Bug 943041 - Don't delay compositing on the compositor thread in ASAP mode (layout.frame_rate == 0). r=BenWa

gfx/layers/ipc/CompositorParent.cpp

@@ -498,6 +498,10 @@ CompositorParent::NotifyShadowTreeTransaction(uint64_t aId, bool aIsFirstPaint)
   ScheduleComposition();
 }
 
+// Used when layout.frame_rate is -1. Needs to be kept in sync with
+// DEFAULT_FRAME_RATE in nsRefreshDriver.cpp.
+static const int32_t kDefaultFrameRate = 60;
+
 void
 CompositorParent::ScheduleComposition()
 {
@@ -510,19 +514,29 @@ CompositorParent::ScheduleComposition()
   if (!initialComposition)
     delta = TimeStamp::Now() - mLastCompose;
 
+  int32_t rate = gfxPlatform::GetPrefLayoutFrameRate();
+  if (rate < 0) {
+    // TODO: The main thread frame scheduling code consults the actual monitor
+    // refresh rate in this case. We should do the same.
+    rate = kDefaultFrameRate;
+  }
+
+  // If rate == 0 (ASAP mode), minFrameDelta must be 0 so there's no delay.
+  TimeDuration minFrameDelta = TimeDuration::FromMilliseconds(
+    rate == 0 ? 0.0 : std::max(0.0, 1000.0 / rate));
+
 #ifdef COMPOSITOR_PERFORMANCE_WARNING
-  mExpectedComposeTime = TimeStamp::Now() + TimeDuration::FromMilliseconds(15);
+  mExpectedComposeTime = TimeStamp::Now() + minFrameDelta;
 #endif
 
   mCurrentCompositeTask = NewRunnableMethod(this, &CompositorParent::Composite);
 
-  // Since 60 fps is the maximum frame rate we can acheive, scheduling composition
-  // events less than 15 ms apart wastes computation..
-  if (!initialComposition && delta.ToMilliseconds() < 15) {
+  if (!initialComposition && delta < minFrameDelta) {
+    TimeDuration delay = minFrameDelta - delta;
 #ifdef COMPOSITOR_PERFORMANCE_WARNING
-    mExpectedComposeTime = TimeStamp::Now() + TimeDuration::FromMilliseconds(15 - delta.ToMilliseconds());
+    mExpectedComposeTime = TimeStamp::Now() + delay;
 #endif
-    ScheduleTask(mCurrentCompositeTask, 15 - delta.ToMilliseconds());
+    ScheduleTask(mCurrentCompositeTask, delay.ToMilliseconds());
   } else {
     ScheduleTask(mCurrentCompositeTask, 0);
   }

gfx/thebes/gfxPlatform.cpp

@@ -58,6 +58,7 @@
 #include "nsTArray.h"
 #include "nsILocaleService.h"
 #include "nsIObserverService.h"
+#include "MainThreadUtils.h"
 
 #include "nsWeakReference.h"
 
@@ -2049,6 +2050,12 @@ InitLayersAccelerationPrefs()
 {
   if (!sLayersAccelerationPrefsInitialized)
   {
+    // If this is called for the first time on a non-main thread, we're screwed.
+    // At the moment there's no explicit guarantee that the main thread calls
+    // this before the compositor thread, but let's at least make the assumption
+    // explicit.
+    MOZ_ASSERT(NS_IsMainThread(), "can only initialize prefs on the main thread");
+
     sPrefLayersOffMainThreadCompositionEnabled = Preferences::GetBool("layers.offmainthreadcomposition.enabled", false);
     sPrefLayersOffMainThreadCompositionTestingEnabled = Preferences::GetBool("layers.offmainthreadcomposition.testing.enabled", false);
     sPrefLayersOffMainThreadCompositionForceEnabled = Preferences::GetBool("layers.offmainthreadcomposition.force-enabled", false);