Read compositor VSync information from platform, when possible

ash/display/display_controller.cc

 // Copyright (c) 2012 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 "ash/display/display_controller.h"
 
 #include <algorithm>
 #include <cmath>
 #include <map>
 
@@ skipping 101 matching lines @@
   ui::SetIntProperty(xwindow, kInternalProp, kCARDINAL, internal);
   ui::SetIntProperty(xwindow, kRotationProp, kCARDINAL, xrandr_rotation);
   ui::SetIntProperty(xwindow,
                      kScaleFactorProp,
                      kCARDINAL,
                      100 * display.device_scale_factor());
 #endif
   scoped_ptr<aura::RootWindowTransformer> transformer(
       internal::CreateRootWindowTransformerForDisplay(root->window(), display));
   root->host()->SetRootWindowTransformer(transformer.Pass());
+
+  internal::DisplayMode mode;
+  if (GetDisplayManager()->GetSelectedModeForDisplayId(display.id(), &mode)) {

[brianderson, 2014/02/03 22:56:04]

Will this be called every frame? If not, how often will it be called?

[sheu, 2014/02/03 23:02:40]

This should be called only when monitor configuration changes.

[brianderson, 2014/02/04 01:39:51]

Does it happen in all the following cases:
When a window changes monitors.
When the monitor lowers its frequency to save power.
When the user manually selects a monitor config.

Also, does this code path only apply to ChromeOS?

[oshima, 2014/02/04 02:07:53]

No
Currently no, although it's not difficult to support this.
(just need to check refresh rate in DisplayManager::UpdateDisplays)
Yes
Yes

[brianderson, 2014/02/04 02:14:36]

Is that beca
For all instances where the answer is "No", is the answer "Yes" for the original
code path?

+    root->host()->compositor()->OnUpdateVSyncParameters(
+        base::TimeTicks(),

[brianderson, 2014/02/03 22:56:04]

Why isn't there timebase information here?

[sheu, 2014/02/03 23:02:40]

We don't have access to timebase information here, so we can't assign it a
value.

+        base::TimeDelta::FromMicroseconds(base::Time::kMicrosecondsPerSecond /
+                                          mode.refresh_rate),
+        true);

[brianderson, 2014/02/03 22:56:04]

Is this the only place where the authoritative interval is true?

[sheu, 2014/02/03 23:02:40]

Yes.

+  }
 }
 
 }  // namespace
 
 namespace internal {
 
 // A utility class to store/restore focused/active window
 // when the display configuration has changed.
 class FocusActivationStore {
  public:
@@ skipping 603 matching lines @@
     std::string name =
         root_windows[i] == primary ? "aura_root_0" : "aura_root_x";
     gfx::AcceleratedWidget xwindow =
         root_windows[i]->GetDispatcher()->host()->GetAcceleratedWidget();
     XStoreName(gfx::GetXDisplay(), xwindow, name.c_str());
   }
 #endif
 }
 
 }  // namespace ash