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<title>Sub-device Interface</title>
<para>This is an <link linkend="experimental">experimental</link>
interface and may change in the future.</para>
<para>The complex nature of V4L2 devices, where hardware is often made of
several integrated circuits that need to interact with each other in a
controlled way, leads to complex V4L2 drivers. The drivers usually reflect
the hardware model in software, and model the different hardware components
as software blocks called sub-devices.</para>
<para>V4L2 sub-devices are usually kernel-only objects. If the V4L2 driver
implements the media device API, they will automatically inherit from media
entities. Applications will be able to enumerate the sub-devices and discover
the hardware topology using the media entities, pads and links enumeration
<para>In addition to make sub-devices discoverable, drivers can also choose
to make them directly configurable by applications. When both the sub-device
driver and the V4L2 device driver support this, sub-devices will feature a
character device node on which ioctls can be called to
<listitem><para>query, read and write sub-devices controls</para></listitem>
<listitem><para>subscribe and unsubscribe to events and retrieve them</para></listitem>
<listitem><para>negotiate image formats on individual pads</para></listitem>
<para>Sub-device character device nodes, conventionally named
<filename>/dev/v4l-subdev*</filename>, use major number 81.</para>
<para>Most V4L2 controls are implemented by sub-device hardware. Drivers
usually merge all controls and expose them through video device nodes.
Applications can control all sub-devices through a single interface.</para>
<para>Complex devices sometimes implement the same control in different
pieces of hardware. This situation is common in embedded platforms, where
both sensors and image processing hardware implement identical functions,
such as contrast adjustment, white balance or faulty pixels correction. As
the V4L2 controls API doesn't support several identical controls in a single
device, all but one of the identical controls are hidden.</para>
<para>Applications can access those hidden controls through the sub-device
node with the V4L2 control API described in <xref linkend="control" />. The
ioctls behave identically as when issued on V4L2 device nodes, with the
exception that they deal only with controls implemented in the sub-device.
<para>Depending on the driver, those controls might also be exposed through
one (or several) V4L2 device nodes.</para>
<para>V4L2 sub-devices can notify applications of events as described in
<xref linkend="event" />. The API behaves identically as when used on V4L2
device nodes, with the exception that it only deals with events generated by
the sub-device. Depending on the driver, those events might also be reported
on one (or several) V4L2 device nodes.</para>
<section id="pad-level-formats">
<title>Pad-level Formats</title>
<warning><para>Pad-level formats are only applicable to very complex device that
need to expose low-level format configuration to user space. Generic V4L2
applications do <emphasis>not</emphasis> need to use the API described in
this section.</para></warning>
<note><para>For the purpose of this section, the term
<wordasword>format</wordasword> means the combination of media bus data
format, frame width and frame height.</para></note>
<para>Image formats are typically negotiated on video capture and
output devices using the format and <link
linkend="vidioc-subdev-g-selection">selection</link> ioctls. The
driver is responsible for configuring every block in the video
pipeline according to the requested format at the pipeline input
and/or output.</para>
<para>For complex devices, such as often found in embedded systems,
identical image sizes at the output of a pipeline can be achieved using
different hardware configurations. One such example is shown on
<xref linkend="pipeline-scaling" />, where
image scaling can be performed on both the video sensor and the host image
processing hardware.</para>
<figure id="pipeline-scaling">
<title>Image Format Negotiation on Pipelines</title>
<imagedata fileref="pipeline.pdf" format="PS" />
<imagedata fileref="pipeline.png" format="PNG" />
<phrase>High quality and high speed pipeline configuration</phrase>
<para>The sensor scaler is usually of less quality than the host scaler, but
scaling on the sensor is required to achieve higher frame rates. Depending
on the use case (quality vs. speed), the pipeline must be configured
differently. Applications need to configure the formats at every point in
the pipeline explicitly.</para>
<para>Drivers that implement the <link linkend="media-controller-intro">media
API</link> can expose pad-level image format configuration to applications.
