Documentation/DocBook/media/v4l/dev-subdev.xml - kernel/msm-4.9 - Gitiles

   <title>Sub-device Interface</title>

   <note>
     <title>Experimental</title>
     <para>This is an <link linkend="experimental">experimental</link>
     interface and may change in the future.</para>
   </note>

   <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
   API.</para>

   <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
   <itemizedlist>
     <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>
   </itemizedlist>
   </para>

   <para>Sub-device character device nodes, conventionally named
   <filename>/dev/v4l-subdev*</filename>, use major number 81.</para>

   <section>
     <title>Controls</title>
     <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>

     <para>Depending on the driver, those controls might also be exposed through
     one (or several) V4L2 device nodes.</para>
   </section>

   <section>
     <title>Events</title>
     <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>

   <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>
       <mediaobject>
 	<imageobject>
 	  <imagedata fileref="pipeline.pdf" format="PS" />
 	</imageobject>
 	<imageobject>
 	  <imagedata fileref="pipeline.png" format="PNG" />
 	</imageobject>
 	<textobject>
 	  <phrase>High quality and high speed pipeline configuration</phrase>
 	</textobject>
       </mediaobject>
     </figure>

     <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
     invalid.</para>

     <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>

     <section>
       <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
       continue.</para>

       <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:
       <itemizedlist>
         <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>
       </itemizedlist>
       </para>

       <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" />
 	  <colspec colname="frontend-0" />
 	  <colspec colname="frontend-1" />
 	  <colspec colname="scaler-0" />
 	  <colspec colname="scaler-1" />
 	  <thead>
 	    <row>
 	      <entry></entry>
 	      <entry>Sensor/0</entry>
 	      <entry>Frontend/0</entry>
 	      <entry>Frontend/1</entry>
 	      <entry>Scaler/0</entry>
 	      <entry>Scaler/1</entry>
 	    </row>
 	  </thead>
 	  <tbody valign="top">
 	    <row>
 	      <entry>Initial state</entry>
 	      <entry>2048x1536</entry>
 	      <entry>-</entry>
 	      <entry>-</entry>
 	      <entry>-</entry>
 	      <entry>-</entry>
 	    </row>
 	    <row>
 	      <entry>Configure frontend input</entry>
 	      <entry>2048x1536</entry>
 	      <entry><emphasis>2048x1536</emphasis></entry>
 	      <entry><emphasis>2046x1534</emphasis></entry>
 	      <entry>-</entry>
 	      <entry>-</entry>
 	    </row>
 	    <row>
 	      <entry>Configure scaler input</entry>
 	      <entry>2048x1536</entry>
 	      <entry>2048x1536</entry>
 	      <entry>2046x1534</entry>
 	      <entry><emphasis>2046x1534</emphasis></entry>
 	      <entry><emphasis>2046x1534</emphasis></entry>
 	    </row>
 	    <row>
 	      <entry>Configure scaler output</entry>
 	      <entry>2048x1536</entry>
 	      <entry>2048x1536</entry>
 	      <entry>2046x1534</entry>
 	      <entry>2046x1534</entry>
 	      <entry><emphasis>1280x960</emphasis></entry>
 	    </row>
 	  </tbody>
 	</tgroup>
       </table>

       <para>
       <orderedlist>
 	<listitem><para>Initial state. The sensor output is set to its native 3MP
 	resolution. Resolutions on the host frontend and scaler input and output
 	pads are undefined.</para></listitem>
 	<listitem><para>The application configures the frontend input pad resolution to
 	2048x1536. The driver propagates the format to the frontend output pad.
 	Note that the propagated output format can be different, as in this case,
 	than the input format, as the hardware might need to crop pixels (for
 	instance when converting a Bayer filter pattern to RGB or YUV).</para></listitem>
 	<listitem><para>The application configures the scaler input pad resolution to
 	2046x1534 to match the frontend output resolution. The driver propagates
 	the format to the scaler output pad.</para></listitem>
 	<listitem><para>The application configures the scaler output pad resolution to
 	1280x960.</para></listitem>
       </orderedlist>
       </para>

       <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.
       </para>
     </section>

     <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
       processing.</para>

       <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
       operation.</para>

       <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>
     </section>

     <section>
       <title>Types of selection targets</title>

       <section>
 	<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>
       </section>

       <section>
 	<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>
       </section>

     </section>

     <section>
       <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>

       <orderedlist>
 	<listitem>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.</listitem>

 	<listitem>Sink pad actual crop selection. The sink pad crop
 	defines the crop performed to the sink pad format.</listitem>

 	<listitem>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
 	rectangle.</listitem>

 	<listitem>Source pad actual crop selection. Crop on the source
 	pad defines crop performed to the image in the sink compose
 	bounds rectangle.</listitem>

 	<listitem>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.</listitem>
       </orderedlist>

       <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>
 	<mediaobject>
 	  <imageobject>
 	    <imagedata fileref="subdev-image-processing-crop.svg"
 	    format="SVG" scale="200" />
 	  </imageobject>
 	</mediaobject>
       </figure>

       <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>
 	<mediaobject>
 	  <imageobject>
 	    <imagedata fileref="subdev-image-processing-scaling-multi-source.svg"
 	    format="SVG" scale="200" />
 	  </imageobject>
 	</mediaobject>
       </figure>

       <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
       it.</para>

       <figure id="subdev-image-processing-full">
 	<title>Image processing in subdevs: scaling and composition
 	with multiple sinks and sources</title>
 	<mediaobject>
 	  <imageobject>
 	    <imagedata fileref="subdev-image-processing-full.svg"
 	    format="SVG" scale="200" />
 	  </imageobject>
 	</mediaobject>
       </figure>

