GPU process clients can perform a "shallow" finish.

cc/output/gl_renderer.cc

 // Copyright 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/output/gl_renderer.h"
 
 #include <algorithm>
 #include <limits>
 #include <set>
 #include <string>
@@ skipping 1994 matching lines @@
         1.f,
         1.f));
   }
 
   SetShaderOpacity(1.f, program->fragment_shader().alpha_location());
 
   GLC(Context(), Context()->bindTexture(GL_TEXTURE_2D, texture_id));
   DrawQuadGeometry(
       frame, draw_matrix, rect, program->vertex_shader().matrix_location());
 }
-
-void GLRenderer::Finish() {
-  TRACE_EVENT0("cc", "GLRenderer::finish");
-  context_->finish();
-}
+//
+//void GLRenderer::Finish() {
+//  TRACE_EVENT0("cc", "GLRenderer::finish");
+//  context_->finish();
+//}
 
 void GLRenderer::SwapBuffers() {
   DCHECK(visible_);
   DCHECK(!is_backbuffer_discarded_);
 
   TRACE_EVENT0("cc", "GLRenderer::SwapBuffers");
   // We're done! Time to swapbuffers!
 
   CompositorFrame compositor_frame;
   compositor_frame.metadata = client_->MakeCompositorFrameMetadata();
@@ skipping 306 matching lines @@
   // Save the buffer to verify the callbacks happen in the expected order.
   pending_async_read_pixels_.front()->buffer = buffer;
 
   if (is_async) {
     unsigned sync_point = context_->insertSyncPoint();
     SyncPointHelper::SignalSyncPoint(
         context_,
         sync_point,
         finished_callback);
   } else {
-    resource_provider_->Finish();
+    // Can this be a flush?

[piman, 2013/06/21 02:31:10]

It can't because we're waiting on the the ReadPixels to be done on the GPU
process side (it's async when GL_PIXEL_PACK_TRANSFER_BUFFER_CHROMIUM != 0).
In the current version, it can be made a shallow flush.

In the future though, we want to have an async readpixels on the GPU process
side (using PBOs), which will need a sort of finish to complete synchronously,
or something.

+    // resource_provider_->Finish();
     finished_callback.Run();
   }
 
   EnforceMemoryPolicy();
 }
 
 void GLRenderer::FinishedReadback(
     const AsyncGetFramebufferPixelsCleanupCallback& cleanup_callback,
     unsigned source_buffer,
     uint8* dest_pixels,
@@ skipping 672 matching lines @@
   // The Skia GPU backend requires a stencil buffer.  See ReinitializeGrCanvas
   // implementation.
   return gr_context_ && context_->getContextAttributes().stencil;
 }
 
 bool GLRenderer::IsContextLost() {
   return (context_->getGraphicsResetStatusARB() != GL_NO_ERROR);
 }
 
 }  // namespace cc