interface-operators-trigger-callback"> |
| <title>trigger callback</title> |
| <para> |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static int snd_xxx_trigger(struct snd_pcm_substream *substream, int cmd); |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| This is called when the pcm is started, stopped or paused. |
| </para> |
| |
| <para> |
| Which action is specified in the second argument, |
| <constant>SNDRV_PCM_TRIGGER_XXX</constant> in |
| <filename><sound/pcm.h></filename>. At least, |
| the <constant>START</constant> and <constant>STOP</constant> |
| commands must be defined in this callback. |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| switch (cmd) { |
| case SNDRV_PCM_TRIGGER_START: |
| /* do something to start the PCM engine */ |
| break; |
| case SNDRV_PCM_TRIGGER_STOP: |
| /* do something to stop the PCM engine */ |
| break; |
| default: |
| return -EINVAL; |
| } |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| When the pcm supports the pause operation (given in the info |
| field of the hardware table), the <constant>PAUSE_PUSH</constant> |
| and <constant>PAUSE_RELEASE</constant> commands must be |
| handled here, too. The former is the command to pause the pcm, |
| and the latter to restart the pcm again. |
| </para> |
| |
| <para> |
| When the pcm supports the suspend/resume operation, |
| regardless of full or partial suspend/resume support, |
| the <constant>SUSPEND</constant> and <constant>RESUME</constant> |
| commands must be handled, too. |
| These commands are issued when the power-management status is |
| changed. Obviously, the <constant>SUSPEND</constant> and |
| <constant>RESUME</constant> commands |
| suspend and resume the pcm substream, and usually, they |
| are identical to the <constant>STOP</constant> and |
| <constant>START</constant> commands, respectively. |
| See the <link linkend="power-management"><citetitle> |
| Power Management</citetitle></link> section for details. |
| </para> |
| |
| <para> |
| As mentioned, this callback is atomic. You cannot call |
| functions which may sleep. |
| The trigger callback should be as minimal as possible, |
| just really triggering the DMA. The other stuff should be |
| initialized hw_params and prepare callbacks properly |
| beforehand. |
| </para> |
| </section> |
| |
| <section id="pcm-interface-operators-pointer-callback"> |
| <title>pointer callback</title> |
| <para> |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static snd_pcm_uframes_t snd_xxx_pointer(struct snd_pcm_substream *substream) |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| This callback is called when the PCM middle layer inquires |
| the current hardware position on the buffer. The position must |
| be returned in frames, |
| ranging from 0 to buffer_size - 1. |
| </para> |
| |
| <para> |
| This is called usually from the buffer-update routine in the |
| pcm middle layer, which is invoked when |
| <function>snd_pcm_period_elapsed()</function> is called in the |
| interrupt routine. Then the pcm middle layer updates the |
| position and calculates the available space, and wakes up the |
| sleeping poll threads, etc. |
| </para> |
| |
| <para> |
| This callback is also atomic. |
| </para> |
| </section> |
| |
| <section id="pcm-interface-operators-copy-silence"> |
| <title>copy and silence callbacks</title> |
| <para> |
| These callbacks are not mandatory, and can be omitted in |
| most cases. These callbacks are used when the hardware buffer |
| cannot be in the normal memory space. Some chips have their |
| own buffer on the hardware which is not mappable. In such a |
| case, you have to transfer the data manually from the memory |
| buffer to the hardware buffer. Or, if the buffer is |
| non-contiguous on both physical and virtual memory spaces, |
| these callbacks must be defined, too. |
| </para> |
| |
| <para> |
| If these two callbacks are defined, copy and set-silence |
| operations are done by them. The detailed will be described in |
| the later section <link |
| linkend="buffer-and-memory"><citetitle>Buffer and Memory |
