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>

1 files changed, 4 insertions, 7 deletions

diff --git a/include/media/media-entity.h b/include/media/media-entity.h
index 8546f13c42a9..33ee595c13f4 100644
--- a/include/media/media-entity.h
+++ b/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;
 }
 
 /**