surfaceflinger: skip composition for empty frames

By not committing the results of composition for empty frames, we
avoid spitting out series of black frames for virtual displays that
don't have visible layers. We still draw one black frame when going
from having layers to not having any. In particular, this avoids
having a series of empty frames due to re-compositing the primary
display in the period between creating the virtual display and adding
layers to it.

Bug: 16786752
Change-Id: I7e9b2ed2e407d8d49c7af736b447d4c6181b0ad8

3 files changed, 28 insertions, 3 deletions

diff --git a/services/surfaceflinger/DisplayDevice.cpp b/services/surfaceflinger/DisplayDevice.cpp
index e5ecf07..88e0dd7 100644
--- a/services/surfaceflinger/DisplayDevice.cpp
+++ b/services/surfaceflinger/DisplayDevice.cpp
@@ -59,7 +59,8 @@ DisplayDevice::DisplayDevice(
         const sp<DisplaySurface>& displaySurface,
         const sp<IGraphicBufferProducer>& producer,
         EGLConfig config)
-    : mFlinger(flinger),
+    : lastCompositionHadVisibleLayers(false),
+      mFlinger(flinger),
       mType(type), mHwcDisplayId(hwcId),
       mDisplayToken(displayToken),
       mDisplaySurface(displaySurface),
diff --git a/services/surfaceflinger/DisplayDevice.h b/services/surfaceflinger/DisplayDevice.h
index f750c6c..620e598 100644
--- a/services/surfaceflinger/DisplayDevice.h
+++ b/services/surfaceflinger/DisplayDevice.h
@@ -53,6 +53,7 @@ public:
     mutable Region swapRegion;
     // region in screen space
     Region undefinedRegion;
+    bool lastCompositionHadVisibleLayers;
 
     enum DisplayType {
         DISPLAY_ID_INVALID = -1,
diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index 08cc506..13a63c4 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -881,9 +881,32 @@ void SurfaceFlinger::rebuildLayerStacks() {
 
 void SurfaceFlinger::setUpHWComposer() {
     for (size_t dpy=0 ; dpy<mDisplays.size() ; dpy++) {
-        bool mustRecompose =
-                !(mDisplays[dpy]->getDirtyRegion(false).isEmpty());
+        bool dirty = !mDisplays[dpy]->getDirtyRegion(false).isEmpty();
+        bool empty = mDisplays[dpy]->getVisibleLayersSortedByZ().size() == 0;
+        bool wasEmpty = !mDisplays[dpy]->lastCompositionHadVisibleLayers;
+
+        // If nothing has changed (!dirty), don't recompose.
+        // If something changed, but we don't currently have any visible layers,
+        //   and didn't when we last did a composition, then skip it this time.
+        // The second rule does two things:
+        // - When all layers are removed from a display, we'll emit one black
+        //   frame, then nothing more until we get new layers.
+        // - When a display is created with a private layer stack, we won't
+        //   emit any black frames until a layer is added to the layer stack.
+        bool mustRecompose = dirty && !(empty && wasEmpty);
+
+        ALOGV_IF(mDisplays[dpy]->getDisplayType() == DisplayDevice::DISPLAY_VIRTUAL,
+                "dpy[%zu]: %s composition (%sdirty %sempty %swasEmpty)", dpy,
+                mustRecompose ? "doing" : "skipping",
+                dirty ? "+" : "-",
+                empty ? "+" : "-",
+                wasEmpty ? "+" : "-");
+
         mDisplays[dpy]->beginFrame(mustRecompose);
+
+        if (mustRecompose) {
+            mDisplays[dpy]->lastCompositionHadVisibleLayers = !empty;
+        }
     }
 
     HWComposer& hwc(getHwComposer());