[PATCH 4/4] drm/i915: Make blocking commits lockless

[Ville Syrjala]

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

Make blocking commits execute locklessly (just as nonblocking
commits do) by scheduling them onto the workqueues as well.
There will be a later flush_work() done by whoever called
the commit hook to make sure the blocking behaviour of the
ioctl/etc. is preserved.

I also wondered about just dropping the locks straight from the
driver, but I guess whoever called us might still depend on
having the locks so that seems like a terrible idea. Also calling
commit_tail() directly when not holding the locks would then
allow eg. two ioctls to execute full modesets in parallel,
which we don't want as we haven't fully evaluated all modeset
codepaths for concurrency issues. Currently we achieve serial
excution with a combination of an ordered workqueue (for
nonblocking commits) and reliance on the singular connection_mutex
(for blocking commits), and a flush_workqueue() to sync between
the two.

So by always scheduling everything onto the workqueues we
don't have to worry about racing between the lockless direct
commit_tail() calls, and we don't need some kind of new atomic
hook that would do said call for us after dropping the locks.
Also less codepaths in general seems like a beneficial thing.

Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Cc: Rob Clark <robdclark at gmail.com>
Cc: Simon Ser <contact at emersion.fr>
Cc: Pekka Paalanen <pekka.paalanen at collabora.com>
Cc: Jonas Ådahl <jadahl at gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
---
 drivers/gpu/drm/i915/display/intel_display.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
index cd617046e0ee..18a5f14e7f41 100644
--- a/drivers/gpu/drm/i915/display/intel_display.c
+++ b/drivers/gpu/drm/i915/display/intel_display.c
@@ -7771,15 +7771,10 @@ static int intel_atomic_commit(struct drm_device *dev,
 	INIT_WORK(&state->base.commit_work, intel_atomic_commit_work);
 
 	i915_sw_fence_commit(&state->commit_ready);
-	if (nonblock && state->modeset) {
+	if (state->modeset)
 		queue_work(dev_priv->display.wq.modeset, &state->base.commit_work);
-	} else if (nonblock) {
+	else
 		queue_work(dev_priv->display.wq.flip, &state->base.commit_work);
-	} else {
-		if (state->modeset)
-			flush_workqueue(dev_priv->display.wq.modeset);
-		intel_atomic_commit_tail(state);
-	}
 
 	return 0;
 }
-- 
2.35.1