Bug 1117870. Compositor unobserve vsync after configurable number of notifications. r=benwa

gfx/layers/ipc/CompositorParent.cpp

@@ -199,6 +199,7 @@ static void SetThreadPriority()
 CompositorVsyncObserver::CompositorVsyncObserver(CompositorParent* aCompositorParent, nsIWidget* aWidget)
   : mNeedsComposite(false)
   , mIsObservingVsync(false)
+  , mVsyncNotificationsSkipped(0)
   , mCompositorParent(aCompositorParent)
   , mCurrentCompositeTaskMonitor("CurrentCompositeTaskMonitor")
   , mCurrentCompositeTask(nullptr)
@@ -285,6 +286,9 @@ CompositorVsyncObserver::Composite(TimeStamp aVsyncTimestamp)
   if (mNeedsComposite && mCompositorParent) {
     mNeedsComposite = false;
     mCompositorParent->CompositeCallback(aVsyncTimestamp);
+    mVsyncNotificationsSkipped = 0;
+  } else if (mVsyncNotificationsSkipped++ > gfxPrefs::CompositorUnobserveCount()) {
+    UnobserveVsync();
   }
 
   DispatchTouchEvents(aVsyncTimestamp);

gfx/layers/ipc/CompositorParent.h

@@ -117,6 +117,7 @@ private:
 
   bool mNeedsComposite;
   bool mIsObservingVsync;
+  int32_t mVsyncNotificationsSkipped;
   nsRefPtr<CompositorParent> mCompositorParent;
   nsRefPtr<CompositorVsyncDispatcher> mCompositorVsyncDispatcher;
 

gfx/thebes/gfxPrefs.h

@@ -219,6 +219,9 @@ private:
   // Use vsync events generated by hardware
   DECL_GFX_PREF(Once, "gfx.vsync.hw-vsync.enabled",            HardwareVsyncEnabled, bool, false);
   DECL_GFX_PREF(Once, "gfx.vsync.compositor",                  VsyncAlignedCompositor, bool, false);
+  // On b2g, in really bad cases, I've seen up to 80 ms delays between touch events and the main thread
+  // processing them. So 80 ms / 16 = 5 vsync events. Double it up just to be on the safe side, so 10.
+  DECL_GFX_PREF(Once, "gfx.vsync.compositor.unobserve-count",  CompositorUnobserveCount, int32_t, 10);
   DECL_GFX_PREF(Once, "gfx.touch.resample",                    TouchResampling, bool, false);
   // These times should be in milliseconds
   DECL_GFX_PREF(Once, "gfx.touch.resample.max-predict",        TouchResampleMaxPredict, int32_t, 8);