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

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

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/ozone/platform/drm/gpu/hardware_display_controller.h"
 
 #include <drm.h>
 #include <string.h>
 #include <xf86drm.h>
 
@@ skipping 53 matching lines @@
   if (HasPendingPageFlips())
     requests_.push_back(
         PageFlipRequest(current_planes_, false, base::Bind(&base::DoNothing)));
 
   return status;
 }
 
 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_);
 }
 
 void HardwareDisplayController::Disable() {
   TRACE_EVENT0("drm", "HDC::Disable");
   for (size_t i = 0; i < crtc_controllers_.size(); ++i)
     crtc_controllers_[i]->Disable();
 
   is_disabled_ = true;
@@ skipping 233 matching lines @@
 
 void HardwareDisplayController::ClearPendingRequests() {
   while (!requests_.empty()) {
     PageFlipRequest request = requests_.front();
     requests_.pop_front();
     request.callback.Run();
   }
 }
 
 }  // namespace ui