314 lines
14 KiB
XML
314 lines
14 KiB
XML
<title>Sub-device Interface</title>
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<note>
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<title>Experimental</title>
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<para>This is an <link linkend="experimental">experimental</link>
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interface and may change in the future.</para>
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</note>
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<para>The complex nature of V4L2 devices, where hardware is often made of
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several integrated circuits that need to interact with each other in a
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controlled way, leads to complex V4L2 drivers. The drivers usually reflect
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the hardware model in software, and model the different hardware components
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as software blocks called sub-devices.</para>
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<para>V4L2 sub-devices are usually kernel-only objects. If the V4L2 driver
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implements the media device API, they will automatically inherit from media
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entities. Applications will be able to enumerate the sub-devices and discover
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the hardware topology using the media entities, pads and links enumeration
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API.</para>
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<para>In addition to make sub-devices discoverable, drivers can also choose
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to make them directly configurable by applications. When both the sub-device
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driver and the V4L2 device driver support this, sub-devices will feature a
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character device node on which ioctls can be called to
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<itemizedlist>
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<listitem><para>query, read and write sub-devices controls</para></listitem>
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<listitem><para>subscribe and unsubscribe to events and retrieve them</para></listitem>
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<listitem><para>negotiate image formats on individual pads</para></listitem>
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</itemizedlist>
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</para>
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<para>Sub-device character device nodes, conventionally named
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<filename>/dev/v4l-subdev*</filename>, use major number 81.</para>
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<section>
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<title>Controls</title>
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<para>Most V4L2 controls are implemented by sub-device hardware. Drivers
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usually merge all controls and expose them through video device nodes.
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Applications can control all sub-devices through a single interface.</para>
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<para>Complex devices sometimes implement the same control in different
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pieces of hardware. This situation is common in embedded platforms, where
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both sensors and image processing hardware implement identical functions,
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such as contrast adjustment, white balance or faulty pixels correction. As
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the V4L2 controls API doesn't support several identical controls in a single
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device, all but one of the identical controls are hidden.</para>
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<para>Applications can access those hidden controls through the sub-device
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node with the V4L2 control API described in <xref linkend="control" />. The
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ioctls behave identically as when issued on V4L2 device nodes, with the
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exception that they deal only with controls implemented in the sub-device.
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</para>
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<para>Depending on the driver, those controls might also be exposed through
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one (or several) V4L2 device nodes.</para>
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</section>
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<section>
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<title>Events</title>
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<para>V4L2 sub-devices can notify applications of events as described in
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<xref linkend="event" />. The API behaves identically as when used on V4L2
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device nodes, with the exception that it only deals with events generated by
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the sub-device. Depending on the driver, those events might also be reported
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on one (or several) V4L2 device nodes.</para>
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</section>
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<section id="pad-level-formats">
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<title>Pad-level Formats</title>
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<warning><para>Pad-level formats are only applicable to very complex device that
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need to expose low-level format configuration to user space. Generic V4L2
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applications do <emphasis>not</emphasis> need to use the API described in
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this section.</para></warning>
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<note><para>For the purpose of this section, the term
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<wordasword>format</wordasword> means the combination of media bus data
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format, frame width and frame height.</para></note>
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<para>Image formats are typically negotiated on video capture and output
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devices using the <link linkend="crop">cropping and scaling</link> ioctls.
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The driver is responsible for configuring every block in the video pipeline
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according to the requested format at the pipeline input and/or
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output.</para>
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<para>For complex devices, such as often found in embedded systems,
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identical image sizes at the output of a pipeline can be achieved using
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different hardware configurations. One such example is shown on
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<xref linkend="pipeline-scaling" />, where
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image scaling can be performed on both the video sensor and the host image
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processing hardware.</para>
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<figure id="pipeline-scaling">
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<title>Image Format Negotiation on Pipelines</title>
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<mediaobject>
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<imageobject>
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<imagedata fileref="pipeline.pdf" format="PS" />
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</imageobject>
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<imageobject>
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<imagedata fileref="pipeline.png" format="PNG" />
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</imageobject>
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<textobject>
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<phrase>High quality and high speed pipeline configuration</phrase>
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</textobject>
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</mediaobject>
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</figure>
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<para>The sensor scaler is usually of less quality than the host scaler, but
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scaling on the sensor is required to achieve higher frame rates. Depending
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on the use case (quality vs. speed), the pipeline must be configured
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differently. Applications need to configure the formats at every point in
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the pipeline explicitly.</para>
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<para>Drivers that implement the <link linkend="media-controller-intro">media
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API</link> can expose pad-level image format configuration to applications.
