[PATCH AUTOSEL 4.20 026/117] drm/atomic-helper: Complete fake_commit->flip_done potentially earlier

[Sasha Levin]

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

[ Upstream commit 2de42f79bb21a412f40ade8831eb6fc445cb78a4 ]

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>
Link: https://patchwork.freedesktop.org/patch/msgid/20181122143412.11655-1-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Signed-off-by: Sasha Levin <sashal at kernel.org>
---
 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 d8b526b7932c..b4e292a56046 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -1445,6 +1445,9 @@ 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);
 
-- 
2.19.1