       <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>

     </section>

   </section>

   &sub-subdev-formats;
	<title>Sub-device Interface</title>

	<note>
	<title>Experimental</title>
	<para>This is an <link linkend="experimental">experimental</link>
	interface and may change in the future.</para>
	</note>

	<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
	API.</para>

	<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
	<itemizedlist>
	<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>
	</itemizedlist>
	</para>

	<para>Sub-device character device nodes, conventionally named
	<filename>/dev/v4l-subdev*</filename>, use major number 81.</para>

	<section>
	<title>Controls</title>
	<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>

	<para>Depending on the driver, those controls might also be exposed through
	one (or several) V4L2 device nodes.</para>
	</section>

	<section>
	<title>Events</title>
	<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>

	<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>
	<mediaobject>
	<imageobject>
	<imagedata fileref="pipeline.pdf" format="PS" />
	</imageobject>
	<imageobject>
	<imagedata fileref="pipeline.png" format="PNG" />
	</imageobject>
	<textobject>
	<phrase>High quality and high speed pipeline configuration</phrase>
	</textobject>
	</mediaobject>
	</figure>

	<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
	invalid.</para>

	<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>

	<section>
	<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
	continue.</para>

	<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:
	<itemizedlist>
	<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>
	</itemizedlist>
	</para>

	<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" />
	<colspec colname="frontend-0" />
	<colspec colname="frontend-1" />
	<colspec colname="scaler-0" />
	<colspec colname="scaler-1" />
	<thead>
	<row>
	<entry></entry>
	<entry>Sensor/0</entry>
	<entry>Frontend/0</entry>
	<entry>Frontend/1</entry>
	<entry>Scaler/0</entry>
	<entry>Scaler/1</entry>
	</row>
	</thead>
	<tbody valign="top">
	<row>
	<entry>Initial state</entry>
	<entry>2048x1536</entry>
	<entry>-</entry>
	<entry>-</entry>
	<entry>-</entry>
	<entry>-</entry>
	</row>
	<row>
	<entry>Configure frontend input</entry>
	<entry>2048x1536</entry>
	<entry><emphasis>2048x1536</emphasis></entry>
	<entry><emphasis>2046x1534</emphasis></entry>
	<entry>-</entry>
	<entry>-</entry>
	</row>
	<row>
	<entry>Configure scaler input</entry>
	<entry>2048x1536</entry>
	<entry>2048x1536</entry>
	<entry>2046x1534</entry>
	<entry><emphasis>2046x1534</emphasis></entry>
	<entry><emphasis>2046x1534</emphasis></entry>
	</row>
	<row>
	<entry>Configure scaler output</entry>
	<entry>2048x1536</entry>
	<entry>2048x1536</entry>
	<entry>2046x1534</entry>
	<entry>2046x1534</entry>
	<entry><emphasis>1280x960</emphasis></entry>
	</row>
	</tbody>
	</tgroup>
	</table>

	<para>
	<orderedlist>
	<listitem><para>Initial state. The sensor output is set to its native 3MP
	resolution. Resolutions on the host frontend and scaler input and output
	pads are undefined.</para></listitem>
	<listitem><para>The application configures the frontend input pad resolution to
	2048x1536. The driver propagates the format to the frontend output pad.
	Note that the propagated output format can be different, as in this case,
	than the input format, as the hardware might need to crop pixels (for
	instance when converting a Bayer filter pattern to RGB or YUV).</para></listitem>
	<listitem><para>The application configures the scaler input pad resolution to
	2046x1534 to match the frontend output resolution. The driver propagates
	the format to the scaler output pad.</para></listitem>
	<listitem><para>The application configures the scaler output pad resolution to
	1280x960.</para></listitem>
	</orderedlist>
	</para>

	<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.
	</para>
	</section>

	<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
	processing.</para>

	<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
	operation.</para>

	<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>
	</section>

	<section>
	<title>Types of selection targets</title>

	<section>
	<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>
	</section>

	<section>
	<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>
	</section>

	</section>

	<section>
	<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>

	<orderedlist>
	<listitem>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.</listitem>

	<listitem>Sink pad actual crop selection. The sink pad crop
	defines the crop performed to the sink pad format.</listitem>

	<listitem>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
	rectangle.</listitem>

	<listitem>Source pad actual crop selection. Crop on the source
	pad defines crop performed to the image in the sink compose
	bounds rectangle.</listitem>

	<listitem>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.</listitem>
	</orderedlist>

	<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>
	<mediaobject>
	<imageobject>
	<imagedata fileref="subdev-image-processing-crop.svg"
	format="SVG" scale="200" />
	</imageobject>
	</mediaobject>
	</figure>

	<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>
	<mediaobject>
	<imageobject>
	<imagedata fileref="subdev-image-processing-scaling-multi-source.svg"
	format="SVG" scale="200" />
	</imageobject>
	</mediaobject>
	</figure>

	<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
	it.</para>

	<figure id="subdev-image-processing-full">
	<title>Image processing in subdevs: scaling and composition
	with multiple sinks and sources</title>
	<mediaobject>
	<imageobject>
	<imagedata fileref="subdev-image-processing-full.svg"
	format="SVG" scale="200" />
	</imageobject>
	</mediaobject>
	</figure>

	<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>

	</section>

	</section>

	&sub-subdev-formats;