Add a backdoor to change FrameTimeline's maxDisplayFrames

This change adds a backdoor entry through SurfaceFlinger to allow
modifying the max number of display frames stored in FrameTimeline. This
is particularly useful for debugging, enabling to take a trace as well
see the dump on the side.

Bug: 169894383
Test: libsurfaceflinger_unittest
Change-Id: I872e240597193d597dac4a9f096b6360edf5cce1
diff --git a/services/surfaceflinger/FrameTimeline/FrameTimeline.cpp b/services/surfaceflinger/FrameTimeline/FrameTimeline.cpp
index 22d9d10..43176a3 100644
--- a/services/surfaceflinger/FrameTimeline/FrameTimeline.cpp
+++ b/services/surfaceflinger/FrameTimeline/FrameTimeline.cpp
@@ -285,7 +285,9 @@
     dumpTable(result, mPredictions, mActuals, indent, mPredictionState, baseTime);
 }
 
-FrameTimeline::FrameTimeline() : mCurrentDisplayFrame(std::make_shared<DisplayFrame>()) {}
+FrameTimeline::FrameTimeline()
+      : mCurrentDisplayFrame(std::make_shared<DisplayFrame>()),
+        mMaxDisplayFrames(kDefaultMaxDisplayFrames) {}
 
 FrameTimeline::DisplayFrame::DisplayFrame()
       : surfaceFlingerPredictions(TimelineItem()),
@@ -438,7 +440,7 @@
 }
 
 void FrameTimeline::finalizeCurrentDisplayFrame() {
-    while (mDisplayFrames.size() >= kMaxDisplayFrames) {
+    while (mDisplayFrames.size() >= mMaxDisplayFrames) {
         // We maintain only a fixed number of frames' data. Pop older frames
         mDisplayFrames.pop_front();
     }
@@ -530,4 +532,17 @@
     }
 }
 
+void FrameTimeline::setMaxDisplayFrames(uint32_t size) {
+    std::lock_guard<std::mutex> lock(mMutex);
+
+    // The size can either increase or decrease, clear everything, to be consistent
+    mDisplayFrames.clear();
+    mPendingPresentFences.clear();
+    mMaxDisplayFrames = size;
+}
+
+void FrameTimeline::reset() {
+    setMaxDisplayFrames(kDefaultMaxDisplayFrames);
+}
+
 } // namespace android::frametimeline::impl