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- <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>
- <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 of ACTUAL type <xref
- linkend="v4l2-subdev-selection-targets">.</xref></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_SUBDEV_SEL_COMPOSE_ACTUAL</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_SUBDEV_SEL_FLAG_SIZE_GE</constant> and
- <constant>V4L2_SUBDEV_SEL_FLAG_SIZE_LE</constant> flags may be
- used to round the image size either up or down. <xref
- linkend="v4l2-subdev-selection-flags"></xref></para>
- </section>
-
- <section>
- <title>Types of selection targets</title>
-
- <section>
- <title>ACTUAL targets</title>
-
- <para>ACTUAL targets reflect the actual hardware configuration
- at any point of time. There is a BOUNDS target
- corresponding to every ACTUAL.</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_SUBDEV_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;