WebVR: use optimized render path with deferred wait

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

 // Copyright 2016 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 "modules/vr/VRDisplay.h"
 
 #include "core/css/StylePropertySet.h"
 #include "core/dom/DOMException.h"
 #include "core/dom/FrameRequestCallback.h"
 #include "core/dom/ScriptedAnimationController.h"
@@ skipping 571 matching lines @@
   }
 
   // There's two types of synchronization needed for submitting frames:
   //
   // - Before submitting, need to wait for the previous frame to be
   //   pulled off the transfer surface to avoid lost frames. This
   //   is currently a compile-time option, normally we always want
   //   to defer this wait to increase parallelism.
   //
   // - After submitting, need to wait for the mailbox to be consumed,
-  //   and must remain inside the execution context while waiting.
-  //   The waitForPreviousTransferToFinish option defers this wait
-  //   until the next frame. That's more efficient, but seems to
-  //   cause wobbly framerates.
-  bool wait_for_previous_transfer_to_finish =
-      RuntimeEnabledFeatures::webVRExperimentalRenderingEnabled();
-
-  if (wait_for_previous_transfer_to_finish) {
+  //   and the image object must remain alive during this time.
+  //   We keep a reference to the image so that we can defer this
+  //   wait. Here, we wait for the previous transfer to complete.
+  {
     TRACE_EVENT0("gpu", "VRDisplay::waitForPreviousTransferToFinish");
     while (pending_submit_frame_) {
       if (!submit_frame_client_binding_.WaitForIncomingMethodCall()) {
         DLOG(ERROR) << "Failed to receive SubmitFrame response";
         break;
       }
     }
   }
 
   TRACE_EVENT_BEGIN0("gpu", "VRDisplay::GetImage");
@@ skipping 13 matching lines @@
   // The AcceleratedStaticBitmapImage must be kept alive until the
   // mailbox is used via createAndConsumeTextureCHROMIUM, the mailbox
   // itself does not keep it alive. We must keep a reference to the
   // 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,
-    // where we'll wait for the transfer to finish before overwriting
-    // it.
-    previous_image_ = std::move(image_ref);
-  }
+  // Save a reference to the image to keep it alive until next frame,
+  // where we'll wait for the transfer to finish before overwriting
+  // it.
+  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");
@@ skipping 24 matching lines @@
 
   did_submit_this_frame_ = true;
   // If we were deferring a rAF-triggered vsync request, do this now.
   if (pending_vrdisplay_raf_)
     RequestVSync();
 
   // If preserveDrawingBuffer is false, must clear now. Normally this
   // happens as part of compositing, but that's not active while
   // presenting, so run the responsible code directly.
   rendering_context_->MarkCompositedAndClearBackbufferIfNeeded();
-
-  // If we're not deferring the wait for transferring the mailbox,
-  // we need to wait for it now to prevent the image going out of
-  // scope before its mailbox is retrieved.
-  if (!wait_for_previous_transfer_to_finish) {
-    TRACE_EVENT0("gpu", "waitForCurrentTransferToFinish");
-    while (pending_submit_frame_) {
-      if (!submit_frame_client_binding_.WaitForIncomingMethodCall()) {
-        DLOG(ERROR) << "Failed to receive SubmitFrame response";
-        break;
-      }
-    }
-  }
 }
 
 void VRDisplay::OnSubmitFrameTransferred() {
   DVLOG(3) << __FUNCTION__;
   pending_submit_frame_ = false;
 }
 
 void VRDisplay::OnSubmitFrameRendered() {
   DVLOG(3) << __FUNCTION__;
   pending_previous_frame_render_ = false;
@@ skipping 257 matching lines @@
   visitor->Trace(stage_parameters_);
   visitor->Trace(eye_parameters_left_);
   visitor->Trace(eye_parameters_right_);
   visitor->Trace(layer_);
   visitor->Trace(rendering_context_);
   visitor->Trace(scripted_animation_controller_);
   visitor->Trace(pending_present_resolvers_);
 }
 
 }  // namespace blink