Surface Synchronization: Distinguish between dependencies and references

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 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 "cc/trees/layer_tree_host_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 1606 matching lines @@
   }
 
   if (GetDrawMode() == DRAW_MODE_RESOURCELESS_SOFTWARE) {
     metadata.is_resourceless_software_draw_with_scroll_or_animation =
         IsActivelyScrolling() || mutator_host_->NeedsTickAnimations();
   }
 
   for (LayerImpl* surface_layer : active_tree_->SurfaceLayers()) {
     SurfaceLayerImpl* surface_layer_impl =
         static_cast<SurfaceLayerImpl*>(surface_layer);
-    metadata.referenced_surfaces.push_back(
-        surface_layer_impl->primary_surface_info().id());
-    // We need to retain a reference to the fallback surface too so that it's
-    // guaranteed to be available when needed.
-    if (surface_layer_impl->fallback_surface_info().is_valid()) {
+    if (settings_.enable_surface_synchronization) {
+      if (surface_layer_impl->fallback_surface_info().is_valid()) {
+        metadata.referenced_surfaces.push_back(
+            surface_layer_impl->fallback_surface_info().id());
+      }

[piman, 2017/04/12 00:09:34]

Should we add a reference to the primary when we don't have a fallback (because
we don't need one, we're embedding an already known surface)?

[Fady Samuel, 2017/04/12 00:19:44]

Currently we have both primary and fallback SurfaceInfos but we'll only emit a
single SurfaceDrawQuad:

https://cs.chromium.org/chromium/src/cc/layers/surface_layer_impl.cc?q=Surfac...

I don't want to add only the primary if a fallback doesn't exist because the
display compositor may not know about that primary yet, and I don't want to add
a reference to a thing that doesn't yet exist.

In Aura-Mus, we generate a big gutter that fills the window on the first frame
if it arrives late. Other OSes like Windows 10, I noticed does something
similar. 

+    } else {
       metadata.referenced_surfaces.push_back(
-          surface_layer_impl->fallback_surface_info().id());
+          surface_layer_impl->primary_surface_info().id());
     }
   }
   if (!InnerViewportScrollLayer())
     return metadata;
 
   metadata.root_overflow_x_hidden |=
       !InnerViewportScrollLayer()->user_scrollable_horizontal();
   metadata.root_overflow_y_hidden |=
       !InnerViewportScrollLayer()->user_scrollable_vertical();
 
@@ skipping 2653 matching lines @@
 }
 
 void LayerTreeHostImpl::UpdateScrollSourceInfo(bool is_wheel_scroll) {
   if (is_wheel_scroll)
     has_scrolled_by_wheel_ = true;
   else
     has_scrolled_by_touch_ = true;
 }
 
 }  // namespace cc