Ozone: Avoid blocking in Swapbuffer Call.

ui/ozone/platform/dri/gbm_surfaceless.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/dri/gbm_surfaceless.h"
 
 #include "ui/ozone/platform/dri/dri_vsync_provider.h"
 #include "ui/ozone/platform/dri/dri_window_delegate.h"
 #include "ui/ozone/platform/dri/gbm_buffer.h"
 #include "ui/ozone/platform/dri/hardware_display_controller.h"
@@ skipping 14 matching lines @@
 bool GbmSurfaceless::ResizeNativeWindow(const gfx::Size& viewport_size) {
   NOTIMPLEMENTED();
   return false;
 }
 
 bool GbmSurfaceless::OnSwapBuffers() {
   HardwareDisplayController* controller = window_delegate_->GetController();
   if (!controller)
     return true;
 
+  // Ensure any previous PageFlip events are handled. Doing this here also makes
+  // sure we don't throttle GPU in case it's still handling previous PageFlip
+  // request. This also makes sure we have at most two frames "in flight". Let's
+  // say we submitted all the commands for render buffer A to GPU and then
+  // requested the driver to display it. GPU might still be rendering buffer A,
+  // so the flip is still pending. However, we might already be busy at work
+  // submitting commands for render buffer B. Once done submitting commands for
+  // buffer B, we queue B for page flip, and continue render for frame C. If we
+  // try page flipping B without handling the events, the driver might return
+  // EBusy(in case it's still handling previous flip) here, and also we don't
+  // want to use any more than triple buffering. Thus, we sleep waiting for A
+  // to show up on the screen, before issuing a flip request for B.
+  controller->WaitForPageFlipEvent();
   bool success = controller->SchedulePageFlip();
-  controller->WaitForPageFlipEvent();
 
   return success;
 }
 
 scoped_ptr<gfx::VSyncProvider> GbmSurfaceless::CreateVSyncProvider() {
   return scoped_ptr<gfx::VSyncProvider>(new DriVSyncProvider(window_delegate_));
 }
 
 }  // namespace ui