When they do, applications can use the &VIDIOC-SUBDEV-G-FMT; and
&VIDIOC-SUBDEV-S-FMT; ioctls. to negotiate formats on a per-pad basis.</para>
<para>Applications are responsible for configuring coherent parameters on
the whole pipeline and making sure that connected pads have compatible
formats. The pipeline is checked for formats mismatch at &VIDIOC-STREAMON;
time, and an &EPIPE; is then returned if the configuration is
<para>Pad-level image format configuration support can be tested by calling
the &VIDIOC-SUBDEV-G-FMT; ioctl on pad 0. If the driver returns an &EINVAL;
pad-level format configuration is not supported by the sub-device.</para>
<title>Format Negotiation</title>
<para>Acceptable formats on pads can (and usually do) depend on a number
of external parameters, such as formats on other pads, active links, or
even controls. Finding a combination of formats on all pads in a video
pipeline, acceptable to both application and driver, can't rely on formats
enumeration only. A format negotiation mechanism is required.</para>
<para>Central to the format negotiation mechanism are the get/set format
operations. When called with the <structfield>which</structfield> argument
set to <constant>V4L2_SUBDEV_FORMAT_TRY</constant>, the
&VIDIOC-SUBDEV-G-FMT; and &VIDIOC-SUBDEV-S-FMT; ioctls operate on a set of
formats parameters that are not connected to the hardware configuration.
Modifying those 'try' formats leaves the device state untouched (this
applies to both the software state stored in the driver and the hardware
state stored in the device itself).</para>
<para>While not kept as part of the device state, try formats are stored
in the sub-device file handles. A &VIDIOC-SUBDEV-G-FMT; call will return
the last try format set <emphasis>on the same sub-device file
handle</emphasis>. Several applications querying the same sub-device at
the same time will thus not interact with each other.</para>
<para>To find out whether a particular format is supported by the device,
applications use the &VIDIOC-SUBDEV-S-FMT; ioctl. Drivers verify and, if
needed, change the requested <structfield>format</structfield> based on
device requirements and return the possibly modified value. Applications
can then choose to try a different format or accept the returned value and
<para>Formats returned by the driver during a negotiation iteration are
guaranteed to be supported by the device. In particular, drivers guarantee
that a returned format will not be further changed if passed to an
&VIDIOC-SUBDEV-S-FMT; call as-is (as long as external parameters, such as
formats on other pads or links' configuration are not changed).</para>
<para>Drivers automatically propagate formats inside sub-devices. When a
try or active format is set on a pad, corresponding formats on other pads
of the same sub-device can be modified by the driver. Drivers are free to
modify formats as required by the device. However, they should comply with
the following rules when possible:
<listitem><para>Formats should be propagated from sink pads to source pads.
Modifying a format on a source pad should not modify the format on any
sink pad.</para></listitem>
<listitem><para>Sub-devices that scale frames using variable scaling factors
should reset the scale factors to default values when sink pads formats
are modified. If the 1:1 scaling ratio is supported, this means that
source pads formats should be reset to the sink pads formats.</para></listitem>
<para>Formats are not propagated across links, as that would involve
propagating them from one sub-device file handle to another. Applications
must then take care to configure both ends of every link explicitly with
compatible formats. Identical formats on the two ends of a link are
guaranteed to be compatible. Drivers are free to accept different formats
matching device requirements as being compatible.</para>
<para><xref linkend="sample-pipeline-config" />
shows a sample configuration sequence for the pipeline described in
<xref linkend="pipeline-scaling" /> (table
columns list entity names and pad numbers).</para>
<table pgwide="0" frame="none" id="sample-pipeline-config">
<title>Sample Pipeline Configuration</title>
<tgroup cols="3">
<colspec colname="what"/>
<colspec colname="sensor-0 format" />
<colspec colname="frontend-0 format" />
<colspec colname="frontend-1 format" />
