diff --git a/services/surfaceflinger/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index b41f414..61fab81 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -646,7 +646,7 @@
 
     // Toggles hardware VSYNC by calling into HWC.
     // TODO(b/241286146): Rename for self-explanatory API.
-    void setVsyncEnabled(bool) override;
+    void setVsyncEnabled(PhysicalDisplayId, bool) override;
     void requestDisplayModes(std::vector<display::DisplayModeRequest>) override;
     void kernelTimerChanged(bool expired) override;
     void triggerOnFrameRateOverridesChanged() override;
@@ -1132,7 +1132,15 @@
     pid_t mPid;
     std::future<void> mRenderEnginePrimeCacheFuture;
 
-    // access must be protected by mStateLock
+    // mStateLock has conventions related to the current thread, because only
+    // the main thread should modify variables protected by mStateLock.
+    // - read access from a non-main thread must lock mStateLock, since the main
+    // thread may modify these variables.
+    // - write access from a non-main thread is not permitted.
+    // - read access from the main thread can use an ftl::FakeGuard, since other
+    // threads must not modify these variables.
+    // - write access from the main thread must lock mStateLock, since another
+    // thread may be reading these variables.
     mutable Mutex mStateLock;
     State mCurrentState{LayerVector::StateSet::Current};
     std::atomic<int32_t> mTransactionFlags = 0;