[PATCH v2 1/3] drm/syncobj: call drm_syncobj_fence_add_wait when WAIT_AVAILABLE flag is set

[Erik Kurzinger]

When waiting for a syncobj timeline point whose fence has not yet been
submitted with the WAIT_FOR_SUBMIT flag, a callback is registered using
drm_syncobj_fence_add_wait and the thread is put to sleep until the
timeout expires. If the fence is submitted before then,
drm_syncobj_add_point will wake up the sleeping thread immediately which
will proceed to wait for the fence to be signaled.

However, if the WAIT_AVAILABLE flag is used instead,
drm_syncobj_fence_add_wait won't get called, meaning the waiting thread
will always sleep for the full timeout duration, even if the fence gets
submitted earlier. If it turns out that the fence *has* been submitted
by the time it eventually wakes up, it will still indicate to userspace
that the wait completed successfully (it won't return -ETIME), but it
will have taken much longer than it should have.

To fix this, we must call drm_syncobj_fence_add_wait if *either* the
WAIT_FOR_SUBMIT flag or the WAIT_AVAILABLE flag is set. The only
difference being that with WAIT_FOR_SUBMIT we will also wait for the
fence to be signaled after it has been submitted while with
WAIT_AVAILABLE we will return immediately.

IGT test patch: https://lists.freedesktop.org/archives/igt-dev/2024-January/067537.html

v1 -> v2: adjust lockdep_assert_none_held_once condition

Fixes: 01d6c3578379 ("drm/syncobj: add support for timeline point wait v8")
Signed-off-by: Erik Kurzinger <ekurzinger at nvidia.com>
---
 drivers/gpu/drm/drm_syncobj.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
index 94ebc71e5be5..97be8b140599 100644
--- a/drivers/gpu/drm/drm_syncobj.c
+++ b/drivers/gpu/drm/drm_syncobj.c
@@ -1058,7 +1058,8 @@ static signed long drm_syncobj_array_wait_timeout(struct drm_syncobj **syncobjs,
 	uint64_t *points;
 	uint32_t signaled_count, i;
 
-	if (flags & DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT)
+	if (flags & (DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT |
+		     DRM_SYNCOBJ_WAIT_FLAGS_WAIT_AVAILABLE))
 		lockdep_assert_none_held_once();
 
 	points = kmalloc_array(count, sizeof(*points), GFP_KERNEL);
@@ -1127,7 +1128,8 @@ static signed long drm_syncobj_array_wait_timeout(struct drm_syncobj **syncobjs,
 	 * fallthough and try a 0 timeout wait!
 	 */
 
-	if (flags & DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT) {
+	if (flags & (DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT |
+		     DRM_SYNCOBJ_WAIT_FLAGS_WAIT_AVAILABLE)) {
 		for (i = 0; i < count; ++i)
 			drm_syncobj_fence_add_wait(syncobjs[i], &entries[i]);
 	}
-- 
2.43.0