Read compositor VSync information from platform, when possible

ui/compositor/compositor_vsync_manager.cc

+// Copyright (c) 2014 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 "ui/compositor/compositor_vsync_manager.h"
+
+// These constants define a reasonable range for a calculated refresh interval.
+// Calculating refreshes out of this range will be considered a fatal error.
+const int64 kMinVsyncIntervalUs = base::Time::kMicrosecondsPerSecond / 400;
+const int64 kMaxVsyncIntervalUs = base::Time::kMicrosecondsPerSecond / 10;
+
+namespace ui {
+
+CompositorVSyncManager::CompositorVSyncManager()
+    : observer_list_(new ObserverListThreadSafe<Observer>()),
+      authoritative_vsync_interval_(base::TimeDelta::FromSeconds(0)) {}
+
+CompositorVSyncManager::~CompositorVSyncManager() {}
+
+void CompositorVSyncManager::SetAuthoritativeVSyncInterval(
+    base::TimeDelta interval) {
+  base::AutoLock lock(vsync_parameters_lock_);
+  authoritative_vsync_interval_ = interval;
+  NotifyObservers(last_timebase_, authoritative_vsync_interval_);

[piman, 2014/02/04 22:52:54]

nit: release the lock before NotifyObservers, to make sure we don't deadlock if
the observer reenters.

+}
+
+void CompositorVSyncManager::UpdateVSyncParameters(base::TimeTicks timebase,
+                                                   base::TimeDelta interval) {
+  {
+    base::AutoLock lock(vsync_parameters_lock_);
+    if (authoritative_vsync_interval_ != base::TimeDelta::FromSeconds(0))
+      interval = authoritative_vsync_interval_;
+  }
+  NotifyObservers(timebase, interval);
+}
+
+void CompositorVSyncManager::AddObserver(Observer* observer) {
+  observer_list_->AddObserver(observer);
+  observer->OnUpdateVSyncParameters(last_timebase_, last_interval_);
+}
+
+void CompositorVSyncManager::RemoveObserver(Observer* observer) {
+  observer_list_->RemoveObserver(observer);
+}
+
+void CompositorVSyncManager::NotifyObservers(base::TimeTicks timebase,
+                                             base::TimeDelta interval) {
+  // The vsync interval reported here controls the repaint rate of the
+  // compositor. If too low, kill the process instead of silently appearing to
+  // hang the UI.
+  if (interval.InMicroseconds() < kMinVsyncIntervalUs ||
+      interval.InMicroseconds() > kMaxVsyncIntervalUs) {
+    LOG(FATAL) << "Calculated bogus refresh interval of "
+               << interval.InMicroseconds() << " us.";
+  }
+
+  last_timebase_ = timebase;
+  last_interval_ = interval;

[piman, 2014/02/04 22:52:54]

acquire lock while modifying these 2.

+  observer_list_->Notify(
+      &CompositorVSyncManager::Observer::OnUpdateVSyncParameters,
+      timebase,
+      interval);
+}
+
+}  // namespace ui