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When they do, applications can use the &VIDIOC-SUBDEV-G-FMT; and
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&VIDIOC-SUBDEV-S-FMT; ioctls. to negotiate formats on a per-pad basis.</para>
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<para>Applications are responsible for configuring coherent parameters on
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the whole pipeline and making sure that connected pads have compatible
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formats. The pipeline is checked for formats mismatch at &VIDIOC-STREAMON;
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time, and an &EPIPE; is then returned if the configuration is
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invalid.</para>
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<para>Pad-level image format configuration support can be tested by calling
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the &VIDIOC-SUBDEV-G-FMT; ioctl on pad 0. If the driver returns an &EINVAL;
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pad-level format configuration is not supported by the sub-device.</para>
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<section>
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<title>Format Negotiation</title>
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<para>Acceptable formats on pads can (and usually do) depend on a number
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of external parameters, such as formats on other pads, active links, or
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even controls. Finding a combination of formats on all pads in a video
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pipeline, acceptable to both application and driver, can't rely on formats
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enumeration only. A format negotiation mechanism is required.</para>
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<para>Central to the format negotiation mechanism are the get/set format
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operations. When called with the <structfield>which</structfield> argument
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set to <constant>V4L2_SUBDEV_FORMAT_TRY</constant>, the
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&VIDIOC-SUBDEV-G-FMT; and &VIDIOC-SUBDEV-S-FMT; ioctls operate on a set of
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formats parameters that are not connected to the hardware configuration.
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Modifying those 'try' formats leaves the device state untouched (this
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applies to both the software state stored in the driver and the hardware
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state stored in the device itself).</para>
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<para>While not kept as part of the device state, try formats are stored
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in the sub-device file handles. A &VIDIOC-SUBDEV-G-FMT; call will return
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the last try format set <emphasis>on the same sub-device file
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handle</emphasis>. Several applications querying the same sub-device at
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the same time will thus not interact with each other.</para>
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<para>To find out whether a particular format is supported by the device,
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applications use the &VIDIOC-SUBDEV-S-FMT; ioctl. Drivers verify and, if
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needed, change the requested <structfield>format</structfield> based on
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device requirements and return the possibly modified value. Applications
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can then choose to try a different format or accept the returned value and
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continue.</para>
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<para>Formats returned by the driver during a negotiation iteration are
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guaranteed to be supported by the device. In particular, drivers guarantee
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that a returned format will not be further changed if passed to an
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&VIDIOC-SUBDEV-S-FMT; call as-is (as long as external parameters, such as
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formats on other pads or links' configuration are not changed).</para>
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<para>Drivers automatically propagate formats inside sub-devices. When a
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try or active format is set on a pad, corresponding formats on other pads
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of the same sub-device can be modified by the driver. Drivers are free to
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modify formats as required by the device. However, they should comply with
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the following rules when possible:
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<itemizedlist>
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<listitem><para>Formats should be propagated from sink pads to source pads.
