zero-copy: Clarify to allocate/destroy GpuMemoryBuffer on any thread and use it on the main thread o

content/common/gpu/gpu_channel_manager.cc

 // Copyright (c) 2012 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 "content/common/gpu/gpu_channel_manager.h"
 
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "content/common/gpu/gpu_channel.h"
 #include "content/common/gpu/gpu_memory_buffer_factory.h"
@@ skipping 41 matching lines @@
     IPC_BEGIN_MESSAGE_MAP(GpuChannelManagerMessageFilter, message)
       IPC_MESSAGE_HANDLER(GpuMsg_CreateGpuMemoryBuffer, OnCreateGpuMemoryBuffer)
       IPC_MESSAGE_UNHANDLED(handled = false)
     IPC_END_MESSAGE_MAP()
     return handled;
   }
 
  protected:
   ~GpuChannelManagerMessageFilter() override {}
 
+  // The UI thread in the browser compositor must remain fast at all times.
+  // Bouncing off only GPU IO is the way to ensure that.

[no sievers, 2015/04/08 18:51:35]

nit: What is this exactly trying to say? That we are handling this on the IO
thread directly, since it's a synchronous method (and we don't want to block the
client for too long)?

[dshwang, 2015/04/09 04:06:18]

Main thread also can handle synchronous message while it requires IO->Main->IO
bouncing. I tried to move GpuMsg_CreateGpuMemoryBuffer handling to main thread
in patch set 1, but alexst@ rejected due to above reason.

I changed the comment to be more understandable.

  // GPU IO thread bounces off GpuMsg_CreateGpuMemoryBuffer message, because
  // the UI thread in the browser process must remain fast at all times.

   void OnCreateGpuMemoryBuffer(
       const GpuMsg_CreateGpuMemoryBuffer_Params& params) {
     TRACE_EVENT2("gpu",
                  "GpuChannelManagerMessageFilter::OnCreateGpuMemoryBuffer",
                  "id", params.id, "client_id", params.client_id);
     sender_->Send(new GpuHostMsg_GpuMemoryBufferCreated(
         gpu_memory_buffer_factory_->CreateGpuMemoryBuffer(
             params.id, params.size, params.format, params.usage,
             params.client_id, params.surface_handle)));
   }
@@ skipping 291 matching lines @@
     OnResourcesRelinquished();
 #endif
   }
 }
 
 void GpuChannelManager::OnResourcesRelinquished() {
   Send(new GpuHostMsg_ResourcesRelinquished());
 }
 
 }  // namespace content