Enable backpressure for BufferStateLayer

The default behaviour of buffer state layer is to drop older
buffers if there are newer buffers that are ready to be presented.

When emulating BufferQueue behavior via the adapter, we want
to queue up buffers without any present timestamps. To solve this,
we introduce a layer state flag to keep the buffer in the transaction
queue if there is already a buffer that is ready to be applied.

Test: atest SurfaceViewBufferTests:BufferPresentationTests
Bug: 176967609
Change-Id: I33f6347bd1c7a2d80dc4214e596bb864abe8c6bf

1 files changed, 17 insertions, 4 deletions

diff --git a/libs/gui/BLASTBufferQueue.cpp b/libs/gui/BLASTBufferQueue.cpp
index 0a3d44d336..490495596b 100644
--- a/libs/gui/BLASTBufferQueue.cpp
+++ b/libs/gui/BLASTBufferQueue.cpp
@@ -146,6 +146,10 @@ BLASTBufferQueue::BLASTBufferQueue(const std::string& name, const sp<SurfaceCont
 
     mTransformHint = mSurfaceControl->getTransformHint();
     mBufferItemConsumer->setTransformHint(mTransformHint);
+    SurfaceComposerClient::Transaction()
+            .setFlags(surface, layer_state_t::eEnableBackpressure,
+                      layer_state_t::eEnableBackpressure)
+            .apply();
 
     mNumAcquired = 0;
     mNumFrameAvailable = 0;
@@ -169,13 +173,20 @@ BLASTBufferQueue::~BLASTBufferQueue() {
 void BLASTBufferQueue::update(const sp<SurfaceControl>& surface, uint32_t width, uint32_t height,
                               int32_t format) {
     std::unique_lock _lock{mMutex};
-    mSurfaceControl = surface;
-
     if (mFormat != format) {
         mFormat = format;
         mBufferItemConsumer->setDefaultBufferFormat(format);
     }
 
+    SurfaceComposerClient::Transaction t;
+    bool applyTransaction = false;
+    if (!SurfaceControl::isSameSurface(mSurfaceControl, surface)) {
+        mSurfaceControl = surface;
+        t.setFlags(mSurfaceControl, layer_state_t::eEnableBackpressure,
+                   layer_state_t::eEnableBackpressure);
+        applyTransaction = true;
+    }
+
     ui::Size newSize(width, height);
     if (mRequestedSize != newSize) {
         mRequestedSize.set(newSize);
@@ -184,13 +195,15 @@ void BLASTBufferQueue::update(const sp<SurfaceControl>& surface, uint32_t width,
             // If the buffer supports scaling, update the frame immediately since the client may
             // want to scale the existing buffer to the new size.
             mSize = mRequestedSize;
-            SurfaceComposerClient::Transaction t;
             t.setFrame(mSurfaceControl,
                        {0, 0, static_cast<int32_t>(mSize.width),
                         static_cast<int32_t>(mSize.height)});
-            t.apply();
+            applyTransaction = true;
         }
     }
+    if (applyTransaction) {
+        t.apply();
+    }
 }
 
 static void transactionCallbackThunk(void* context, nsecs_t latchTime,