Add documentation related to multiplexed streams.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@xxxxxxxxxxxxxxxx>
---
.../driver-api/media/v4l2-subdev.rst | 8 +
.../userspace-api/media/v4l/dev-subdev.rst | 165 ++++++++++++++++++
2 files changed, 173 insertions(+)
diff --git a/Documentation/driver-api/media/v4l2-subdev.rst b/Documentation/driver-api/media/v4l2-subdev.rst
index db012729f8e3..1d986e814b4e 100644
--- a/Documentation/driver-api/media/v4l2-subdev.rst
+++ b/Documentation/driver-api/media/v4l2-subdev.rst
@@ -546,6 +546,14 @@ subdev drivers. In these cases the called subdev ops must also handle the NULL
case. This can be easily managed by the use of
v4l2_subdev_validate_and_lock_state() helper.
+Streams, multiplexed media pads and internal routing
+----------------------------------------------------
+
+A subdevice driver can implement support for multiplexed streams by setting
+the V4L2_SUBDEV_FL_MULTIPLEXED subdev flag and implementing support for
+centrally managed subdev active state, routing and stream based
+configuration.
+
V4L2 sub-device functions and data structures
---------------------------------------------
diff --git a/Documentation/userspace-api/media/v4l/dev-subdev.rst b/Documentation/userspace-api/media/v4l/dev-subdev.rst
index a67c2749089a..fd042afeddd6 100644
--- a/Documentation/userspace-api/media/v4l/dev-subdev.rst
+++ b/Documentation/userspace-api/media/v4l/dev-subdev.rst
@@ -503,3 +503,168 @@ source pads.
:maxdepth: 1
subdev-formats
+
+Streams, multiplexed media pads and internal routing
+----------------------------------------------------
+
+Commonly V4L2 subdevices support only separate video streams, that is, each
+link in the media graph and each pad in a subdevice pass through a single
+video stream. 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, a single
+bus carries multiple streams. Thus a camera could output two streams, a pixel
+stream and a metadata stream, and a bridge subdev could route the streams
+from multiple input pads into a single output pad.
+
+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 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 configuration for 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.
+
+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), and outputs two streams, pixel data and metadata.
+
+- Multiplexer bridge (Bridge). The bridge has two sink pads, connected to the
+ sensors (pads 0, 1), and one source pad (pad 2), which outputs all 4
+ streams.
+
+- Receiver in the SoC (Receiver). The receiver has a single sink pad (pad 0),
+ connected to the bridge, and four source pads (pads 1-4), going to the DMA
+ engine. The receiver demultiplexes the incoming streams to the four source
+ pads.
+
+- Four DMA Engines in the SoC (DMA Engine). 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:: Sensor routing table (identical on both sensors)
+ :header-rows: 1
+
+ * - Sink Pad/Stream
+ - Source Pad/Stream
+ - Routing Flags
+ - Comments
+ * - 0/0 (unused)
+ - 0/0
+ - V4L2_SUBDEV_ROUTE_FL_ACTIVE | V4L2_SUBDEV_ROUTE_FL_SOURCE
+ - Pixel data stream. Source route, i.e. the sink fields are unused.
+ * - 0/0 (unused)
+ - 0/1
+ - V4L2_SUBDEV_ROUTE_FL_ACTIVE | V4L2_SUBDEV_ROUTE_FL_SOURCE
+ - Metadata stream. Source route, i.e. the sink fields are unused.
+
+.. 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
+ * - 0/1
+ - 2/1
+ - V4L2_SUBDEV_ROUTE_FL_ACTIVE
+ - Metadata stream from Sensor A
+ * - 1/0
+ - 2/2
+ - V4L2_SUBDEV_ROUTE_FL_ACTIVE
+ - Pixel data stream from Sensor B
+ * - 1/1
+ - 2/3
+ - V4L2_SUBDEV_ROUTE_FL_ACTIVE
+ - Metadata 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
+ - Metadata stream from Sensor A
+ * - 0/2
+ - 3/0
+ - V4L2_SUBDEV_ROUTE_FL_ACTIVE
+ - Pixel data stream from Sensor B
+ * - 0/3
+ - 4/0
+ - V4L2_SUBDEV_ROUTE_FL_ACTIVE
+ - Metadata 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 VIDIOC_SUBDEV_S_FMT
+to configure each stream endpoint in each subdev.
--
2.25.1