| Management</citetitle></link>. |
| </para> |
| </section> |
| |
| <section id="pcm-interface-operators-ack"> |
| <title>ack callback</title> |
| <para> |
| This callback is also not mandatory. This callback is called |
| when the appl_ptr is updated in read or write operations. |
| Some drivers like emu10k1-fx and cs46xx need to track the |
| current appl_ptr for the internal buffer, and this callback |
| is useful only for such a purpose. |
| </para> |
| <para> |
| This callback is atomic. |
| </para> |
| </section> |
| |
| <section id="pcm-interface-operators-page-callback"> |
| <title>page callback</title> |
| |
| <para> |
| This callback is optional too. This callback is used |
| mainly for non-contiguous buffers. The mmap calls this |
| callback to get the page address. Some examples will be |
| explained in the later section <link |
| linkend="buffer-and-memory"><citetitle>Buffer and Memory |
| Management</citetitle></link>, too. |
| </para> |
| </section> |
| </section> |
| |
| <section id="pcm-interface-interrupt-handler"> |
| <title>Interrupt Handler</title> |
| <para> |
| The rest of pcm stuff is the PCM interrupt handler. The |
| role of PCM interrupt handler in the sound driver is to update |
| the buffer position and to tell the PCM middle layer when the |
| buffer position goes across the prescribed period size. To |
| inform this, call the <function>snd_pcm_period_elapsed()</function> |
| function. |
| </para> |
| |
| <para> |
| There are several types of sound chips to generate the interrupts. |
| </para> |
| |
| <section id="pcm-interface-interrupt-handler-boundary"> |
| <title>Interrupts at the period (fragment) boundary</title> |
| <para> |
| This is the most frequently found type: the hardware |
| generates an interrupt at each period boundary. |
| In this case, you can call |
| <function>snd_pcm_period_elapsed()</function> at each |
| interrupt. |
| </para> |
| |
| <para> |
| <function>snd_pcm_period_elapsed()</function> takes the |
| substream pointer as its argument. Thus, you need to keep the |
| substream pointer accessible from the chip instance. For |
| example, define substream field in the chip record to hold the |
| current running substream pointer, and set the pointer value |
| at open callback (and reset at close callback). |
| </para> |
| |
| <para> |
| If you acquire a spinlock in the interrupt handler, and the |
| lock is used in other pcm callbacks, too, then you have to |
| release the lock before calling |
| <function>snd_pcm_period_elapsed()</function>, because |
| <function>snd_pcm_period_elapsed()</function> calls other pcm |
| callbacks inside. |
| </para> |
| |
| <para> |
| Typical code would be like: |
| |
| <example> |
| <title>Interrupt Handler Case #1</title> |
| <programlisting> |
| <![CDATA[ |
| static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id) |
| { |
| struct mychip *chip = dev_id; |
| spin_lock(&chip->lock); |
| .... |
| if (pcm_irq_invoked(chip)) { |
| /* call updater, unlock before it */ |
| spin_unlock(&chip->lock); |
| snd_pcm_period_elapsed(chip->substream); |
| spin_lock(&chip->lock); |
| /* acknowledge the interrupt if necessary */ |
| } |
| .... |
| spin_unlock(&chip->lock); |
| return IRQ_HANDLED; |
| } |
| ]]> |
| </programlisting> |
| </example> |
| </para> |
| </section> |
| |
| <section id="pcm-interface-interrupt-handler-timer"> |
| <title>High frequency timer interrupts</title> |
| <para> |
| This happens when the hardware doesn't generate interrupts |
| at the period boundary but issues timer interrupts at a fixed |
| timer rate (e.g. es1968 or ymfpci drivers). |
| In this case, you need to check the current hardware |
| position and accumulate the processed sample length at each |
| interrupt. When the accumulated size exceeds the period |
| size, call |
| <function>snd_pcm_period_elapsed()</function> and reset the |
| accumulator. |
| </para> |
| |
| <para> |
| Typical code would be like the following. |
| |
| <example> |
| <title>Interrupt Handler Case #2</title> |
| <programlisting> |
| <![CDATA[ |
| static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id) |
