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-  <title>Sub-device Interface</title>
-
-  <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 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" />
-	  <thead>
-	    <row>
-	      <entry></entry>
-	      <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>
-	    </row>
-	  </thead>
-	  <tbody valign="top">
-	    <row>
-	      <entry>Initial state</entry>
-	      <entry>2048x1536/SGRBG8_1X8</entry>
-	      <entry>(default)</entry>
-	      <entry>(default)</entry>
-	      <entry>(default)</entry>
-	      <entry>(default)</entry>
-	      <entry>(default)</entry>
-	    </row>
-	    <row>
-	      <entry>Configure frontend sink format</entry>
-	      <entry>2048x1536/SGRBG8_1X8</entry>
-	      <entry><emphasis>2048x1536/SGRBG8_1X8</emphasis></entry>
-	      <entry><emphasis>2046x1534/SGRBG8_1X8</emphasis></entry>
-	      <entry>(default)</entry>
-	      <entry>(default)</entry>
-	      <entry>(default)</entry>
-	    </row>
-	    <row>
-	      <entry>Configure scaler sink format</entry>
-	      <entry>2048x1536/SGRBG8_1X8</entry>
-	      <entry>2048x1536/SGRBG8_1X8</entry>
-	      <entry>2046x1534/SGRBG8_1X8</entry>
-	      <entry><emphasis>2046x1534/SGRBG8_1X8</emphasis></entry>
-	      <entry><emphasis>0,0/2046x1534</emphasis></entry>
-	      <entry><emphasis>2046x1534/SGRBG8_1X8</emphasis></entry>
-	    </row>
-	    <row>
-	      <entry>Configure scaler sink compose selection</entry>
-	      <entry>2048x1536/SGRBG8_1X8</entry>
-	      <entry>2048x1536/SGRBG8_1X8</entry>
-	      <entry>2046x1534/SGRBG8_1X8</entry>
-	      <entry>2046x1534/SGRBG8_1X8</entry>
-	      <entry><emphasis>0,0/1280x960</emphasis></entry>
-	      <entry><emphasis>1280x960/SGRBG8_1X8</emphasis></entry>
-	    </row>
-	  </tbody>
-	</tgroup>
-      </table>
-
-      <para>
-      <orderedlist>
-	<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
-	format.</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><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
-	rectangle.</para></listitem>
-
-	<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>
-      </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;