[media] media: Add link_validate() op to check links to the sink pad

The purpose of the link_validate() op is to allow an entity driver to ensure
that the properties of the pads at the both ends of the link are suitable
for starting the pipeline. link_validate is called on sink pads on active
links which belong to the active part of the graph.

Signed-off-by: Sakari Ailus <[e-mail]>
Acked-by: Laurent Pinchart <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

Documentation/media-framework.txt

···
 Calls to media_entity_pipeline_start() can be nested. The pipeline pointer must
 be identical for all nested calls to the function.
 
+media_entity_pipeline_start() may return an error. In that case, it will
+clean up any the changes it did by itself.
+
 When stopping the stream, drivers must notify the entities with
 
 	media_entity_pipeline_stop(struct media_entity *entity);
···
 changing entities configuration parameters) drivers can explicitly check the
 media_entity stream_count field to find out if an entity is streaming. This
 operation must be done with the media_device graph_mutex held.
+
+
+Link validation
+---------------
+
+Link validation is performed by media_entity_pipeline_start() for any
+entity which has sink pads in the pipeline. The
+media_entity::link_validate() callback is used for that purpose. In
+link_validate() callback, entity driver should check that the properties of
+the source pad of the connected entity and its own sink pad match. It is up
+to the type of the entity (and in the end, the properties of the hardware)
+what matching actually means.
+
+Subsystems should facilitate link validation by providing subsystem specific
+helper functions to provide easy access for commonly needed information, and
+in the end provide a way to use driver-specific callbacks.

drivers/media/media-entity.c

···
  * pipeline pointer must be identical for all nested calls to
  * media_entity_pipeline_start().
  */
-void media_entity_pipeline_start(struct media_entity *entity,
-				 struct media_pipeline *pipe)
+__must_check int media_entity_pipeline_start(struct media_entity *entity,
+					     struct media_pipeline *pipe)
 {
 	struct media_device *mdev = entity->parent;
 	struct media_entity_graph graph;
+	struct media_entity *entity_err = entity;
+	int ret;
 
 	mutex_lock(&mdev->graph_mutex);
 
 	media_entity_graph_walk_start(&graph, entity);
 
 	while ((entity = media_entity_graph_walk_next(&graph))) {
+		unsigned int i;
+
 		entity->stream_count++;
 		WARN_ON(entity->pipe && entity->pipe != pipe);
 		entity->pipe = pipe;
+
+		/* Already streaming --- no need to check. */
+		if (entity->stream_count > 1)
+			continue;
+
+		if (!entity->ops || !entity->ops->link_validate)
+			continue;
+
+		for (i = 0; i < entity->num_links; i++) {
+			struct media_link *link = &entity->links[i];
+
+			/* Is this pad part of an enabled link? */
+			if (!(link->flags & MEDIA_LNK_FL_ENABLED))
+				continue;
+
+			/* Are we the sink or not? */
+			if (link->sink->entity != entity)
+				continue;
+
+			ret = entity->ops->link_validate(link);
+			if (ret < 0 && ret != -ENOIOCTLCMD)
+				goto error;
+		}
 	}
 
 	mutex_unlock(&mdev->graph_mutex);
+
+	return 0;
+
+error:
+	/*
+	 * Link validation on graph failed. We revert what we did and
+	 * return the error.
+	 */
+	media_entity_graph_walk_start(&graph, entity_err);
+
+	while ((entity_err = media_entity_graph_walk_next(&graph))) {
+		entity_err->stream_count--;
+		if (entity_err->stream_count == 0)
+			entity_err->pipe = NULL;
+
+		/*
+		 * We haven't increased stream_count further than this
+		 * so we quit here.
+		 */
+		if (entity_err == entity)
+			break;
+	}
+
+	mutex_unlock(&mdev->graph_mutex);
+
+	return ret;
 }
 EXPORT_SYMBOL_GPL(media_entity_pipeline_start);
 

include/media/media-entity.h

···
 	int (*link_setup)(struct media_entity *entity,
 			  const struct media_pad *local,
 			  const struct media_pad *remote, u32 flags);
+	int (*link_validate)(struct media_link *link);
 };
 
 struct media_entity {
···
 		struct media_entity *entity);
 struct media_entity *
 media_entity_graph_walk_next(struct media_entity_graph *graph);
-void media_entity_pipeline_start(struct media_entity *entity,
-		struct media_pipeline *pipe);
+__must_check int media_entity_pipeline_start(struct media_entity *entity,
+					     struct media_pipeline *pipe);
 void media_entity_pipeline_stop(struct media_entity *entity);
 
 #define media_entity_call(entity, operation, args...)			\