| { |
| struct mychip *chip = dev_id; |
| spin_lock(&chip->lock); |
| .... |
| if (pcm_irq_invoked(chip)) { |
| unsigned int last_ptr, size; |
| /* get the current hardware pointer (in frames) */ |
| last_ptr = get_hw_ptr(chip); |
| /* calculate the processed frames since the |
| * last update |
| */ |
| if (last_ptr < chip->last_ptr) |
| size = runtime->buffer_size + last_ptr |
| - chip->last_ptr; |
| else |
| size = last_ptr - chip->last_ptr; |
| /* remember the last updated point */ |
| chip->last_ptr = last_ptr; |
| /* accumulate the size */ |
| chip->size += size; |
| /* over the period boundary? */ |
| if (chip->size >= runtime->period_size) { |
| /* reset the accumulator */ |
| chip->size %= runtime->period_size; |
| /* call updater */ |
| spin_unlock(&chip->lock); |
| snd_pcm_period_elapsed(substream); |
| spin_lock(&chip->lock); |
| } |
| /* acknowledge the interrupt if necessary */ |
| } |
| .... |
| spin_unlock(&chip->lock); |
| return IRQ_HANDLED; |
| } |
| ]]> |
| </programlisting> |
| </example> |
| </para> |
| </section> |
| |
| <section id="pcm-interface-interrupt-handler-both"> |
| <title>On calling <function>snd_pcm_period_elapsed()</function></title> |
| <para> |
| In both cases, even if more than one period are elapsed, you |
| don't have to call |
| <function>snd_pcm_period_elapsed()</function> many times. Call |
| only once. And the pcm layer will check the current hardware |
| pointer and update to the latest status. |
| </para> |
| </section> |
| </section> |
| |
| <section id="pcm-interface-atomicity"> |
| <title>Atomicity</title> |
| <para> |
| One of the most important (and thus difficult to debug) problems |
| in kernel programming are race conditions. |
| In the Linux kernel, they are usually avoided via spin-locks, mutexes |
| or semaphores. In general, if a race condition can happen |
| in an interrupt handler, it has to be managed atomically, and you |
| have to use a spinlock to protect the critical session. If the |
| critical section is not in interrupt handler code and |
| if taking a relatively long time to execute is acceptable, you |
| should use mutexes or semaphores instead. |
| </para> |
| |
| <para> |
| As already seen, some pcm callbacks are atomic and some are |
| not. For example, the <parameter>hw_params</parameter> callback is |
| non-atomic, while <parameter>trigger</parameter> callback is |
| atomic. This means, the latter is called already in a spinlock |
| held by the PCM middle layer. Please take this atomicity into |
| account when you choose a locking scheme in the callbacks. |
| </para> |
| |
| <para> |
| In the atomic callbacks, you cannot use functions which may call |
| <function>schedule</function> or go to |
| <function>sleep</function>. Semaphores and mutexes can sleep, |
| and hence they cannot be used inside the atomic callbacks |
| (e.g. <parameter>trigger</parameter> callback). |
| To implement some delay in such a callback, please use |
| <function>udelay()</function> or <function>mdelay()</function>. |
| </para> |
| |
| <para> |
| All three atomic callbacks (trigger, pointer, and ack) are |
| called with local interrupts disabled. |
| </para> |
| |
| </section> |
| <section id="pcm-interface-constraints"> |
| <title>Constraints</title> |
| <para> |
| If your chip supports unconventional sample rates, or only the |
| limited samples, you need to set a constraint for the |
| condition. |
| </para> |
| |
| <para> |
| For example, in order to restrict the sample rates in the some |
| supported values, use |
| <function>snd_pcm_hw_constraint_list()</function>. |
| You need to call this function in the open callback. |
| |
| <example> |
| <title>Example of Hardware Constraints</title> |
| <programlisting> |
| <![CDATA[ |
| static unsigned int rates[] = |
| {4000, 10000, 22050, 44100}; |
| static struct snd_pcm_hw_constraint_list constraints_rates = { |
| .count = ARRAY_SIZE(rates), |
| .list = rates, |
| .mask = 0, |
| }; |
| |
| static int snd_mychip_pcm_open(struct snd_pcm_substream *substream) |
| { |
| int err; |
| .... |
