[media] media: Entity graph traversal

Add media entity graph traversal. The traversal follows enabled links by
depth first. Traversing graph backwards is prevented by comparing the next
possible entity in the graph with the previous one. Multiply connected
graphs are thus not supported.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Vimarsh Zutshi <vimarsh.zutshi@gmail.com>
Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

Documentation/media-framework.txt

@@ -216,3 +216,45 @@ Links have flags that describe the link capabilities and state.
 	modified at runtime. If MEDIA_LNK_FL_IMMUTABLE is set, then
 	MEDIA_LNK_FL_ENABLED must also be set since an immutable link is always
 	enabled.
+
+
+Graph traversal
+---------------
+
+The media framework provides APIs to iterate over entities in a graph.
+
+To iterate over all entities belonging to a media device, drivers can use the
+media_device_for_each_entity macro, defined in include/media/media-device.h.
+
+	struct media_entity *entity;
+
+	media_device_for_each_entity(entity, mdev) {
+		/* entity will point to each entity in turn */
+		...
+	}
+
+Drivers might also need to iterate over all entities in a graph that can be
+reached only through enabled links starting at a given entity. The media
+framework provides a depth-first graph traversal API for that purpose.
+
+Note that graphs with cycles (whether directed or undirected) are *NOT*
+supported by the graph traversal API. To prevent infinite loops, the graph
+traversal code limits the maximum depth to MEDIA_ENTITY_ENUM_MAX_DEPTH,
+currently defined as 16.
+
+Drivers initiate a graph traversal by calling
+
+	media_entity_graph_walk_start(struct media_entity_graph *graph,
+				      struct media_entity *entity);
+
+The graph structure, provided by the caller, is initialized to start graph
+traversal at the given entity.
+
+Drivers can then retrieve the next entity by calling
+
+	media_entity_graph_walk_next(struct media_entity_graph *graph);
+
+When the graph traversal is complete the function will return NULL.
+
+Graph traversal can be interrupted at any moment. No cleanup function call is
+required and the graph structure can be freed normally.