|ASoC jack detection
|ALSA has a standard API for representing physical jacks to user space,
|the kernel side of which can be seen in include/sound/jack.h. ASoC
|provides a version of this API adding two additional features:
| - It allows more than one jack detection method to work together on one
| user visible jack. In embedded systems it is common for multiple
| to be present on a single jack but handled by separate bits of
| - Integration with DAPM, allowing DAPM endpoints to be updated
| automatically based on the detected jack status (eg, turning off the
| headphone outputs if no headphones are present).
|This is done by splitting the jacks up into three things working
|together: the jack itself represented by a struct snd_soc_jack, sets of
|snd_soc_jack_pins representing DAPM endpoints to update and blocks of
|code providing jack reporting mechanisms.
|For example, a system may have a stereo headset jack with two reporting
|mechanisms, one for the headphone and one for the microphone. Some
|systems won't be able to use their speaker output while a headphone is
|connected and so will want to make sure to update both speaker and
|headphone when the headphone jack status changes.
|The jack - struct snd_soc_jack
|This represents a physical jack on the system and is what is visible to
|user space. The jack itself is completely passive, it is set up by the
|machine driver and updated by jack detection methods.
|Jacks are created by the machine driver calling snd_soc_jack_new().
|These represent a DAPM pin to update depending on some of the status
|bits supported by the jack. Each snd_soc_jack has zero or more of these
|which are updated automatically. They are created by the machine driver
|and associated with the jack using snd_soc_jack_add_pins(). The status
|of the endpoint may configured to be the opposite of the jack status if
|required (eg, enabling a built in microphone if a microphone is not
|connected via a jack).
|Jack detection methods
|Actual jack detection is done by code which is able to monitor some
|input to the system and update a jack by calling snd_soc_jack_report(),
|specifying a subset of bits to update. The jack detection code should
|be set up by the machine driver, taking configuration for the jack to
|update and the set of things to report when the jack is connected.
|Often this is done based on the status of a GPIO - a handler for this is
|provided by the snd_soc_jack_add_gpio() function. Other methods are
|also available, for example integrated into CODECs. One example of
|CODEC integrated jack detection can be see in the WM8350 driver.
|Each jack may have multiple reporting mechanisms, though it will need at
|least one to be useful.
|These are all hooked together by the machine driver depending on the
|system hardware. The machine driver will set up the snd_soc_jack and
|the list of pins to update then set up one or more jack detection
|mechanisms to update that jack based on their current status.