| err = snd_pcm_hw_constraint_list(substream->runtime, 0, |
| SNDRV_PCM_HW_PARAM_RATE, |
| &constraints_rates); |
| if (err < 0) |
| return err; |
| .... |
| } |
| ]]> |
| </programlisting> |
| </example> |
| </para> |
| |
| <para> |
| There are many different constraints. |
| Look at <filename>sound/pcm.h</filename> for a complete list. |
| You can even define your own constraint rules. |
| For example, let's suppose my_chip can manage a substream of 1 channel |
| if and only if the format is S16_LE, otherwise it supports any format |
| specified in the <structname>snd_pcm_hardware</structname> structure (or in any |
| other constraint_list). You can build a rule like this: |
| |
| <example> |
| <title>Example of Hardware Constraints for Channels</title> |
| <programlisting> |
| <![CDATA[ |
| static int hw_rule_channels_by_format(struct snd_pcm_hw_params *params, |
| struct snd_pcm_hw_rule *rule) |
| { |
| struct snd_interval *c = hw_param_interval(params, |
| SNDRV_PCM_HW_PARAM_CHANNELS); |
| struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT); |
| struct snd_interval ch; |
| |
| snd_interval_any(&ch); |
| if (f->bits[0] == SNDRV_PCM_FMTBIT_S16_LE) { |
| ch.min = ch.max = 1; |
| ch.integer = 1; |
| return snd_interval_refine(c, &ch); |
| } |
| return 0; |
| } |
| ]]> |
| </programlisting> |
| </example> |
| </para> |
| |
| <para> |
| Then you need to call this function to add your rule: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_CHANNELS, |
| hw_rule_channels_by_format, NULL, |
| SNDRV_PCM_HW_PARAM_FORMAT, -1); |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| The rule function is called when an application sets the PCM |
| format, and it refines the number of channels accordingly. |
| But an application may set the number of channels before |
| setting the format. Thus you also need to define the inverse rule: |
| |
| <example> |
| <title>Example of Hardware Constraints for Formats</title> |
| <programlisting> |
| <![CDATA[ |
| static int hw_rule_format_by_channels(struct snd_pcm_hw_params *params, |
| struct snd_pcm_hw_rule *rule) |
| { |
| struct snd_interval *c = hw_param_interval(params, |
| SNDRV_PCM_HW_PARAM_CHANNELS); |
| struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT); |
| struct snd_mask fmt; |
| |
| snd_mask_any(&fmt); /* Init the struct */ |
| if (c->min < 2) { |
| fmt.bits[0] &= SNDRV_PCM_FMTBIT_S16_LE; |
| return snd_mask_refine(f, &fmt); |
| } |
| return 0; |
| } |
| ]]> |
| </programlisting> |
| </example> |
| </para> |
| |
| <para> |
| ...and in the open callback: |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_FORMAT, |
| hw_rule_format_by_channels, NULL, |
| SNDRV_PCM_HW_PARAM_CHANNELS, -1); |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| I won't give more details here, rather I |
| would like to say, <quote>Luke, use the source.</quote> |
| </para> |
| </section> |
| |
| </chapter> |
| |
| |
| <!-- ****************************************************** --> |
| <!-- Control Interface --> |
| <!-- ****************************************************** --> |
| <chapter id="control-interface"> |
| <title>Control Interface</title> |
| |
| <section id="control-interface-general"> |
| <title>General</title> |
| <para> |
| The control interface is used widely for many switches, |
| sliders, etc. which are accessed from user-space. Its most |
| important use is the mixer interface. In other words, since ALSA |
| 0.9.x, all the mixer stuff is implemented on the control kernel API. |
| </para> |
| |
| <para> |
| ALSA has a well-defined AC97 control module. If your chip |
| supports only the AC97 and nothing else, you can skip this |
| section. |
| </para> |
| |
| <para> |
| The control API is defined in |
| <filename><sound/control.h></filename>. |
| Include this file if you want to add your own controls. |
| </para> |
| </section> |
| |
| <section id="control-interface-definition"> |
| <title>Definition of Controls</title> |
| <para> |
| To create a new control, you need to define the |
| following three |
| callbacks: <structfield>info</structfield>, |
| <structfield>get</structfield> and |
