media: Documentation: add multiplexed streams documentation

Add documentation related to multiplexed streams.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

Documentation/driver-api/media/v4l2-subdev.rst

@@ -593,6 +593,14 @@ before calling v4l2_subdev_init_finalize():
 
 This shares the driver's private mutex between the controls and the states.
 
+Streams, multiplexed media pads and internal routing
+----------------------------------------------------
+
+A subdevice driver can implement support for multiplexed streams by setting
+the V4L2_SUBDEV_FL_STREAMS subdev flag and implementing support for
+centrally managed subdev active state, routing and stream based
+configuration.
+
 V4L2 sub-device functions and data structures
 ---------------------------------------------
 

Documentation/userspace-api/media/v4l/dev-subdev.rst

@@ -503,3 +503,141 @@ source pads.
     :maxdepth: 1
 
     subdev-formats
+
+Streams, multiplexed media pads and internal routing
+----------------------------------------------------
+
+Commonly V4L2 subdevices support only separate video streams, that is, only a
+single stream can pass through a media link and a media pad. Thus each pad
+contains a format configuration for that single stream. In some cases 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 subdevices have routing
+tables which describe how the incoming streams from sink pads are routed to the
+source pads.
+
+A stream ID (often just "stream") is a media link-local identifier for a stream.
+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 subdevice.
+
+A stream at a specific point in the media pipeline is identified with the
+subdev and a (pad, stream) pair. For subdevices that do not support
+multiplexed streams the 'stream' is always 0.
+
+Configuring streams
+^^^^^^^^^^^^^^^^^^^
+
+The configuration of the streams is done individually for each subdevice and
+the validity of the streams between subdevices is validated when the pipeline
+is started.
+
+There are three steps in configuring the streams:
+
+1) Set up links. Connect the pads between subdevices using the :ref:`Media
+Controller API <media_controller>`
+
+2) Routing. The routing table for the subdevice must be set with
+:ref:`VIDIOC_SUBDEV_S_ROUTING <VIDIOC_SUBDEV_G_ROUTING>` ioctl. Note that
+setting the routing table will reset all the stream configurations in a media
+entity.
+
+3) Configure streams. Each route endpoint must be configured
+with :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>`.
+
+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 subdevices,
+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 streams
+
+After configuring the routing table, the next step is configuring the streams.
+This step is similar to configuring the pads in a non-multiplexed streams
+setup, with the difference that we need to configure each (pad, stream) pair
+(i.e. route endpoint) instead of just a pad.
+
+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 subdev.