Perform glReadPixels with PBOs in the gpu, if PBOs are available.

gpu/command_buffer/service/feature_info.cc

Index: gpu/command_buffer/service/feature_info.cc
diff --git a/gpu/command_buffer/service/feature_info.cc b/gpu/command_buffer/service/feature_info.cc
index 5d9584827d4fe4d36f1259d9877707d6c7f99fed..4dc3545da17dab414091cdcd0da558850b04651e 100644
--- a/gpu/command_buffer/service/feature_info.cc
+++ b/gpu/command_buffer/service/feature_info.cc
@@ -117,7 +117,8 @@ FeatureInfo::FeatureFlags::FeatureFlags()
       enable_shader_name_hashing(false),
       enable_samplers(false),
       ext_draw_buffers(false),
-      ext_frag_depth(false) {
+      ext_frag_depth(false),
+      use_async_readpixels(false) {
 }
 
 FeatureInfo::Workarounds::Workarounds() :
@@ -623,6 +624,17 @@ void FeatureInfo::AddFeatures(const CommandLine& command_line) {
     feature_flags_.ext_frag_depth = true;
   }
 
+  bool ui_gl_fence_works =
+      extensions.Contains("GL_NV_fence") ||
+      extensions.Contains("GL_ARB_sync");
+
+  if (ui_gl_fence_works &&
+      extensions.Contains("GL_ARB_pixel_buffer_object") &&
+      !command_line.HasSwitch(switches::kForceSynchronousGLReadPixels) &&

[piman, 2013/06/26 22:17:08]

You don't need this any more. You have
--gpu-driver-bug-workarounds='disable_async_readpixels'

Also, looking at the origin of this flag (in
content/browser/renderer_host/compositing_iosurface_mac.mm), it sounds like you
need a blacklisting entry on intel drivers on older macs, where PBOs are exposed
but crash - I think you need to add it to
gpu/config/gpu_driver_bug_list_json.cc. zmo@ can probably help you.

[hubbe, 2013/06/28 22:17:49]

Removed the HasSwitch call.
I'm trying to find out if the workaround is strictly needed. We did some
investigating, and it would seem that the code that had a problem didn't
actually check if the driver claimed to support PBOs or now. I need to talk to
alpha (who wrote the workaround) and figure out if that particular mac/driver
combo actually lists PBOs as supported or not.

[hubbe, 2013/06/29 00:13:56]

Talked to alpha, and it seems that the workaround is indeed needed, so I went
ahead and added it. Not sure if anybody around here has a mac like that to test
it on though...

+      !workarounds_.disable_async_readpixels) {
+    feature_flags_.use_async_readpixels = true;
+  }
+
   if (!disallowed_features_.swap_buffer_complete_callback)
     AddExtensionString("GL_CHROMIUM_swapbuffers_complete_callback");