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diff --git a/Documentation/DocBook/v4l/dev-subdev.xml b/Documentation/DocBook/v4l/dev-subdev.xml deleted file mode 100644 index 05c8fef..0000000 --- a/Documentation/DocBook/v4l/dev-subdev.xml +++ /dev/null @@ -1,313 +0,0 @@ - <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; |