dev-subdev.xml

  <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 <link linkend="crop">cropping and scaling</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_TRY</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>
      <title>Cropping and scaling</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 a video sensor or 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>The crop rectangle is retrieved and set using the
      &VIDIOC-SUBDEV-G-CROP; and &VIDIOC-SUBDEV-S-CROP; ioctls. Like for pad
      formats, drivers store try and active crop rectangles. The format
      negotiation mechanism applies to crop settings as well.</para>

      <para>On input pads, cropping is applied relatively 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. The crop rectangle be entirely containted
      inside the input image size.</para>

      <para>Input crop rectangle are reset to their default value when the input
      image format is modified. Drivers should use the input image size as the
      crop rectangle default value, but hardware requirements may prevent this.
      </para>

      <para>Cropping behaviour on output pads is not defined.</para>

    </section>
  </section>

  &sub-subdev-formats;