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

Buffers are expected to be scheduled/released in FIFO order. When
enabling a display we want to re-use a buffer to avoid flashes. As
such we need to make sure that the buffer used for modesetting is the
last buffer queued.

BUG=chrome-os-partner:38267

Review URL: https://codereview.chromium.org/1041823002

Cr-Commit-Position: refs/heads/master@{#322673}

2 files changed, 10 insertions, 2 deletions

diff --git a/ui/ozone/platform/drm/gpu/hardware_display_controller.cc b/ui/ozone/platform/drm/gpu/hardware_display_controller.cc
index 3f31b6e..4371963 100644
--- a/ui/ozone/platform/drm/gpu/hardware_display_controller.cc
+++ b/ui/ozone/platform/drm/gpu/hardware_display_controller.cc
@@ -56,7 +56,6 @@ bool HardwareDisplayController::Modeset(const OverlayPlane& primary,
   mode_ = mode;
 
   current_planes_ = std::vector<OverlayPlane>(1, primary);
-  pending_planes_.clear();
   ClearPendingRequests();
 
   // Because a page flip is pending we need to leave some state for the
@@ -71,7 +70,13 @@ 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
+    primary = OverlayPlane::GetPrimaryPlane(current_planes_);
 
   return Modeset(*primary, mode_);
 }
diff --git a/ui/ozone/platform/drm/gpu/hardware_display_controller.h b/ui/ozone/platform/drm/gpu/hardware_display_controller.h
index ee1d606..52095d5 100644
--- a/ui/ozone/platform/drm/gpu/hardware_display_controller.h
+++ b/ui/ozone/platform/drm/gpu/hardware_display_controller.h
@@ -180,7 +180,10 @@ class OZONE_EXPORT HardwareDisplayController
   // Buffers need to be declared first so that they are destroyed last. Needed
   // since the controllers may reference the buffers.
   OverlayPlaneList current_planes_;
+  // Planes currently being queued without having SchedulePageFlip() called.
   OverlayPlaneList pending_planes_;
+  // Plane lists for which SchedulePageFlip() was called. They are waiting for
+  // previous page flip events to be completed before they can be serviced.
   std::deque<PageFlipRequest> requests_;
   scoped_refptr<ScanoutBuffer> cursor_buffer_;