<colspec colname="scaler-0 format" />
<colspec colname="scaler-0 compose" />
<colspec colname="scaler-1 format" />
<entry>Sensor/0 format</entry>
<entry>Frontend/0 format</entry>
<entry>Frontend/1 format</entry>
<entry>Scaler/0 format</entry>
<entry>Scaler/0 compose selection rectangle</entry>
<entry>Scaler/1 format</entry>
<tbody valign="top">
<entry>Initial state</entry>
<entry>Configure frontend sink format</entry>
<entry>Configure scaler sink format</entry>
<entry>Configure scaler sink compose selection</entry>
<listitem><para>Initial state. The sensor source pad format is
set to its native 3MP size and V4L2_MBUS_FMT_SGRBG8_1X8
media bus code. Formats on the host frontend and scaler sink
and source pads have the default values, as well as the
compose rectangle on the scaler's sink pad.</para></listitem>
<listitem><para>The application configures the frontend sink
pad format's size to 2048x1536 and its media bus code to
V4L2_MBUS_FMT_SGRBG_1X8. The driver propagates the format to
the frontend source pad.</para></listitem>
<listitem><para>The application configures the scaler sink pad
format's size to 2046x1534 and the media bus code to
V4L2_MBUS_FMT_SGRBG_1X8 to match the frontend source size and
media bus code. The media bus code on the sink pad is set to
V4L2_MBUS_FMT_SGRBG_1X8. The driver propagates the size to the
compose selection rectangle on the scaler's sink pad, and the
format to the scaler source pad.</para></listitem>
<listitem><para>The application configures the size of the compose
selection rectangle of the scaler's sink pad 1280x960. The driver
propagates the size to the scaler's source pad
<para>When satisfied with the try results, applications can set the active
formats by setting the <structfield>which</structfield> argument to
<constant>V4L2_SUBDEV_FORMAT_ACTIVE</constant>. Active formats are changed
exactly as try formats by drivers. To avoid modifying the hardware state
during format negotiation, applications should negotiate try formats first
and then modify the active settings using the try formats returned during
the last negotiation iteration. This guarantees that the active format
will be applied as-is by the driver without being modified.
<section id="v4l2-subdev-selections">
<title>Selections: cropping, scaling and composition</title>
<para>Many sub-devices support cropping frames on their input or output
pads (or possible even on both). Cropping is used to select the area of
interest in an image, typically on an image sensor or a video decoder. It can
also be used as part of digital zoom implementations to select the area of
the image that will be scaled up.</para>
<para>Crop settings are defined by a crop rectangle and represented in a
&v4l2-rect; by the coordinates of the top left corner and the rectangle
size. Both the coordinates and sizes are expressed in pixels.</para>
<para>As for pad formats, drivers store try and active
rectangles for the selection targets <xref
linkend="v4l2-selections-common" />.</para>
<para>On sink pads, cropping is applied relative to the
current pad format. The pad format represents the image size as
received by the sub-device from the previous block in the
pipeline, and the crop rectangle represents the sub-image that
will be transmitted further inside the sub-device for
<para>The scaling operation changes the size of the image by
scaling it to new dimensions. The scaling ratio isn't specified
explicitly, but is implied from the original and scaled image
sizes. Both sizes are represented by &v4l2-rect;.</para>
<para>Scaling support is optional. When supported by a subdev,
the crop rectangle on the subdev's sink pad is scaled to the
size configured using the &VIDIOC-SUBDEV-S-SELECTION; IOCTL
using <constant>V4L2_SEL_TGT_COMPOSE</constant>
selection target on the same pad. If the subdev supports scaling
but not composing, the top and left values are not used and must
always be set to zero.</para>
<para>On source pads, cropping is similar to sink pads, with the
exception that the source size from which the cropping is
performed, is the COMPOSE rectangle on the sink pad. In both
sink and source pads, the crop rectangle must be entirely
contained inside the source image size for the crop
<para>The drivers should always use the closest possible
rectangle the user requests on all selection targets, unless
specifically told otherwise.
