aw: Add and use DrawGL kModeSync

android_webview/native/aw_contents.cc

Index: android_webview/native/aw_contents.cc
diff --git a/android_webview/native/aw_contents.cc b/android_webview/native/aw_contents.cc
index 32cc8d1a4de9b85bde117fc05a2975615f2e386b..cc5e6a3cb28a4b77871c6532929bad3e40c8612f 100644
--- a/android_webview/native/aw_contents.cc
+++ b/android_webview/native/aw_contents.cc
@@ -342,6 +342,12 @@ jlong AwContents::GetAwDrawGLViewContext(JNIEnv* env, jobject obj) {
 }
 
 void AwContents::DrawGL(AwDrawGLInfo* draw_info) {
+  if (draw_info->mode == AwDrawGLInfo::kModeSync) {
+    if (hardware_renderer_)
+      hardware_renderer_->CommitFrame();

[hush (inactive), 2014/07/25 18:08:07]

CommitFrame will just copy the child draw result (framedata) to the parent and
recreate the delegated layer in parent if needed.
Then in the next kModeDraw, parent compositor will draw the frame, issue GL
commands, etc.

Do I understand it correctly?

Is the following sequence of events guaranteed?
ondraw --> kModeSync --> kModeDraw

And you always have a 1 to 1 correspondence between kModeSync and kModeDraw,
right?

[boliu, 2014/07/25 18:11:02]

Everything above here is correct.
No. kModeDraw can happen more times compared to kModeSync, it will just keep
drawing the same frame. onDraw and kModeSync are 1-to-1 though.

[hush (inactive), 2014/07/25 18:31:00]

Okay.
Imagine a scenario where onDraw happens every 16ms, but DrawGL is ridiculously
slow.
Without this change, the new frame data that is produced on UI thread just keeps
getting overwritten by the next onDraw because DrawGL is slow and hasn't
finished the previous frame yet. In this case, DrawGL will draw an old frame.

With this change, the parent compositor will always have the most recent
framedata due to kModeSync, so the parent still drop some frames because UI is
producing frames faster than RT can handle, but will always use the most recent
frame when it draws.

Is that right?
On 2014/07/25 18:11:02, boliu wrote:

[boliu, 2014/07/25 18:41:51]

Yes. But "old frame" isn't a good charaterization. It's a two stage pipeline.
Before this change, we advanced the pipeline in DrawGL. So "old frame" just
means the second stage of the pipeline
Can't happen. Remember I said onDraw and kModeSync are 1-to-1. Framework will
block the UI thread after onDraw until kModeSync happens.

+    return;
+  }
+
   {
     GLViewRendererManager* manager = GLViewRendererManager::GetInstance();
     base::AutoLock lock(render_thread_lock_);
@@ -366,6 +372,7 @@ void AwContents::DrawGL(AwDrawGLInfo* draw_info) {
 
   if (!hardware_renderer_) {
     hardware_renderer_.reset(new HardwareRenderer(&shared_renderer_state_));
+    hardware_renderer_->CommitFrame();
   }
 
   hardware_renderer_->DrawGL(state_restore.stencil_enabled(),