WebVR unstuffing: delay "rendered" notification

chrome/browser/android/vr_shell/vr_shell_gl.cc

Index: chrome/browser/android/vr_shell/vr_shell_gl.cc
diff --git a/chrome/browser/android/vr_shell/vr_shell_gl.cc b/chrome/browser/android/vr_shell/vr_shell_gl.cc
index 65cc835187302e13ebd1d9266e54d0b50d144d13..ebc784c4bfb900c7642c7e26af55114794d0faf7 100644
--- a/chrome/browser/android/vr_shell/vr_shell_gl.cc
+++ b/chrome/browser/android/vr_shell/vr_shell_gl.cc
@@ -423,15 +423,6 @@ void VrShellGl::OnWebVRFrameAvailable() {
   TRACE_EVENT1("gpu", "VrShellGl::OnWebVRFrameAvailable", "frame", frame_index);
   pending_frames_.pop();
 
-  // It is legal for the WebVR client to submit a new frame now, since
-  // we've consumed the image. TODO(klausw): would timing be better if
-  // we move the "rendered" notification after draw, or suppress
-  // the next vsync until that's done?
-
-  if (submit_client_) {
-    submit_client_->OnSubmitFrameRendered();
-  }
-
   DrawFrame(frame_index);
 }
 
@@ -1066,6 +1057,14 @@ void VrShellGl::DrawFrame(int16_t frame_index) {
     surface_->SwapBuffers();
   }
 
+  // Report rendering completion to WebVR so that it's permitted to submit
+  // a fresh frame. We could do this earlier, as soon as the frame got pulled
+  // off the transfer surface, but that appears to result in overstuffed
+  // buffers.
+  if (submit_client_) {
+    submit_client_->OnSubmitFrameRendered();
+  }
+
 #if DCHECK_IS_ON()
   // After saving the timestamp, fps will be available via GetFPS().
   // TODO(vollick): enable rendering of this framerate in a HUD.
@@ -1466,7 +1465,6 @@ void VrShellGl::OnVSync() {
     SendVSync(time, base::ResetAndReturn(&callback_));
   } else {
     pending_vsync_ = true;
-    pending_time_ = time;
   }
   if (!ShouldDrawWebVr()) {
     DrawFrame(-1);
@@ -1490,7 +1488,8 @@ void VrShellGl::GetVSync(const GetVSyncCallback& callback) {
     return;
   }
   pending_vsync_ = false;
-  SendVSync(pending_time_, callback);
+  base::TimeDelta time = base::TimeTicks::Now() - vsync_timebase_;

[mthiesse, 2017/05/23 00:14:17]

I don't think we want to do this with the timestamps. The purpose of storing the
timestamp is to ensure we get a consistent interval in the timestamps so that
animations feel smooth - even if they're slightly behind what the current
timestamp is.

What you could do is check if the pending timestamp is more than a frame old,
and up it by one vsync interval though.

+  SendVSync(time, callback);
 }
 
 void VrShellGl::UpdateVSyncInterval(int64_t timebase_nanos,