Bug 1119850 - Modify Vsync Timestamp on Mac to be the previous vsync timestamp. r=mstange

gfx/thebes/VsyncSource.h

@@ -30,6 +30,14 @@ public:
       void AddCompositorVsyncDispatcher(mozilla::CompositorVsyncDispatcher* aCompositorVsyncDispatcher);
       void RemoveCompositorVsyncDispatcher(mozilla::CompositorVsyncDispatcher* aCompositorVsyncDispatcher);
       // Notified when this display's vsync occurs, on the vsync thread
+      // The aVsyncTimestamp should normalize to the Vsync time that just occured
+      // However, different platforms give different vsync notification times.
+      // b2g - The vsync timestamp of the previous frame that was just displayed
+      // OSX - The vsync timestamp of the upcoming frame, in the future
+      // TODO: Windows / Linux. DOCUMENT THIS WHEN IMPLEMENTING ON THOSE PLATFORMS
+      // Android: TODO
+      // All platforms should normalize to the vsync that just occured.
+      // Large parts of Gecko assume TimeStamps should not be in the future such as animations
       virtual void NotifyVsync(mozilla::TimeStamp aVsyncTimestamp);
 
       // These should all only be called on the main thread

gfx/thebes/gfxPlatformMac.cpp

@@ -418,14 +418,7 @@ static CVReturn VsyncCallback(CVDisplayLinkRef aDisplayLink,
                               const CVTimeStamp* aOutputTime,
                               CVOptionFlags aFlagsIn,
                               CVOptionFlags* aFlagsOut,
-                              void* aDisplayLinkContext)
-{
-  VsyncSource::Display* display = (VsyncSource::Display*) aDisplayLinkContext;
-  int64_t timestamp = aOutputTime->hostTime;
-  mozilla::TimeStamp vsyncTime = mozilla::TimeStamp::FromSystemTime(timestamp);
-  display->NotifyVsync(vsyncTime);
-  return kCVReturnSuccess;
-}
+                              void* aDisplayLinkContext);
 
 class OSXVsyncSource MOZ_FINAL : public VsyncSource
 {
@@ -439,7 +432,6 @@ public:
     return mGlobalDisplay;
   }
 
-protected:
   class OSXDisplay MOZ_FINAL : public VsyncSource::Display
   {
   public:
@@ -471,6 +463,7 @@ protected:
         return;
       }
 
+      mPreviousTimestamp = TimeStamp::Now();
       if (CVDisplayLinkStart(mDisplayLink) != kCVReturnSuccess) {
         NS_WARNING("Could not activate the display link");
         mDisplayLink = nullptr;
@@ -494,6 +487,13 @@ protected:
       return mDisplayLink != nullptr;
     }
 
+    // The vsync timestamps given by the CVDisplayLinkCallback are
+    // in the future for the NEXT frame. Large parts of Gecko, such
+    // as animations assume a timestamp at either now or in the past.
+    // Normalize the timestamps given to the VsyncDispatchers to the vsync
+    // that just occured, not the vsync that is upcoming.
+    TimeStamp mPreviousTimestamp;
+
   private:
     // Manages the display link render thread
     CVDisplayLinkRef   mDisplayLink;
@@ -507,6 +507,26 @@ private:
   OSXDisplay mGlobalDisplay;
 }; // OSXVsyncSource
 
+static CVReturn VsyncCallback(CVDisplayLinkRef aDisplayLink,
+                              const CVTimeStamp* aNow,
+                              const CVTimeStamp* aOutputTime,
+                              CVOptionFlags aFlagsIn,
+                              CVOptionFlags* aFlagsOut,
+                              void* aDisplayLinkContext)
+{
+  // Executed on OS X hardware vsync thread
+  OSXVsyncSource::OSXDisplay* display = (OSXVsyncSource::OSXDisplay*) aDisplayLinkContext;
+  int64_t nextVsyncTimestamp = aOutputTime->hostTime;
+  mozilla::TimeStamp nextVsync = mozilla::TimeStamp::FromSystemTime(nextVsyncTimestamp);
+
+  mozilla::TimeStamp previousVsync = display->mPreviousTimestamp;
+  display->mPreviousTimestamp = nextVsync;
+  MOZ_ASSERT(TimeStamp::Now() > previousVsync);
+
+  display->NotifyVsync(previousVsync);
+  return kCVReturnSuccess;
+}
+
 already_AddRefed<mozilla::gfx::VsyncSource>
 gfxPlatformMac::CreateHardwareVsyncSource()
 {

widget/VsyncDispatcher.h

@@ -40,11 +40,6 @@ public:
   CompositorVsyncDispatcher();
 
   // Called on the vsync thread when a hardware vsync occurs
-  // The aVsyncTimestamp can mean different things depending on the platform:
-  // b2g - The vsync timestamp of the previous frame that was just displayed
-  // OSX - The vsync timestamp of the upcoming frame
-  // TODO: Windows / Linux. DOCUMENT THIS WHEN IMPLEMENTING ON THOSE PLATFORMS
-  // Android: TODO
   void NotifyVsync(TimeStamp aVsyncTimestamp);
 
   // Compositor vsync observers must be added/removed on the compositor thread