DisplayEventDispatcher: optimize binder calls

Use DisplayEventReciever::setVsyncRate instead of
DisplayEventReciever::requestNextVsync to accommodate a one-time
registration to vsync events when the app registers back to back
callbacks. With this approach we can save all the binder calls during
a running animation.

Test: run TouchLatency test app and observe systrace
Bug: 162096692
Change-Id: Ib415f2aade40a3d23a2b4c77e5746e5e13666d23

1 files changed, 49 insertions, 21 deletions

diff --git a/libs/gui/DisplayEventDispatcher.cpp b/libs/gui/DisplayEventDispatcher.cpp
index c6c9a8f7a7..2ad484add3 100644
--- a/libs/gui/DisplayEventDispatcher.cpp
+++ b/libs/gui/DisplayEventDispatcher.cpp
@@ -36,7 +36,9 @@ static const size_t EVENT_BUFFER_SIZE = 100;
 DisplayEventDispatcher::DisplayEventDispatcher(
         const sp<Looper>& looper, ISurfaceComposer::VsyncSource vsyncSource,
         ISurfaceComposer::EventRegistrationFlags eventRegistration)
-      : mLooper(looper), mReceiver(vsyncSource, eventRegistration), mWaitingForVsync(false) {
+      : mLooper(looper),
+        mReceiver(vsyncSource, eventRegistration),
+        mVsyncState(VsyncState::Unregistered) {
     ALOGV("dispatcher %p ~ Initializing display event dispatcher.", this);
 }
 
@@ -66,26 +68,37 @@ void DisplayEventDispatcher::dispose() {
 }
 
 status_t DisplayEventDispatcher::scheduleVsync() {
-    if (!mWaitingForVsync) {
-        ALOGV("dispatcher %p ~ Scheduling vsync.", this);
-
-        // Drain all pending events.
-        nsecs_t vsyncTimestamp;
-        PhysicalDisplayId vsyncDisplayId;
-        uint32_t vsyncCount;
-        VsyncEventData vsyncEventData;
-        if (processPendingEvents(&vsyncTimestamp, &vsyncDisplayId, &vsyncCount, &vsyncEventData)) {
-            ALOGE("dispatcher %p ~ last event processed while scheduling was for %" PRId64 "", this,
-                  ns2ms(static_cast<nsecs_t>(vsyncTimestamp)));
-        }
+    switch (mVsyncState) {
+        case VsyncState::Unregistered: {
+            ALOGV("dispatcher %p ~ Scheduling vsync.", this);
+
+            // Drain all pending events.
+            nsecs_t vsyncTimestamp;
+            PhysicalDisplayId vsyncDisplayId;
+            uint32_t vsyncCount;
+            VsyncEventData vsyncEventData;
+            if (processPendingEvents(&vsyncTimestamp, &vsyncDisplayId, &vsyncCount,
+                                     &vsyncEventData)) {
+                ALOGE("dispatcher %p ~ last event processed while scheduling was for %" PRId64 "",
+                      this, ns2ms(static_cast<nsecs_t>(vsyncTimestamp)));
+            }
 
-        status_t status = mReceiver.requestNextVsync();
-        if (status) {
-            ALOGW("Failed to request next vsync, status=%d", status);
-            return status;
-        }
+            status_t status = mReceiver.setVsyncRate(1);
+            if (status) {
+                ALOGW("Failed to set vsync rate, status=%d", status);
+                return status;
+            }
 
-        mWaitingForVsync = true;
+            mVsyncState = VsyncState::RegisteredAndWaitingForVsync;
+            break;
+        }
+        case VsyncState::Registered: {
+            mVsyncState = VsyncState::RegisteredAndWaitingForVsync;
+            break;
+        }
+        case VsyncState::RegisteredAndWaitingForVsync: {
+            break;
+        }
     }
     return OK;
 }
@@ -123,8 +136,23 @@ int DisplayEventDispatcher::handleEvent(int, int events, void*) {
               ", displayId=%s, count=%d, vsyncId=%" PRId64,
               this, ns2ms(vsyncTimestamp), to_string(vsyncDisplayId).c_str(), vsyncCount,
               vsyncEventData.id);
-        mWaitingForVsync = false;
-        dispatchVsync(vsyncTimestamp, vsyncDisplayId, vsyncCount, vsyncEventData);
+        switch (mVsyncState) {
+            case VsyncState::Unregistered:
+                ALOGW("Received unexpected VSYNC event");
+                break;
+            case VsyncState::RegisteredAndWaitingForVsync:
+                mVsyncState = VsyncState::Registered;
+                dispatchVsync(vsyncTimestamp, vsyncDisplayId, vsyncCount, vsyncEventData);
+                break;
+            case VsyncState::Registered:
+                status_t status = mReceiver.setVsyncRate(0);
+                if (status) {
+                    ALOGW("Failed to reset vsync rate, status=%d", status);
+                    return status;
+                }
+                mVsyncState = VsyncState::Unregistered;
+                break;
+        }
     }
 
     return 1; // keep the callback