diff --git a/Documentation/userspace-api/media/v4l/dev-subdev.rst b/Documentation/userspace-api/media/v4l/dev-subdev.rst
index b76e02e54512..0f9eda3187f3 100644
--- a/Documentation/userspace-api/media/v4l/dev-subdev.rst
+++ b/Documentation/userspace-api/media/v4l/dev-subdev.rst
@@ -572,6 +572,29 @@ 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 have a fixed routing configuration. 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. ``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
 ^^^^^^^^^^^^^^^^^^^