From b7a0549c983bef103ce07eb3af5905febc6e538e Mon Sep 17 00:00:00 2001
From: Jesse Hall <jessehall@google.com>
Date: Thu, 14 Aug 2014 15:45:06 -0700
Subject: [PATCH] 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
---
 services/surfaceflinger/DisplayDevice.cpp  |  3 ++-
 services/surfaceflinger/DisplayDevice.h    |  1 +
 services/surfaceflinger/SurfaceFlinger.cpp | 27 ++++++++++++++++++++--
 3 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/services/surfaceflinger/DisplayDevice.cpp b/services/surfaceflinger/DisplayDevice.cpp
index e5ecf0778..88e0dd70e 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 f750c6c82..620e5983c 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 08cc50636..13a63c4df 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());