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Modifying a format on a source pad should not modify the format on any
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sink pad.</para></listitem>
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<listitem><para>Sub-devices that scale frames using variable scaling factors
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should reset the scale factors to default values when sink pads formats
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are modified. If the 1:1 scaling ratio is supported, this means that
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source pads formats should be reset to the sink pads formats.</para></listitem>
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</itemizedlist>
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</para>
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<para>Formats are not propagated across links, as that would involve
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propagating them from one sub-device file handle to another. Applications
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must then take care to configure both ends of every link explicitly with
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compatible formats. Identical formats on the two ends of a link are
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guaranteed to be compatible. Drivers are free to accept different formats
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matching device requirements as being compatible.</para>
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<para><xref linkend="sample-pipeline-config" />
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shows a sample configuration sequence for the pipeline described in
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<xref linkend="pipeline-scaling" /> (table
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columns list entity names and pad numbers).</para>
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<table pgwide="0" frame="none" id="sample-pipeline-config">
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<title>Sample Pipeline Configuration</title>
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<tgroup cols="3">
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<colspec colname="what"/>
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<colspec colname="sensor-0" />
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<colspec colname="frontend-0" />
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<colspec colname="frontend-1" />
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<colspec colname="scaler-0" />
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<colspec colname="scaler-1" />
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<thead>
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<row>
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<entry></entry>
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<entry>Sensor/0</entry>
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<entry>Frontend/0</entry>
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<entry>Frontend/1</entry>
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<entry>Scaler/0</entry>
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<entry>Scaler/1</entry>
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</row>
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</thead>
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<tbody valign="top">
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<row>
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<entry>Initial state</entry>
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<entry>2048x1536</entry>
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<entry>-</entry>
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<entry>-</entry>
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<entry>-</entry>
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<entry>-</entry>
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</row>
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<row>
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<entry>Configure frontend input</entry>
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<entry>2048x1536</entry>
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<entry><emphasis>2048x1536</emphasis></entry>
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<entry><emphasis>2046x1534</emphasis></entry>
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<entry>-</entry>
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<entry>-</entry>
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</row>
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<row>
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<entry>Configure scaler input</entry>
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<entry>2048x1536</entry>
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<entry>2048x1536</entry>
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<entry>2046x1534</entry>
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<entry><emphasis>2046x1534</emphasis></entry>
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<entry><emphasis>2046x1534</emphasis></entry>
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</row>
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<row>
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<entry>Configure scaler output</entry>
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<entry>2048x1536</entry>
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<entry>2048x1536</entry>
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<entry>2046x1534</entry>
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<entry>2046x1534</entry>
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<entry><emphasis>1280x960</emphasis></entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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<orderedlist>
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<listitem><para>Initial state. The sensor output is set to its native 3MP
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resolution. Resolutions on the host frontend and scaler input and output
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pads are undefined.</para></listitem>
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<listitem><para>The application configures the frontend input pad resolution to
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2048x1536. The driver propagates the format to the frontend output pad.
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Note that the propagated output format can be different, as in this case,
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than the input format, as the hardware might need to crop pixels (for
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instance when converting a Bayer filter pattern to RGB or YUV).</para></listitem>
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<listitem><para>The application configures the scaler input pad resolution to
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2046x1534 to match the frontend output resolution. The driver propagates
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the format to the scaler output pad.</para></listitem>
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<listitem><para>The application configures the scaler output pad resolution to
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1280x960.</para></listitem>
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</orderedlist>
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</para>
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<para>When satisfied with the try results, applications can set the active
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formats by setting the <structfield>which</structfield> argument to
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<constant>V4L2_SUBDEV_FORMAT_ACTIVE</constant>. Active formats are changed
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exactly as try formats by drivers. To avoid modifying the hardware state
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during format negotiation, applications should negotiate try formats first
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and then modify the active settings using the try formats returned during
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the last negotiation iteration. This guarantees that the active format
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will be applied as-is by the driver without being modified.
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</para>
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</section>
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<section>
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<title>Cropping and scaling</title>
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<para>Many sub-devices support cropping frames on their input or output
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pads (or possible even on both). Cropping is used to select the area of
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interest in an image, typically on a video sensor or video decoder. It can
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also be used as part of digital zoom implementations to select the area of
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the image that will be scaled up.</para>
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<para>Crop settings are defined by a crop rectangle and represented in a
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&v4l2-rect; by the coordinates of the top left corner and the rectangle
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size. Both the coordinates and sizes are expressed in pixels.</para>
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<para>The crop rectangle is retrieved and set using the
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&VIDIOC-SUBDEV-G-CROP; and &VIDIOC-SUBDEV-S-CROP; ioctls. Like for pad
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formats, drivers store try and active crop rectangles. The format
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negotiation mechanism applies to crop settings as well.</para>
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<para>On input pads, cropping is applied relatively to the current pad
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format. The pad format represents the image size as received by the
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sub-device from the previous block in the pipeline, and the crop rectangle
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represents the sub-image that will be transmitted further inside the
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sub-device for processing. The crop rectangle be entirely containted
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inside the input image size.</para>
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<para>Input crop rectangle are reset to their default value when the input
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image format is modified. Drivers should use the input image size as the
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crop rectangle default value, but hardware requirements may prevent this.
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</para>
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<para>Cropping behaviour on output pads is not defined.</para>
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</section>
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</section>
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&sub-subdev-formats;
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