WebVR: flush before requesting sync token for mailbox

third_party/WebKit/Source/modules/vr/VRDisplay.cpp

Index: third_party/WebKit/Source/modules/vr/VRDisplay.cpp
diff --git a/third_party/WebKit/Source/modules/vr/VRDisplay.cpp b/third_party/WebKit/Source/modules/vr/VRDisplay.cpp
index 5122c99d42d313daa98f7a7ad95c259908966e61..3201ab4b09aef52ea62219a363d4ac17c5b6a456 100644
--- a/third_party/WebKit/Source/modules/vr/VRDisplay.cpp
+++ b/third_party/WebKit/Source/modules/vr/VRDisplay.cpp
@@ -523,7 +523,9 @@ void VRDisplay::submitFrame() {
     return;
   }
 
+  TRACE_EVENT_BEGIN0("gpu", "VRDisplay::Flush_1");
   context_gl_->Flush();
+  TRACE_EVENT_END0("gpu", "VRDisplay::Flush_1");
 
   // Check if the canvas got resized, if yes send a bounds update.
   int current_width = rendering_context_->drawingBufferWidth();
@@ -560,8 +562,10 @@ void VRDisplay::submitFrame() {
     }
   }
 
+  TRACE_EVENT_BEGIN0("gpu", "VRDisplay::GetImage");
   RefPtr<Image> image_ref = rendering_context_->GetImage(
       kPreferAcceleration, kSnapshotReasonCreateImageBitmap);
+  TRACE_EVENT_END0("gpu", "VRDisplay::GetImage");
 
   // Hardware-accelerated rendering should always be texture backed,
   // as implemented by AcceleratedStaticBitmapImage. Ensure this is
@@ -578,7 +582,9 @@ void VRDisplay::submitFrame() {
   // image until the mailbox was consumed.
   StaticBitmapImage* static_image =
       static_cast<StaticBitmapImage*>(image_ref.Get());
+  TRACE_EVENT_BEGIN0("gpu", "VRDisplay::EnsureMailbox");
   static_image->EnsureMailbox();
+  TRACE_EVENT_END0("gpu", "VRDisplay::EnsureMailbox");
 
   if (wait_for_previous_transfer_to_finish) {
     // Save a reference to the image to keep it alive until next frame,
@@ -587,9 +593,22 @@ void VRDisplay::submitFrame() {
     previous_image_ = std::move(image_ref);
   }
 
+  // Create mailbox and sync token for transfer.
+  TRACE_EVENT_BEGIN0("gpu", "VRDisplay::GetMailbox");
+  auto mailbox = static_image->GetMailbox();
+  TRACE_EVENT_END0("gpu", "VRDisplay::GetMailbox");
+  // Flush to avoid black screen flashes which appear to be related to
+  // "fence sync must be flushed before generating sync token" GL errors.
+  TRACE_EVENT_BEGIN0("gpu", "VRDisplay::Flush_2");
+  context_gl_->Flush();
+  TRACE_EVENT_END0("gpu", "VRDisplay::Flush_2");
+  auto sync_token = static_image->GetSyncToken();
+
   // Wait for the previous render to finish, to avoid losing frames in the
   // Android Surface / GLConsumer pair. TODO(klausw): make this tunable?
-  // Other devices may have different preferences.
+  // Other devices may have different preferences. Do this step as late
+  // as possible before SubmitFrame to ensure we can do as much work as
+  // possible in parallel with the previous frame's rendering.
   {
     TRACE_EVENT0("gpu", "waitForPreviousRenderToFinish");
     while (pending_previous_frame_render_) {
@@ -602,10 +621,11 @@ void VRDisplay::submitFrame() {
 
   pending_previous_frame_render_ = true;
   pending_submit_frame_ = true;
-  display_->SubmitFrame(
-      vr_frame_id_,
-      gpu::MailboxHolder(static_image->GetMailbox(),
-                         static_image->GetSyncToken(), GL_TEXTURE_2D));
+
+  TRACE_EVENT_BEGIN0("gpu", "VRDisplay::SubmitFrame");
+  display_->SubmitFrame(vr_frame_id_,
+                        gpu::MailboxHolder(mailbox, sync_token, GL_TEXTURE_2D));
+  TRACE_EVENT_END0("gpu", "VRDisplay::SubmitFrame");
 
   // If preserveDrawingBuffer is false, must clear now. Normally this
   // happens as part of compositing, but that's not active while