| <structfield>put</structfield>. Then, define a |
| struct <structname>snd_kcontrol_new</structname> record, such as: |
| |
| <example> |
| <title>Definition of a Control</title> |
| <programlisting> |
| <![CDATA[ |
| static struct snd_kcontrol_new my_control = { |
| .iface = SNDRV_CTL_ELEM_IFACE_MIXER, |
| .name = "PCM Playback Switch", |
| .index = 0, |
| .access = SNDRV_CTL_ELEM_ACCESS_READWRITE, |
| .private_value = 0xffff, |
| .info = my_control_info, |
| .get = my_control_get, |
| .put = my_control_put |
| }; |
| ]]> |
| </programlisting> |
| </example> |
| </para> |
| |
| <para> |
| The <structfield>iface</structfield> field specifies the control |
| type, <constant>SNDRV_CTL_ELEM_IFACE_XXX</constant>, which |
| is usually <constant>MIXER</constant>. |
| Use <constant>CARD</constant> for global controls that are not |
| logically part of the mixer. |
| If the control is closely associated with some specific device on |
| the sound card, use <constant>HWDEP</constant>, |
| <constant>PCM</constant>, <constant>RAWMIDI</constant>, |
| <constant>TIMER</constant>, or <constant>SEQUENCER</constant>, and |
| specify the device number with the |
| <structfield>device</structfield> and |
| <structfield>subdevice</structfield> fields. |
| </para> |
| |
| <para> |
| The <structfield>name</structfield> is the name identifier |
| string. Since ALSA 0.9.x, the control name is very important, |
| because its role is classified from its name. There are |
| pre-defined standard control names. The details are described in |
| the <link linkend="control-interface-control-names"><citetitle> |
| Control Names</citetitle></link> subsection. |
| </para> |
| |
| <para> |
| The <structfield>index</structfield> field holds the index number |
| of this control. If there are several different controls with |
| the same name, they can be distinguished by the index |
| number. This is the case when |
| several codecs exist on the card. If the index is zero, you can |
| omit the definition above. |
| </para> |
| |
| <para> |
| The <structfield>access</structfield> field contains the access |
| type of this control. Give the combination of bit masks, |
| <constant>SNDRV_CTL_ELEM_ACCESS_XXX</constant>, there. |
| The details will be explained in |
| the <link linkend="control-interface-access-flags"><citetitle> |
| Access Flags</citetitle></link> subsection. |
| </para> |
| |
| <para> |
| The <structfield>private_value</structfield> field contains |
| an arbitrary long integer value for this record. When using |
| the generic <structfield>info</structfield>, |
| <structfield>get</structfield> and |
| <structfield>put</structfield> callbacks, you can pass a value |
| through this field. If several small numbers are necessary, you can |
| combine them in bitwise. Or, it's possible to give a pointer |
| (casted to unsigned long) of some record to this field, too. |
| </para> |
| |
| <para> |
| The <structfield>tlv</structfield> field can be used to provide |
| metadata about the control; see the |
| <link linkend="control-interface-tlv"> |
| <citetitle>Metadata</citetitle></link> subsection. |
| </para> |
| |
| <para> |
| The other three are |
| <link linkend="control-interface-callbacks"><citetitle> |
| callback functions</citetitle></link>. |
| </para> |
| </section> |
| |
| <section id="control-interface-control-names"> |
| <title>Control Names</title> |
| <para> |
| There are some standards to define the control names. A |
| control is usually defined from the three parts as |
| <quote>SOURCE DIRECTION FUNCTION</quote>. |
| </para> |
| |
| <para> |
| The first, <constant>SOURCE</constant>, specifies the source |
| of the control, and is a string such as <quote>Master</quote>, |
| <quote>PCM</quote>, <quote>CD</quote> and |
| <quote>Line</quote>. There are many pre-defined sources. |
| </para> |
| |
| <para> |
| The second, <constant>DIRECTION</constant>, is one of the |
| following strings according to the direction of the control: |
| <quote>Playback</quote>, <quote>Capture</quote>, <quote>Bypass |
| Playback</quote> and <quote>Bypass Capture</quote>. Or, it can |
