[PATCH 1/1] media: Documentation: Interaction between routes, formats and selections

[Sakari Ailus <sakari.ailus@linux.intel.com>]

Document how setting up routes interacts with formats and selections.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
---
Moi,

This could be added on top of the set.

Comments are welcome.

 .../userspace-api/media/v4l/dev-subdev.rst    | 48 ++++++++++++++-----
 1 file changed, 37 insertions(+), 11 deletions(-)

diff --git a/Documentation/userspace-api/media/v4l/dev-subdev.rst b/Documentation/userspace-api/media/v4l/dev-subdev.rst
index 5075b1828b32d..830235eb01598 100644
--- a/Documentation/userspace-api/media/v4l/dev-subdev.rst
+++ b/Documentation/userspace-api/media/v4l/dev-subdev.rst
@@ -406,6 +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
 ---------------------------------------------
 
@@ -507,12 +509,12 @@ source pads.
 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.
+Commonly V4L2 subdevices do not support multiple, unrelated video streams.
+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
@@ -539,15 +541,37 @@ 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.
+A stream ID 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.
+of the subdevice. The same stream ID is used to refer to the stream on
+both pads of the link on all ioctls operating on pads.
 
 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
+subdev and a pad--stream pair. For subdevices that do not support
 multiplexed streams the 'stream' is always 0.
 
+Interaction between routes, formats and selections
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The addition of routes to the V4L2 sub-device interface moves the
+sub-device formats and selections from pads to pad--stream pairs. Instead
+of the sub-device wide merging of streams from all source pads towards all
+sink pads, this takes place separately for each route. The stream ID on
+the sink pad for each configured route is used to configure format and
+selections on the sink pad. Similarly, the stream ID on the source pad of
+each configured route is used to configure format and selections on the
+source pad.
+
+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
+sink pad, however, only a single route is allowed.
+
+Pad--stream pairs are not static but are replaced by calls to the
+:ref:`VIDIOC_SUBDEV_S_ROUTING <VIDIOC_SUBDEV_G_ROUTING>` ioctl. This also
+applies to stream format and selection configurations which that are
+reverted to driver defaults.
+
 Configuring streams
 ^^^^^^^^^^^^^^^^^^^
 
@@ -565,8 +589,10 @@ Controller API <media_controller>`
 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>`.
+3) Configure formats and selections on routes. Each route is configured
+separately as documented plain subdevices in :ref:`<format-propagation>`.
+The stream ID is set to the same stream ID used on sink and source pads of
+the :ref:`VIDIOC_SUBDEV_S_ROUTING <VIDIOC_SUBDEV_G_ROUTING>` ioctl.
 
 Multiplexed streams setup example
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-- 
2.30.2