[PATCH v5 19/20] drm/i915: 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.

Cc: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
---
 drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
index 1ed7475de454..25ba2765d27d 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
@@ -2240,6 +2240,7 @@ static int eb_move_to_gpu(struct i915_execbuffer *eb)
 		struct i915_vma *vma = ev->vma;
 		unsigned int flags = ev->flags;
 		struct drm_i915_gem_object *obj = vma->obj;
+		bool async, write;
 
 		assert_vma_held(vma);
 
@@ -2271,7 +2272,10 @@ static int eb_move_to_gpu(struct i915_execbuffer *eb)
 				flags &= ~EXEC_OBJECT_ASYNC;
 		}
 
-		if (err == 0 && !(flags & EXEC_OBJECT_ASYNC)) {
+		async = flags & EXEC_OBJECT_ASYNC;
+		write = flags & EXEC_OBJECT_WRITE;
+
+		if (err == 0 && (!async || write)) {
 			err = i915_request_await_object
 				(eb->request, obj, flags & EXEC_OBJECT_WRITE);
 		}
-- 
2.32.0