media/v4l/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 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;