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Diffstat (limited to 'Documentation/userspace-api/media/v4l/dev-subdev.rst')
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diff --git a/Documentation/userspace-api/media/v4l/dev-subdev.rst b/Documentation/userspace-api/media/v4l/dev-subdev.rst index fd1de0a73a9f..a4f1df7093e8 100644 --- a/Documentation/userspace-api/media/v4l/dev-subdev.rst +++ b/Documentation/userspace-api/media/v4l/dev-subdev.rst @@ -29,6 +29,8 @@ will feature a character device node on which ioctls can be called to - negotiate image formats on individual pads +- inspect and modify internal data routing between pads of the same entity + Sub-device character device nodes, conventionally named ``/dev/v4l-subdev*``, use major number 81. @@ -404,6 +406,8 @@ pixel array is not rectangular but cross-shaped or round. The maximum size may also be smaller than the BOUNDS rectangle. +.. _format-propagation: + Order of configuration and format propagation --------------------------------------------- @@ -501,3 +505,165 @@ source pads. :maxdepth: 1 subdev-formats + +Streams, multiplexed media pads and internal routing +---------------------------------------------------- + +Simple V4L2 sub-devices do not support multiple, unrelated video streams, +and only a single stream can pass through a media link and a media pad. +Thus each pad contains a format and selection configuration for that +single stream. A subdev can do stream processing and split a stream into +two or compose two streams into one, but the inputs and outputs for the +subdev are still a single stream per pad. + +Some hardware, e.g. MIPI CSI-2, support multiplexed streams, that is, multiple +data streams are transmitted on the same bus, which is represented by a media +link connecting a transmitter source pad with a sink pad on the receiver. For +example, a camera sensor can produce two distinct streams, a pixel stream and a +metadata stream, which are transmitted on the multiplexed data bus, represented +by a media link which connects the single sensor's source pad with the receiver +sink pad. The stream-aware receiver will de-multiplex the streams received on +the its sink pad and allows to route them individually to one of its source +pads. + +Subdevice drivers that support multiplexed streams are compatible with +non-multiplexed subdev drivers, but, of course, require a routing configuration +where the link between those two types of drivers contains only a single +stream. + +Understanding streams +^^^^^^^^^^^^^^^^^^^^^ + +A stream is a stream of content (e.g. pixel data or metadata) flowing through +the media pipeline from a source (e.g. a sensor) towards the final sink (e.g. a +receiver and demultiplexer in a SoC). Each media link carries all the enabled +streams from one end of the link to the other, and sub-devices have routing +tables which describe how the incoming streams from sink pads are routed to the +source pads. + +A stream ID is a media pad-local identifier for a stream. Streams IDs of +the same stream must be equal on both ends of a link. In other words, +a particular stream ID must exist on both sides of a media +link, but another stream ID can be used for the same stream at the other side +of the sub-device. + +A stream at a specific point in the media pipeline is identified by the +sub-device and a (pad, stream) pair. For sub-devices that do not support +multiplexed streams the 'stream' field is always 0. + +Interaction between routes, streams, formats and selections +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The addition of streams to the V4L2 sub-device interface moves the sub-device +formats and selections from pads to (pad, stream) pairs. Besides the +usual pad, also the stream ID needs to be provided for setting formats and +selections. The order of configuring formats and selections along a stream is +the same as without streams (see :ref:`format-propagation`). + +Instead of the sub-device wide merging of streams from all sink pads +towards all source pads, data flows for each route are separate from each +other. Any number of routes from streams on sink pads towards streams on +source pads is allowed, to the extent supported by drivers. For every +stream on a source pad, however, only a single route is allowed. + +Any configurations of a stream within a pad, such as format or selections, +are independent of similar configurations on other streams. This is +subject to change in the future. + +Configuring streams +^^^^^^^^^^^^^^^^^^^ + +The configuration of the streams is done individually for each sub-device and +the validity of the streams between sub-devices is validated when the pipeline +is started. + +There are three steps in configuring the streams: + +1) Set up links. Connect the pads between sub-devices using the :ref:`Media +Controller API <media_controller>` + +2) Streams. Streams are declared and their routing is configured by +setting the routing table for the sub-device using +:ref:`VIDIOC_SUBDEV_S_ROUTING <VIDIOC_SUBDEV_G_ROUTING>` ioctl. Note that +setting the routing table will reset formats and selections in the +sub-device to default values. + +3) Configure formats and selections. Formats and selections of each stream +are configured separately as documented for plain sub-devices in +:ref:`format-propagation`. The stream ID is set to the same stream ID +associated with either sink or source pads of routes configured using the +:ref:`VIDIOC_SUBDEV_S_ROUTING <VIDIOC_SUBDEV_G_ROUTING>` ioctl. + +Multiplexed streams setup example +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +A simple example of a multiplexed stream setup might be as follows: + +- Two identical sensors (Sensor A and Sensor B). Each sensor has a single source + pad (pad 0) which carries a pixel data stream. + +- Multiplexer bridge (Bridge). The bridge has two sink pads, connected to the + sensors (pads 0, 1), and one source pad (pad 2), which outputs two streams. + +- Receiver in the SoC (Receiver). The receiver has a single sink pad (pad 0), + connected to the bridge, and two source pads (pads 1-2), going to the DMA + engine. The receiver demultiplexes the incoming streams to the source pads. + +- DMA Engines in the SoC (DMA Engine), one for each stream. Each DMA engine is + connected to a single source pad in the receiver. + +The sensors, the bridge and the receiver are modeled as V4L2 sub-devices, +exposed to userspace via /dev/v4l-subdevX device nodes. The DMA engines are +modeled as V4L2 devices, exposed to userspace via /dev/videoX nodes. + +To configure this pipeline, the userspace must take the following steps: + +1) Set up media links between entities: connect the sensors to the bridge, +bridge to the receiver, and the receiver to the DMA engines. This step does +not differ from normal non-multiplexed media controller setup. + +2) Configure routing + +.. flat-table:: Bridge routing table + :header-rows: 1 + + * - Sink Pad/Stream + - Source Pad/Stream + - Routing Flags + - Comments + * - 0/0 + - 2/0 + - V4L2_SUBDEV_ROUTE_FL_ACTIVE + - Pixel data stream from Sensor A + * - 1/0 + - 2/1 + - V4L2_SUBDEV_ROUTE_FL_ACTIVE + - Pixel data stream from Sensor B + +.. flat-table:: Receiver routing table + :header-rows: 1 + + * - Sink Pad/Stream + - Source Pad/Stream + - Routing Flags + - Comments + * - 0/0 + - 1/0 + - V4L2_SUBDEV_ROUTE_FL_ACTIVE + - Pixel data stream from Sensor A + * - 0/1 + - 2/0 + - V4L2_SUBDEV_ROUTE_FL_ACTIVE + - Pixel data stream from Sensor B + +3) Configure formats and selections + +After configuring routing, the next step is configuring the formats and +selections for the streams. This is similar to performing this step without +streams, with just one exception: the ``stream`` field needs to be assigned +to the value of the stream ID. + +A common way to accomplish this is to start from the sensors and propagate the +configurations along the stream towards the receiver, +using :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` ioctls to configure each +stream endpoint in each sub-device. |