[PATCH v3] media: rcar-vin: Allow independent VIN link enablement

[Steve Longerbeam <slongerbeam@gmail.com>]

From: Steve Longerbeam <slongerbeam@gmail.com>

There is a block of code in rvin_group_link_notify() that prevents
enabling a link to a VIN node if any entity in the media graph 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 block 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 block 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 code block 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>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
---
Changes in v3:
- Removed RFC from subject and added Niklas' Reviewed-by.
  No functional changes.
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..aef8d8dab6ab 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, 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