[PATCH v4 15/18] drm/etnaviv: Don't break exclusive fence ordering

[Daniel Vetter <daniel.vetter@ffwll.ch>]

There's only one exclusive slot, and we must not break the ordering.
Adding a new exclusive fence drops all previous fences from the
dma_resv. To avoid violating the signalling order we err on the side of
over-synchronizing by waiting for the existing fences, even if
userspace asked us to ignore them.

A better fix would be to us a dma_fence_chain or _array like e.g.
amdgpu now uses, but it probably makes sense to lift this into
dma-resv.c code as a proper concept, so that drivers don't have to
hack up their own solution each on their own. Hence go with the simple
fix for now.

Another option is the fence import ioctl from Jason:

https://lore.kernel.org/dri-devel/20210610210925.642582-7-jason@jlekstrand.net/

v2: Improve commit message per Lucas' suggestion.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Russell King <linux+etnaviv@armlinux.org.uk>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: etnaviv@lists.freedesktop.org
---
 drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c b/drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c
index 5b97ce1299ad..07454db4b150 100644
--- a/drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c
+++ b/drivers/gpu/drm/etnaviv/etnaviv_gem_submit.c
@@ -178,18 +178,20 @@ static int submit_fence_sync(struct etnaviv_gem_submit *submit)
 	for (i = 0; i < submit->nr_bos; i++) {
 		struct etnaviv_gem_submit_bo *bo = &submit->bos[i];
 		struct dma_resv *robj = bo->obj->base.resv;
+		bool write = bo->flags & ETNA_SUBMIT_BO_WRITE;
 
-		if (!(bo->flags & ETNA_SUBMIT_BO_WRITE)) {
+		if (!(write)) {
 			ret = dma_resv_reserve_shared(robj, 1);
 			if (ret)
 				return ret;
 		}
 
-		if (submit->flags & ETNA_SUBMIT_NO_IMPLICIT)
+		/* exclusive fences must be ordered */
+		if (submit->flags & ETNA_SUBMIT_NO_IMPLICIT && !write)
 			continue;
 
 		ret = drm_sched_job_await_implicit(&submit->sched_job, &bo->obj->base,
-						   bo->flags & ETNA_SUBMIT_BO_WRITE);
+						   write);
 		if (ret)
 			return ret;
 	}
-- 
2.32.0