[v2,4/9] media: uapi: v4l: Document source routes

diff --git a/Documentation/userspace-api/media/v4l/dev-subdev.rst b/Documentation/userspace-api/media/v4l/dev-subdev.rst
index d737c9530dbd..0e3ca6d6d81f 100644
--- a/Documentation/userspace-api/media/v4l/dev-subdev.rst
+++ b/Documentation/userspace-api/media/v4l/dev-subdev.rst
@@ -551,6 +551,27 @@  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.
 
+.. _v4l2-subdev-source-routes:
+
+Internal pads and source routes
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Cases where a single sub-device source pad is traversed by multiple streams one
+or more of which originate from within the sub-device itself are special as
+there is no external sink pad for such routes. In those cases, the sources of
+the internally generated streams are represented by internal sink pads, which
+are sink pads that have the :ref:`MEDIA_PAD_FL_INTERNAL <MEDIA-PAD-FL-INTERNAL>`
+pad flag set.
+
+Internal pads have all the properties of an external pad, including formats and
+selections. The format in this case is the source format of the stream. An
+internal pad always has a single stream only (0).
+
+/Source routes/ are routes from an internal sink pad to a(n external) source
+pad. Generally source routes are not modifiable but they can be activated and
+deactivated using the :ref:`V4L2_SUBDEV_ROUTE_FL_ACTIVE
+<v4l2-subdev-routing-flags>` flag, depending on driver capabilities.
+
 Interaction between routes, streams, formats and selections
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -666,3 +687,99 @@  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.
+
+Internal pads setup example
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A simple example of a multiplexed stream setup might be as follows:
+
+- A camera sensor, with one source pad (0) and two internal sink pads (source of
+  image data (1) and source of embedded data (2)) and two source routes, one
+  from each internal sink pad towards the (external) source pad. The embedded
+  data stream needs to be enabled by activating the related route.
+
+- Multiplexer bridge (Bridge). The bridge has one sink pad, connected to the
+  sensor (pad 0), and one source pad (pad 1), 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 sensor, 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:: Camera sensor
+    :header-rows: 1
+
+    * - Sink Pad/Stream
+      - Source Pad/Stream
+      - Routing Flags
+      - Comments
+    * - 1/0
+      - 0/0
+      - V4L2_SUBDEV_ROUTE_FL_ACTIVE
+      - Pixel data stream from internal sink pad
+    * - 2/0
+      - 0/1
+      - V4L2_SUBDEV_ROUTE_FL_ACTIVE
+      - Metadata stream from internal sink pad
+
+.. flat-table:: Bridge 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 camera sensor
+    * - 0/1
+      - 1/1
+      - V4L2_SUBDEV_ROUTE_FL_ACTIVE
+      - Metadata stream from camera sensor
+
+.. 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 camera sensor
+    * - 0/1
+      - 2/0
+      - V4L2_SUBDEV_ROUTE_FL_ACTIVE
+      - Metadata stream from camera sensor
+
+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.
+
+The options available in sensor's routing configuration are dictated by hardware
+capabilities: typically camera sensors always produce image data stream while
+the embedded data stream typically can be either enabled or disabled.