[PATCH v9 11/46] media: Documentation: Document S_ROUTING behaviour

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

Document S_ROUTING behaviour for different devices.

Generally in devices that produce streams the streams are static and some
can be enabled and disabled, whereas in devices that just transport them
or write them to memory, more configurability is allowed. Document this.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Julien Massot <julien.massot@collabora.com>
---
 .../userspace-api/media/v4l/dev-subdev.rst    | 24 +++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/Documentation/userspace-api/media/v4l/dev-subdev.rst b/Documentation/userspace-api/media/v4l/dev-subdev.rst
index d30dcb9e2537..de8dfd4f11a5 100644
--- a/Documentation/userspace-api/media/v4l/dev-subdev.rst
+++ b/Documentation/userspace-api/media/v4l/dev-subdev.rst
@@ -593,6 +593,30 @@ 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.
 
+Device types and routing setup
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Different kinds of sub-devices have differing behaviour for route activation,
+depending on the hardware. In all cases, however, only routes that have the
+``V4L2_SUBDEV_STREAM_FL_ACTIVE`` flag set are active.
+
+Devices generating the streams may allow enabling and disabling some of the
+routes or the configuration is fixed. If the routes can be disabled, not
+declaring the routes (or declaring them without
+``VIDIOC_SUBDEV_STREAM_FL_ACTIVE`` flag set) in ``VIDIOC_SUBDEV_S_ROUTING`` will
+disable the routes while the sub-device driver retains the streams and their
+format and selection configuration. The ``VIDIOC_SUBDEV_S_ROUTING`` will still
+return such routes back to the user in the routes array, with the
+``V4L2_SUBDEV_STREAM_FL_ACTIVE`` flag unset.
+
+Devices transporting the streams almost always have more configurability with
+respect to routing. Typically any route between the sub-device's sink and source
+pads is possible, and multiple routes (usually up to certain limited number) may
+be active simultaneously. For such devices, no routes are created by the driver
+and user-created routes are fully replaced when ``VIDIOC_SUBDEV_S_ROUTING`` is
+called on the sub-device. Such newly created routes have the device's default
+configuration for format and selection rectangles.
+
 Configuring streams
 ^^^^^^^^^^^^^^^^^^^
 
-- 
2.39.2