| be omitted, meaning both playback and capture directions. |
| </para> |
| |
| <para> |
| The third, <constant>FUNCTION</constant>, is one of the |
| following strings according to the function of the control: |
| <quote>Switch</quote>, <quote>Volume</quote> and |
| <quote>Route</quote>. |
| </para> |
| |
| <para> |
| The example of control names are, thus, <quote>Master Capture |
| Switch</quote> or <quote>PCM Playback Volume</quote>. |
| </para> |
| |
| <para> |
| There are some exceptions: |
| </para> |
| |
| <section id="control-interface-control-names-global"> |
| <title>Global capture and playback</title> |
| <para> |
| <quote>Capture Source</quote>, <quote>Capture Switch</quote> |
| and <quote>Capture Volume</quote> are used for the global |
| capture (input) source, switch and volume. Similarly, |
| <quote>Playback Switch</quote> and <quote>Playback |
| Volume</quote> are used for the global output gain switch and |
| volume. |
| </para> |
| </section> |
| |
| <section id="control-interface-control-names-tone"> |
| <title>Tone-controls</title> |
| <para> |
| tone-control switch and volumes are specified like |
| <quote>Tone Control - XXX</quote>, e.g. <quote>Tone Control - |
| Switch</quote>, <quote>Tone Control - Bass</quote>, |
| <quote>Tone Control - Center</quote>. |
| </para> |
| </section> |
| |
| <section id="control-interface-control-names-3d"> |
| <title>3D controls</title> |
| <para> |
| 3D-control switches and volumes are specified like <quote>3D |
| Control - XXX</quote>, e.g. <quote>3D Control - |
| Switch</quote>, <quote>3D Control - Center</quote>, <quote>3D |
| Control - Space</quote>. |
| </para> |
| </section> |
| |
| <section id="control-interface-control-names-mic"> |
| <title>Mic boost</title> |
| <para> |
| Mic-boost switch is set as <quote>Mic Boost</quote> or |
| <quote>Mic Boost (6dB)</quote>. |
| </para> |
| |
| <para> |
| More precise information can be found in |
| <filename>Documentation/sound/alsa/ControlNames.txt</filename>. |
| </para> |
| </section> |
| </section> |
| |
| <section id="control-interface-access-flags"> |
| <title>Access Flags</title> |
| |
| <para> |
| The access flag is the bitmask which specifies the access type |
| of the given control. The default access type is |
| <constant>SNDRV_CTL_ELEM_ACCESS_READWRITE</constant>, |
| which means both read and write are allowed to this control. |
| When the access flag is omitted (i.e. = 0), it is |
| considered as <constant>READWRITE</constant> access as default. |
| </para> |
| |
| <para> |
| When the control is read-only, pass |
| <constant>SNDRV_CTL_ELEM_ACCESS_READ</constant> instead. |
| In this case, you don't have to define |
| the <structfield>put</structfield> callback. |
| Similarly, when the control is write-only (although it's a rare |
| case), you can use the <constant>WRITE</constant> flag instead, and |
| you don't need the <structfield>get</structfield> callback. |
| </para> |
| |
| <para> |
| If the control value changes frequently (e.g. the VU meter), |
| <constant>VOLATILE</constant> flag should be given. This means |
| that the control may be changed without |
| <link linkend="control-interface-change-notification"><citetitle> |
| notification</citetitle></link>. Applications should poll such |
| a control constantly. |
| </para> |
| |
| <para> |
| When the control is inactive, set |
| the <constant>INACTIVE</constant> flag, too. |
| There are <constant>LOCK</constant> and |
| <constant>OWNER</constant> flags to change the write |
| permissions. |
| </para> |
| |
| </section> |
| |
| <section id="control-interface-callbacks"> |
| <title>Callbacks</title> |
| |
| <section id="control-interface-callbacks-info"> |
| <title>info callback</title> |
| <para> |
| The <structfield>info</structfield> callback is used to get |
| detailed information on this control. This must store the |
| values of the given struct <structname>snd_ctl_elem_info</structname> |
| object. For example, for a boolean control with a single |
| element: |
| |
| <example> |
| <title>Example of info callback</title> |
