media: Documentation: mc: add definitions for stream and pipeline

The doc talks about streams and pipelines, but doesn't really define
them. This is an attempt to define them according to my understanding.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

Documentation/driver-api/media/mc-core.rst

@@ -214,6 +214,18 @@ Link properties can be modified at runtime by calling
 Pipelines and media streams
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+A media stream is a stream of pixels or metadata originating from one or more
+source devices (such as a sensors) and flowing through media entity pads
+towards the final sinks. The stream can be modified on the route by the
+devices (e.g. scaling or pixel format conversions), or it can be split into
+multiple branches, or multiple branches can be merged.
+
+A media pipeline is a set of media streams which are interdependent. This
+interdependency can be caused by the hardware (e.g. configuration of a second
+stream cannot be changed if the first stream has been enabled) or by the driver
+due to the software design. Most commonly a media pipeline consists of a single
+stream which does not branch.
+
 When starting streaming, drivers must notify all entities in the pipeline to
 prevent link states from being modified during streaming by calling
 :c:func:`media_pipeline_start()`.