[Intel-gfx] [PATCH 1/2] drm/atomic-helper: Complete fake_commit->flip_done potentially earlier

[Ville Syrjala]

From: Ville Syrjälä <ville.syrjala at linux.intel.com>

Consider the following scenario:
1. nonblocking enable crtc
2. wait for the event
3. nonblocking disable crtc

On i915 this can lead to a spurious -EBUSY from step 3 on
account of non-enabled planes getting the fake_commit in step 1
and we don't complete the fake_commit-> flip_done until
drm_atomic_helper_commit_hw_done() which can happen a long
time after the flip event was sent out.

This will become somewhat easy to hit on SKL+ once we start
to add all the planes for the crtc to every modeset commit
for the purposes of forcing a watermark register programming
[1].

To make the race a little less pronounced let's complete
fake_commit->flip_done after drm_atomic_helper_wait_for_flip_done().
For the single crtc case this should make the race quite
theoretical, assuming drm_atomic_helper_wait_for_flip_done()
actually has to wait for the real commit flip_done. In case
the real commit flip_done gets completed singificantly before
drm_atomic_helper_wait_for_flip_done(), or we are dealing with
multiple crtcs whose vblanks don't line up nicely the race still
exists.

[1] https://patchwork.freedesktop.org/patch/262670/

Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Fixes: 080de2e5be2d ("drm/atomic: Check for busy planes/connectors before setting the commit")
Testcase: igt/kms_cursor_legacy/*nonblocking-modeset-vs-cursor-atomic
Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
---
 drivers/gpu/drm/drm_atomic_helper.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index fa95f9974f6d..47398662b895 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -1460,6 +1460,9 @@ void drm_atomic_helper_wait_for_flip_done(struct drm_device *dev,
 			DRM_ERROR("[CRTC:%d:%s] flip_done timed out\n",
 				  crtc->base.id, crtc->name);
 	}
+
+	if (old_state->fake_commit)
+		complete_all(&old_state->fake_commit->flip_done);
 }
 EXPORT_SYMBOL(drm_atomic_helper_wait_for_flip_done);
 
-- 
2.18.1