[RFC PATCH v2] media: rcar-vin: Allow independent VIN link enablement

[Steve Longerbeam <slongerbeam@gmail.com>]

There is a block of code in rvin_group_link_notify() that loops through
all entities in the media graph, and prevents enabling a link to a VIN
node if any entity is in use. This prevents enabling a VIN link even if
there is an in-use entity somewhere in the graph that is independent of
the link's pipeline.

For example, the code will prevent enabling a link from the first
rcar-csi2 receiver to a VIN node even if there is an enabled link
somewhere far upstream on the second independent rcar-csi2 receiver
pipeline.

If this code is meant to prevent modifying a link if any entity in the
graph is actively involved in streaming (because modifying the CHSEL
register fields can disrupt any/all running streams), then the entities
stream counts should be checked rather than the use counts.

(There is already such a check in __media_entity_setup_link() that verifies
the stream_count of the link's source and sink entities are both zero,
but that is insufficient, since there should be no running streams in
the entire graph).

Modify the media_device_for_each_entity() loop to check the entity
stream_count instead of the use_count, and elaborate on the comment.
VIN node links can now be enabled even if there are other independent
in-use entities that are not streaming.

Fixes: c0cc5aef31 ("media: rcar-vin: add link notify for Gen3")

Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
---
Changes in v2:
- bring back the media_device_for_each_entity() loop but check the
  stream_count not the use_count.
---
 drivers/media/platform/rcar-vin/rcar-core.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/media/platform/rcar-vin/rcar-core.c b/drivers/media/platform/rcar-vin/rcar-core.c
index f0719ce24b97..6dd6b11c1b2b 100644
--- a/drivers/media/platform/rcar-vin/rcar-core.c
+++ b/drivers/media/platform/rcar-vin/rcar-core.c
@@ -131,9 +131,13 @@ static int rvin_group_link_notify(struct media_link *link, u32 flags,
 	    !is_media_entity_v4l2_video_device(link->sink->entity))
 		return 0;
 
-	/* If any entity is in use don't allow link changes. */
+	/*
+	 * Don't allow link changes if any entity in the graph is
+	 * streaming, because modifying the CHSEL register fields
+	 * can disrupt running streams.
+	 */
 	media_device_for_each_entity(entity, &group->mdev)
-		if (entity->use_count)
+		if (entity->stream_count)
 			return -EBUSY;
 
 	mutex_lock(&group->lock);
-- 
2.17.1