[Ozone-Drm] Pick pending buffer over current buffer when modesetting

ui/ozone/platform/drm/gpu/hardware_display_controller.cc

Index: ui/ozone/platform/drm/gpu/hardware_display_controller.cc
diff --git a/ui/ozone/platform/drm/gpu/hardware_display_controller.cc b/ui/ozone/platform/drm/gpu/hardware_display_controller.cc
index 3f31b6ef962e7507af8b2ce0ecb58115cc4afbbc..6a7e2c8e7f3a052886a91e11458d13ae2e63e13b 100644
--- a/ui/ozone/platform/drm/gpu/hardware_display_controller.cc
+++ b/ui/ozone/platform/drm/gpu/hardware_display_controller.cc
@@ -71,7 +71,15 @@ bool HardwareDisplayController::Modeset(const OverlayPlane& primary,
 bool HardwareDisplayController::Enable() {
   TRACE_EVENT0("drm", "HDC::Enable");
   DCHECK(!current_planes_.empty());
-  const OverlayPlane* primary = OverlayPlane::GetPrimaryPlane(current_planes_);
+
+  const OverlayPlane* primary = nullptr;
+  // Use the last scheduled buffer to modeset to preserve request order.
+  if (!requests_.empty())
+    primary = OverlayPlane::GetPrimaryPlane(requests_.back().planes);
+  else if (!pending_planes_.empty())
+    primary = OverlayPlane::GetPrimaryPlane(pending_planes_);
+  else
+    primary = OverlayPlane::GetPrimaryPlane(current_planes_);

[spang, 2015/03/27 21:26:11]

I agree with Dominic.. is it really necessary to do anything here while swaps
are already in flight? Can't we just wait a minute for things to settle before
queuing up more work?

Trying to figure out whatever the last thing we did was and re-do it seems super
confusing to me.

[dnicoara, 2015/03/27 21:28:21]

The Browser treats display configuration and buffer swaps as orthogonal
operations. If we wait, we can wait for ever since the compositors will continue
pushing new frames.

 
   return Modeset(*primary, mode_);
 }