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diff --git a/Documentation/DocBook/v4l/dev-subdev.xml b/Documentation/DocBook/v4l/dev-subdev.xml new file mode 100644 index 0000000..21caff6 --- /dev/null +++ b/Documentation/DocBook/v4l/dev-subdev.xml @@ -0,0 +1,313 @@ + <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 Negotation 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; |