gpu: Add EGL fence support.

ui/gl/gl_surface_egl.cc

Index: ui/gl/gl_surface_egl.cc
diff --git a/ui/gl/gl_surface_egl.cc b/ui/gl/gl_surface_egl.cc
index 3b81daf83285a476768cb724f7facde9bbca8950..d8783f40efc83a7eaaf392170a7d8d6c9b2550f7 100644
--- a/ui/gl/gl_surface_egl.cc
+++ b/ui/gl/gl_surface_egl.cc
@@ -8,6 +8,7 @@
 #include <android/native_window_jni.h>
 #endif
 
+#include "base/debug/trace_event.h"
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/message_loop.h"
@@ -221,7 +222,8 @@ NativeViewGLSurfaceEGL::NativeViewGLSurfaceEGL(bool software,
     : window_(window),
       surface_(NULL),
       supports_post_sub_buffer_(false),
-      config_(NULL) {
+      config_(NULL),
+      max_frame_latency_(0) {
   software_ = software;
 #if defined(OS_ANDROID)
   if (window)
@@ -368,12 +370,33 @@ bool NativeViewGLSurfaceEGL::IsOffscreen() {
 }
 
 bool NativeViewGLSurfaceEGL::SwapBuffers() {
+  // Create a fence for this frame.
+  if (max_frame_latency_ > 0) {
+    DCHECK_LE(frame_fences_.size(), max_frame_latency_ - 1);
+    EGLSyncKHR fence = eglCreateSyncKHR(GetDisplay(), EGL_SYNC_FENCE_KHR, NULL);

[Sami, 2013/04/24 10:44:18]

I remember one driver where sync object creation was expensive. Might be worth
taking a quick trace to see how long this takes.

[epennerAtGoogle, 2013/04/24 19:33:38]

Will do.

+    frame_fences_.push_back(fence);
+  }
+
   if (!eglSwapBuffers(GetDisplay(), surface_)) {
     DVLOG(1) << "eglSwapBuffers failed with error "
              << GetLastEGLErrorString();
     return false;
   }
 
+  // SwapBuffers above will block above according to the driver's
+  // maximum latency. We now block again according to our own.
+  DCHECK_LE(frame_fences_.size(), max_frame_latency_);
+  if (frame_fences_.size() == max_frame_latency_) {
+    TRACE_EVENT0("cc", "NativeViewGLSurfaceEGL eglClientWaitSyncKHR");
+    // We shouldn't need the flush flag, as SwapBuffers
+    // always triggers a flush.
+    EGLint flags = 0; //EGL_SYNC_FLUSH_COMMANDS_BIT_KHR;
+    EGLTimeKHR time = EGL_FOREVER_KHR;
+    EGLSyncKHR fence = frame_fences_.front();
+    eglClientWaitSyncKHR(GetDisplay(), fence, flags, time);

[Sami, 2013/04/24 10:44:18]

Call eglDestroySyncKHR after waiting here to avoid leaking the fence.

For extra paranoia please also add a DCHECK() for the result of the wait.

[epennerAtGoogle, 2013/04/24 19:33:38]

Oops! Thanks!

+    frame_fences_.pop_front();
+  }
+
   return true;
 }
 
@@ -439,7 +462,18 @@ VSyncProvider* NativeViewGLSurfaceEGL::GetVSyncProvider() {
   return vsync_provider_.get();
 }
 
+void NativeViewGLSurfaceEGL::SetMaximumFrameLatency(unsigned int frames) {
+  while (frame_fences_.size() > frames) {
+    eglDestroySyncKHR(GetDisplay(), frame_fences_.front());
+    frame_fences_.pop_front();
+  }
+  max_frame_latency_ = frames;
+}
+
 NativeViewGLSurfaceEGL::~NativeViewGLSurfaceEGL() {
+  SetMaximumFrameLatency(0);
+  DCHECK(!frame_fences_.size());
+
   Destroy();
 #if defined(OS_ANDROID)
   if (window_)