| <programlisting> |
| <![CDATA[ |
| static int snd_myctl_mono_info(struct snd_kcontrol *kcontrol, |
| struct snd_ctl_elem_info *uinfo) |
| { |
| uinfo->type = SNDRV_CTL_ELEM_TYPE_BOOLEAN; |
| uinfo->count = 1; |
| uinfo->value.integer.min = 0; |
| uinfo->value.integer.max = 1; |
| return 0; |
| } |
| ]]> |
| </programlisting> |
| </example> |
| </para> |
| |
| <para> |
| The <structfield>type</structfield> field specifies the type |
| of the control. There are <constant>BOOLEAN</constant>, |
| <constant>INTEGER</constant>, <constant>ENUMERATED</constant>, |
| <constant>BYTES</constant>, <constant>IEC958</constant> and |
| <constant>INTEGER64</constant>. The |
| <structfield>count</structfield> field specifies the |
| number of elements in this control. For example, a stereo |
| volume would have count = 2. The |
| <structfield>value</structfield> field is a union, and |
| the values stored are depending on the type. The boolean and |
| integer types are identical. |
| </para> |
| |
| <para> |
| The enumerated type is a bit different from others. You'll |
| need to set the string for the currently given item index. |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol, |
| struct snd_ctl_elem_info *uinfo) |
| { |
| static char *texts[4] = { |
| "First", "Second", "Third", "Fourth" |
| }; |
| uinfo->type = SNDRV_CTL_ELEM_TYPE_ENUMERATED; |
| uinfo->count = 1; |
| uinfo->value.enumerated.items = 4; |
| if (uinfo->value.enumerated.item > 3) |
| uinfo->value.enumerated.item = 3; |
| strcpy(uinfo->value.enumerated.name, |
| texts[uinfo->value.enumerated.item]); |
| return 0; |
| } |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| Some common info callbacks are available for your convenience: |
| <function>snd_ctl_boolean_mono_info()</function> and |
| <function>snd_ctl_boolean_stereo_info()</function>. |
| Obviously, the former is an info callback for a mono channel |
| boolean item, just like <function>snd_myctl_mono_info</function> |
| above, and the latter is for a stereo channel boolean item. |
| </para> |
| |
| </section> |
| |
| <section id="control-interface-callbacks-get"> |
| <title>get callback</title> |
| |
| <para> |
| This callback is used to read the current value of the |
| control and to return to user-space. |
| </para> |
| |
| <para> |
| For example, |
| |
| <example> |
| <title>Example of get callback</title> |
| <programlisting> |
| <![CDATA[ |
| static int snd_myctl_get(struct snd_kcontrol *kcontrol, |
| struct snd_ctl_elem_value *ucontrol) |
| { |
| struct mychip *chip = snd_kcontrol_chip(kcontrol); |
| ucontrol->value.integer.value[0] = get_some_value(chip); |
| return 0; |
| } |
| ]]> |
| </programlisting> |
| </example> |
| </para> |
| |
| <para> |
| The <structfield>value</structfield> field depends on |
| the type of control as well as on the info callback. For example, |
| the sb driver uses this field to store the register offset, |
| the bit-shift and the bit-mask. The |
| <structfield>private_value</structfield> field is set as follows: |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| .private_value = reg | (shift << 16) | (mask << 24) |
| ]]> |
| </programlisting> |
| </informalexample> |
| and is retrieved in callbacks like |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static int snd_sbmixer_get_single(struct snd_kcontrol *kcontrol, |
| struct snd_ctl_elem_value *ucontrol) |
| { |
| int reg = kcontrol->private_value & 0xff; |
| int shift = (kcontrol->private_value >> 16) & 0xff; |
| int mask = (kcontrol->private_value >> 24) & 0xff; |
| .... |
| } |
| ]]> |
| </programlisting> |
| </informalexample> |
| </para> |
| |
| <para> |
| In the <structfield>get</structfield> callback, |
| you have to fill all the elements if the |
| control has more than one elements, |
| i.e. <structfield>count</structfield> > 1. |
| In the example above, we filled only one element |
| (<structfield>value.integer.value[0]</structfield>) since it's |
| assumed as <structfield>count</structfield> = 1. |
| </para> |
| </section> |
| |
| <section id="control-interface-callbacks-put"> |
| <title>put callback</title> |
