am b7702c5c: Merge "BufferQueueProducer: Throttle EGL frame production." into lmp-mr1-dev

* commit 'b7702c5ce7f010fa5f19646fd79f28ff04e7014b':
  BufferQueueProducer: Throttle EGL frame production.

include/gui/BufferQueueProducer.h

@@ -197,6 +197,12 @@ private:
 
     uint32_t mStickyTransform;
 
+    // This saves the fence from the last queueBuffer, such that the
+    // next queueBuffer call can throttle buffer production. The prior
+    // queueBuffer's fence is not nessessarily available elsewhere,
+    // since the previous buffer might have already been acquired.
+    sp<Fence> mLastQueueBufferFence;
+
 }; // class BufferQueueProducer
 
 } // namespace android

libs/gui/BufferQueueProducer.cpp

@@ -38,7 +38,8 @@ BufferQueueProducer::BufferQueueProducer(const sp<BufferQueueCore>& core) :
     mCore(core),
     mSlots(core->mSlots),
     mConsumerName(),
-    mStickyTransform(0) {}
+    mStickyTransform(0),
+    mLastQueueBufferFence(Fence::NO_FENCE) {}
 
 BufferQueueProducer::~BufferQueueProducer() {}
 
@@ -644,6 +645,15 @@ status_t BufferQueueProducer::queueBuffer(int slot,
         ATRACE_INT(mCore->mConsumerName.string(), mCore->mQueue.size());
     } // Autolock scope
 
+    // Wait without lock held
+    if (mCore->mConnectedApi == NATIVE_WINDOW_API_EGL) {
+        // Waiting here allows for two full buffers to be queued but not a
+        // third. In the event that frames take varying time, this makes a
+        // small trade-off in favor of latency rather than throughput.
+        mLastQueueBufferFence->waitForever("Throttling EGL Production");
+        mLastQueueBufferFence = fence;
+    }
+
     // Call back without lock held
     if (listener != NULL) {
         listener->onFrameAvailable();