diff --git a/include/gui/SurfaceTextureClient.h b/include/gui/SurfaceTextureClient.h
index c0e90b3..6644751 100644
--- a/include/gui/SurfaceTextureClient.h
+++ b/include/gui/SurfaceTextureClient.h
@@ -26,6 +26,7 @@
 
 #include <utils/RefBase.h>
 #include <utils/threads.h>
+#include <utils/KeyedVector.h>
 
 struct ANativeWindow_Buffer;
 
@@ -113,6 +114,11 @@
     void freeAllBuffers();
     int getSlotFromBufferLocked(android_native_buffer_t* buffer) const;
 
+    struct BufferSlot {
+        sp<GraphicBuffer> buffer;
+        Region dirtyRegion;
+    };
+
     // mSurfaceTexture is the interface to the surface texture server. All
     // operations on the surface texture client ultimately translate into
     // interactions with the server using this interface.
@@ -124,7 +130,7 @@
     // slot that has not yet been used. The buffer allocated to a slot will also
     // be replaced if the requested buffer usage or geometry differs from that
     // of the buffer allocated to a slot.
-    sp<GraphicBuffer> mSlots[NUM_BUFFER_SLOTS];
+    BufferSlot mSlots[NUM_BUFFER_SLOTS];
 
     // mReqWidth is the buffer width that will be requested at the next dequeue
     // operation. It is initialized to 1.
@@ -189,8 +195,10 @@
     // must be used from the lock/unlock thread
     sp<GraphicBuffer>           mLockedBuffer;
     sp<GraphicBuffer>           mPostedBuffer;
-    mutable Region              mOldDirtyRegion;
     bool                        mConnectedToCpu;
+
+    // must be accessed from lock/unlock thread only
+    Region mDirtyRegion;
 };
 
 }; // namespace android
diff --git a/libs/gui/SurfaceTextureClient.cpp b/libs/gui/SurfaceTextureClient.cpp
index 99c025f..b37d821 100644
--- a/libs/gui/SurfaceTextureClient.cpp
+++ b/libs/gui/SurfaceTextureClient.cpp
@@ -171,7 +171,7 @@
              result);
         return result;
     }
-    sp<GraphicBuffer>& gbuf(mSlots[buf]);
+    sp<GraphicBuffer>& gbuf(mSlots[buf].buffer);
     if (result & ISurfaceTexture::RELEASE_ALL_BUFFERS) {
         freeAllBuffers();
     }
@@ -204,7 +204,8 @@
         android_native_buffer_t* buffer) const {
     bool dumpedState = false;
     for (int i = 0; i < NUM_BUFFER_SLOTS; i++) {
-        if (mSlots[i] != NULL && mSlots[i]->handle == buffer->handle) {
+        if (mSlots[i].buffer != NULL &&
+                mSlots[i].buffer->handle == buffer->handle) {
             return i;
         }
     }
@@ -586,7 +587,7 @@
 
 void SurfaceTextureClient::freeAllBuffers() {
     for (int i = 0; i < NUM_BUFFER_SLOTS; i++) {
-        mSlots[i] = 0;
+        mSlots[i].buffer = 0;
     }
 }
 
@@ -691,19 +692,32 @@
 
             if (canCopyBack) {
                 // copy the area that is invalid and not repainted this round
-                const Region copyback(mOldDirtyRegion.subtract(newDirtyRegion));
+                const Region copyback(mDirtyRegion.subtract(newDirtyRegion));
                 if (!copyback.isEmpty())
                     copyBlt(backBuffer, frontBuffer, copyback);
             } else {
                 // if we can't copy-back anything, modify the user's dirty
                 // region to make sure they redraw the whole buffer
                 newDirtyRegion.set(bounds);
+                mDirtyRegion.clear();
+                Mutex::Autolock lock(mMutex);
+                for (size_t i=0 ; i<NUM_BUFFER_SLOTS ; i++) {
+                    mSlots[i].dirtyRegion.clear();
+                }
             }
 
-            // keep track of the are of the buffer that is "clean"
-            // (ie: that will be redrawn)
-            mOldDirtyRegion = newDirtyRegion;
 
+            { // scope for the lock
+                Mutex::Autolock lock(mMutex);
+                int backBufferSlot(getSlotFromBufferLocked(backBuffer.get()));
+                if (backBufferSlot >= 0) {
+                    Region& dirtyRegion(mSlots[backBufferSlot].dirtyRegion);
+                    mDirtyRegion.subtract(dirtyRegion);
+                    dirtyRegion = newDirtyRegion;
+                }
+            }
+
+            mDirtyRegion.orSelf(newDirtyRegion);
             if (inOutDirtyBounds) {
                 *inOutDirtyBounds = newDirtyRegion.getBounds();
             }
diff --git a/services/surfaceflinger/SurfaceTextureLayer.cpp b/services/surfaceflinger/SurfaceTextureLayer.cpp
index d7fd39e..2b647bb 100644
--- a/services/surfaceflinger/SurfaceTextureLayer.cpp
+++ b/services/surfaceflinger/SurfaceTextureLayer.cpp
@@ -40,10 +40,6 @@
             outWidth, outHeight, outTransform);
     if (err == NO_ERROR) {
         switch(api) {
-            case NATIVE_WINDOW_API_CPU:
-                // SurfaceTextureClient supports only 2 buffers for CPU connections
-                this->setBufferCountServer(2);
-                break;
             case NATIVE_WINDOW_API_MEDIA:
             case NATIVE_WINDOW_API_CAMERA:
                 // Camera preview and videos are rate-limited on the producer