<constant>V4L2_SEL_FLAG_GE</constant> and
<constant>V4L2_SEL_FLAG_LE</constant> flags may be
used to round the image size either up or down. <xref
linkend="v4l2-selection-flags" /></para>
<title>Types of selection targets</title>
<title>Actual targets</title>
<para>Actual targets (without a postfix) reflect the actual
hardware configuration at any point of time. There is a BOUNDS
target corresponding to every actual target.</para>
<title>BOUNDS targets</title>
<para>BOUNDS targets is the smallest rectangle that contains all
valid actual rectangles. It may not be possible to set the actual
rectangle as large as the BOUNDS rectangle, however. This may be
because e.g. a sensor's pixel array is not rectangular but
cross-shaped or round. The maximum size may also be smaller than the
BOUNDS rectangle.</para>
<title>Order of configuration and format propagation</title>
<para>Inside subdevs, the order of image processing steps will
always be from the sink pad towards the source pad. This is also
reflected in the order in which the configuration must be
performed by the user: the changes made will be propagated to
any subsequent stages. If this behaviour is not desired, the
user must set
<constant>V4L2_SEL_FLAG_KEEP_CONFIG</constant> flag. This
flag causes no propagation of the changes are allowed in any
circumstances. This may also cause the accessed rectangle to be
adjusted by the driver, depending on the properties of the
underlying hardware.</para>
<para>The coordinates to a step always refer to the actual size
of the previous step. The exception to this rule is the source
compose rectangle, which refers to the sink compose bounds
rectangle --- if it is supported by the hardware.</para>
<listitem><para>Sink pad format. The user configures the sink pad
format. This format defines the parameters of the image the
entity receives through the pad for further processing.</para></listitem>
<listitem><para>Sink pad actual crop selection. The sink pad crop
defines the crop performed to the sink pad format.</para></listitem>
<listitem><para>Sink pad actual compose selection. The size of the
sink pad compose rectangle defines the scaling ratio compared
to the size of the sink pad crop rectangle. The location of
the compose rectangle specifies the location of the actual
sink compose rectangle in the sink compose bounds
<listitem><para>Source pad actual crop selection. Crop on the source
pad defines crop performed to the image in the sink compose
bounds rectangle.</para></listitem>
<listitem><para>Source pad format. The source pad format defines the
output pixel format of the subdev, as well as the other
parameters with the exception of the image width and height.
Width and height are defined by the size of the source pad
actual crop selection.</para></listitem>
<para>Accessing any of the above rectangles not supported by the
subdev will return <constant>EINVAL</constant>. Any rectangle
referring to a previous unsupported rectangle coordinates will
instead refer to the previous supported rectangle. For example,
if sink crop is not supported, the compose selection will refer
to the sink pad format dimensions instead.</para>
<figure id="subdev-image-processing-crop">
<title>Image processing in subdevs: simple crop example</title>
<imagedata fileref="subdev-image-processing-crop.svg"
format="SVG" scale="200" />
<para>In the above example, the subdev supports cropping on its
sink pad. To configure it, the user sets the media bus format on
the subdev's sink pad. Now the actual crop rectangle can be set
on the sink pad --- the location and size of this rectangle
reflect the location and size of a rectangle to be cropped from
the sink format. The size of the sink crop rectangle will also
be the size of the format of the subdev's source pad.</para>
<figure id="subdev-image-processing-scaling-multi-source">
<title>Image processing in subdevs: scaling with multiple sources</title>
<imagedata fileref="subdev-image-processing-scaling-multi-source.svg"
format="SVG" scale="200" />
<para>In this example, the subdev is capable of first cropping,
then scaling and finally cropping for two source pads
individually from the resulting scaled image. The location of
the scaled image in the cropped image is ignored in sink compose
target. Both of the locations of the source crop rectangles
refer to the sink scaling rectangle, independently cropping an
area at location specified by the source crop rectangle from
<figure id="subdev-image-processing-full">
<title>Image processing in subdevs: scaling and composition
with multiple sinks and sources</title>
<imagedata fileref="subdev-image-processing-full.svg"
format="SVG" scale="200" />
<para>The subdev driver supports two sink pads and two source
pads. The images from both of the sink pads are individually
cropped, then scaled and further composed on the composition
bounds rectangle. From that, two independent streams are cropped
and sent out of the subdev from the source pads.</para>