| |
| <para> |
| This callback is used to write a value from user-space. |
| </para> |
| |
| <para> |
| For example, |
| |
| <example> |
| <title>Example of put callback</title> |
| <programlisting> |
| <![CDATA[ |
| static int snd_myctl_put(struct snd_kcontrol *kcontrol, |
| struct snd_ctl_elem_value *ucontrol) |
| { |
| struct mychip *chip = snd_kcontrol_chip(kcontrol); |
| int changed = 0; |
| if (chip->current_value != |
| ucontrol->value.integer.value[0]) { |
| change_current_value(chip, |
| ucontrol->value.integer.value[0]); |
| changed = 1; |
| } |
| return changed; |
| } |
| ]]> |
| </programlisting> |
| </example> |
| |
| As seen above, you have to return 1 if the value is |
| changed. If the value is not changed, return 0 instead. |
| If any fatal error happens, return a negative error code as |
| usual. |
| </para> |
| |
| <para> |
| As in the <structfield>get</structfield> callback, |
| when the control has more than one elements, |
| all elements must be evaluated in this callback, too. |
| </para> |
| </section> |
| |
| <section id="control-interface-callbacks-all"> |
| <title>Callbacks are not atomic</title> |
| <para> |
| All these three callbacks are basically not atomic. |
| </para> |
| </section> |
| </section> |
| |
| <section id="control-interface-constructor"> |
| <title>Constructor</title> |
| <para> |
| When everything is ready, finally we can create a new |
| control. To create a control, there are two functions to be |
| called, <function>snd_ctl_new1()</function> and |
| <function>snd_ctl_add()</function>. |
| </para> |
| |
| <para> |
| In the simplest way, you can do like this: |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| err = snd_ctl_add(card, snd_ctl_new1(&my_control, chip)); |
| if (err < 0) |
| return err; |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| where <parameter>my_control</parameter> is the |
| struct <structname>snd_kcontrol_new</structname> object defined above, and chip |
| is the object pointer to be passed to |
| kcontrol->private_data |
| which can be referred to in callbacks. |
| </para> |
| |
| <para> |
| <function>snd_ctl_new1()</function> allocates a new |
| <structname>snd_kcontrol</structname> instance, |
| and <function>snd_ctl_add</function> assigns the given |
| control component to the card. |
| </para> |
| </section> |
| |
| <section id="control-interface-change-notification"> |
| <title>Change Notification</title> |
| <para> |
| If you need to change and update a control in the interrupt |
| routine, you can call <function>snd_ctl_notify()</function>. For |
| example, |
| |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_VALUE, id_pointer); |
| ]]> |
| </programlisting> |
| </informalexample> |
| |
| This function takes the card pointer, the event-mask, and the |
| control id pointer for the notification. The event-mask |
| specifies the types of notification, for example, in the above |
| example, the change of control values is notified. |
| The id pointer is the pointer of struct <structname>snd_ctl_elem_id</structname> |
| to be notified. |
| You can find some examples in <filename>es1938.c</filename> or |
| <filename>es1968.c</filename> for hardware volume interrupts. |
| </para> |
| </section> |
| |
| <section id="control-interface-tlv"> |
| <title>Metadata</title> |
| <para> |
| To provide information about the dB values of a mixer control, use |
| on of the <constant>DECLARE_TLV_xxx</constant> macros from |
| <filename><sound/tlv.h></filename> to define a variable |
| containing this information, set the<structfield>tlv.p |
| </structfield> field to point to this variable, and include the |
| <constant>SNDRV_CTL_ELEM_ACCESS_TLV_READ</constant> flag in the |
| <structfield>access</structfield> field; like this: |
| <informalexample> |
| <programlisting> |
| <![CDATA[ |
| static DECLARE_TLV_DB_SCALE(db_scale_my_control, -4050, 150, 0); |
| |
| static struct snd_kcontrol_new my_control = { |
| ... |
| .access = SNDRV_CTL_ELEM_ACCESS_READWRITE | |
| SNDRV_CTL_ELEM_ACCESS_TLV_READ, |
| ... |
| .tlv.p = db_scale_my_control, |
|