media: media-entity: Simplify media_pipeline_start()

The media_pipeline_start() function has two purposes: it constructs a
pipeline by recording the entities that are part of it, gathered from a
graph walk, and validate the media links. The pipeline pointer is stored
in the media_entity structure as part of this process, and the entity's
stream count is increased, to record that the entity is streaming.

When multiple video nodes are present in a pipeline,
media_pipeline_start() is typically called on all of them, with the same
pipeline pointer. This is taken into account in media_pipeline_start()
by skipping validation for entities that are already part of the
pipeline, while returning an error if an entity is part of a different
pipeline.

It turns out that this process is overly complicated. When
media_pipeline_start() is called for the first time, it constructs the
full pipeline, adding all entities and validating all the links.
Subsequent calls to media_pipeline_start() are then nearly no-ops, they
only increase the stream count on the pipeline and on all entities.

The media_entity stream_count field is used for two purposes: checking
if the entity is streaming, and detecting when a call to
media_pipeline_stop() balances needs to reset the entity pipe pointer to
NULL. The former can easily be replaced by a check of the pipe pointer.

Simplify media_pipeline_start() by avoiding the pipeline walk on all
calls but the first one, and drop the media_entity stream_count field.
media_pipeline_stop() is updated accordingly.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
[Sakari Ailus: Drop redundant '!= NULL' as discussed]
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>

drivers/media/mc/mc-entity.c

@@ -396,11 +396,14 @@ __must_check int __media_pipeline_start(struct media_entity *entity,
 	struct media_link *link;
 	int ret;
 
-	if (!pipe->streaming_count++) {
+	if (pipe->streaming_count) {
+		pipe->streaming_count++;
+		return 0;
+	}
+
 	ret = media_graph_walk_init(&pipe->graph, mdev);
 	if (ret)
-			goto error_graph_walk_start;
-	}
+		return ret;
 
 	media_graph_walk_start(&pipe->graph, entity);
 
@@ -408,8 +411,6 @@ __must_check int __media_pipeline_start(struct media_entity *entity,
 		DECLARE_BITMAP(active, MEDIA_ENTITY_MAX_PADS);
 		DECLARE_BITMAP(has_no_links, MEDIA_ENTITY_MAX_PADS);
 
-		entity->stream_count++;
-
 		if (entity->pipe && entity->pipe != pipe) {
 			pr_err("Pipe active for %s. Can't start for %s\n",
 				entity->name,
@@ -418,12 +419,12 @@ __must_check int __media_pipeline_start(struct media_entity *entity,
 			goto error;
 		}
 
-		entity->pipe = pipe;
-
 		/* Already streaming --- no need to check. */
-		if (entity->stream_count > 1)
+		if (entity->pipe)
 			continue;
 
+		entity->pipe = pipe;
+
 		if (!entity->ops || !entity->ops->link_validate)
 			continue;
 
@@ -479,6 +480,8 @@ __must_check int __media_pipeline_start(struct media_entity *entity,
 		}
 	}
 
+	pipe->streaming_count++;
+
 	return 0;
 
 error:
@@ -489,23 +492,16 @@ __must_check int __media_pipeline_start(struct media_entity *entity,
 	media_graph_walk_start(graph, entity_err);
 
 	while ((entity_err = media_graph_walk_next(graph))) {
-		/* Sanity check for negative stream_count */
-		if (!WARN_ON_ONCE(entity_err->stream_count <= 0)) {
-			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.
+		 * We haven't started entities further than this so we quit
+		 * here.
 		 */
 		if (entity_err == entity)
 			break;
 	}
 
-error_graph_walk_start:
-	if (!--pipe->streaming_count)
 	media_graph_walk_cleanup(graph);
 
 	return ret;
@@ -537,18 +533,14 @@ void __media_pipeline_stop(struct media_entity *entity)
 	if (WARN_ON(!pipe))
 		return;
 
+	if (--pipe->streaming_count)
+		return;
+
 	media_graph_walk_start(graph, entity);
 
-	while ((entity = media_graph_walk_next(graph))) {
-		/* Sanity check for negative stream_count */
-		if (!WARN_ON_ONCE(entity->stream_count <= 0)) {
-			entity->stream_count--;
-			if (entity->stream_count == 0)
+	while ((entity = media_graph_walk_next(graph)))
 		entity->pipe = NULL;
-		}
-	}
 
-	if (!--pipe->streaming_count)
 	media_graph_walk_cleanup(graph);
 
 }

include/media/media-entity.h

@@ -268,7 +268,6 @@ enum media_entity_type {
  * @pads:	Pads array with the size defined by @num_pads.
  * @links:	List of data links.
  * @ops:	Entity operations.
- * @stream_count: Stream count for the entity.
  * @use_count:	Use count for the entity.
  * @pipe:	Pipeline this entity belongs to.
  * @info:	Union with devnode information.  Kept just for backward
@@ -283,10 +282,9 @@ enum media_entity_type {
  *
  * .. note::
  *
- *    @stream_count and @use_count reference counts must never be
- *    negative, but are signed integers on purpose: a simple ``WARN_ON(<0)``
- *    check can be used to detect reference count bugs that would make them
- *    negative.
+ *    The @use_count reference count must never be negative, but is a signed
+ *    integer on purpose: a simple ``WARN_ON(<0)`` check can be used to detect
+ *    reference count bugs that would make it negative.
  */
 struct media_entity {
 	struct media_gobj graph_obj;	/* must be first field in struct */
@@ -305,7 +303,6 @@ struct media_entity {
 
 	const struct media_entity_operations *ops;
 
-	int stream_count;
 	int use_count;
 
 	struct media_pipeline *pipe;
@@ -867,7 +864,7 @@ struct media_pad *media_entity_remote_pad(const struct media_pad *pad);
  */
 static inline bool media_entity_is_streaming(const struct media_entity *entity)
 {
-	return entity->stream_count > 0;
+	return entity->pipe;
 }
 
 /**