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

From: Steve Longerbeam
Date: Fri Jan 04 2019 - 19:49:10 EST

Hi Niklas,

How about a patch that simply replaces the entity use_count check with a stream_count check?

As in:

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,
ÂÂÂÂ ÂÂÂ return 0;

-ÂÂÂ /* If any entity is in use don't allow link changes. */
+ÂÂÂ /*
+ÂÂÂ Â* Don't allow link changes if any entity in the graph is
+ÂÂÂ Â* streaming, modifying the CHSEL register fields can disrupt
+ÂÂÂ Â* running streams.
+ÂÂÂ Â*/
ÂÂÂÂ media_device_for_each_entity(entity, &group->mdev)
-ÂÂÂ ÂÂÂ if (entity->use_count)
+ÂÂÂ ÂÂÂ if (entity->stream_count)

ÂÂÂÂ mutex_lock(&group->lock);

And that might be overkilll, maybe only the stream_count's of the VIN entities need to be checked.


On 12/29/18 3:37 PM, Steve Longerbeam wrote:
Hi Niklas,

On 12/26/18 4:51 PM, Niklas SÃderlund wrote:
Hi Steve,

Thanks for your patch.

On 2018-12-25 15:27:25 -0800, Steve Longerbeam wrote:
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

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.
Unfortunately this is by design and needed due to the hardware design.
The different VIN endpoints shares a configuration register which
controls the routing from the CSI-2 receivers to the VIN (register name
CHSEL). Modifying the CHSEL register which is what happens when a link
is enabled/disabled can have side effects on running streams even if
they are not shown to be dependent in the media graph.

Ok, understood, modifying CHSEL register can adversely affect running streams.

There is a CHSEL register in VIN0 which controls the routing from all
CSI-2 receivers to VIN0-3 and a CHSEL register in VIN4 which controls
the same for VIN4-7.

If this code block is meant to prevent modifying a link if the
link is actively involved in streaming, 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.
For the reason above the check in __media_entity_setup_link() is not
enough :-( This register sharing is my least favorite thing about the
VIN on Gen3 and forces the driver to become more complex as all VIN
instances needs to know about each other and interact.

Given above I understand why the stream count checks in __media_entity_setup_link() are insufficient, because only the requested link's source stream count is checked, and not the other CSI-2 receiver for example.

But why check the use counts of every entity upstream from the VIN sources? Why not check only the VIN source entities stream counts (both CSI-2 receivers and/or parallel devices), and ignore entities upstream from those?

And why are the use counts checked, it seems it should be the stream counts that should be checked.

Remove the code block so that VIN node links can be enabled even if
there are other independent in-use entities.
There is room for some improvement in this area disregarding the odd
hardware design. It *could* be allowed to change a link terminating in
VIN4-7 even if there is a stream running for one or more in VIN0-3.

I would be interested to test such a patch but to allow any link change
which is allowed by __media_entity_setup_link() is unfortunately not
possible, as I understand it. Maybe someone more clever then me can find
ways to unlock even more then just the split between VIN0-3 and VIn4-7.

In essence the CHSEL register can not be changed if it's involved in a
running pipeline even if the end result would be that the running
pipeline would look the same. This is possible as there are multiple
CHSEL settings where the same source is connected to a specific VIN
while other members of the "subgroup of VINs" (e.g. VIN0-3) is routed to
something else for the two CHSEL settings.

Right, so rvin_group_link_notify() determines whether the requested VIN link enable will result in a valid set of CSI2->VIN links for the given hardware, using the CHSEL bitmask tables. Which is why it seems it is the stream counts that should be checked as mentioned above, rather than the use counts, because the CHSEL bitmask checks are validating the set of enabled links, and the only remaining checks are to verify no streams are running on either CSI-2 receiver.


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

Signed-off-by: Steve Longerbeam <slongerbeam@xxxxxxxxx>
 drivers/media/platform/rcar-vin/rcar-core.c | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/drivers/media/platform/rcar-vin/rcar-core.c b/drivers/media/platform/rcar-vin/rcar-core.c
index f0719ce24b97..b2c9a876969e 100644
--- a/drivers/media/platform/rcar-vin/rcar-core.c
+++ b/drivers/media/platform/rcar-vin/rcar-core.c
@@ -116,7 +116,6 @@ static int rvin_group_link_notify(struct media_link *link, u32 flags,
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ struct rvin_group, mdev);
ÂÂÂÂÂ unsigned int master_id, channel, mask_new, i;
ÂÂÂÂÂ unsigned int mask = ~0;
-ÂÂÂ struct media_entity *entity;
ÂÂÂÂÂ struct video_device *vdev;
ÂÂÂÂÂ struct media_pad *csi_pad;
ÂÂÂÂÂ struct rvin_dev *vin = NULL;
@@ -131,11 +130,6 @@ static int rvin_group_link_notify(struct media_link *link, u32 flags,
ÂÂÂÂÂÂÂÂÂ return 0;
 - /* If any entity is in use don't allow link changes. */
-ÂÂÂ media_device_for_each_entity(entity, &group->mdev)
-ÂÂÂÂÂÂÂ if (entity->use_count)
ÂÂÂÂÂ mutex_lock(&group->lock);
 Â /* Find the master VIN that controls the routes. */