Force GPU switch with CGLSetVirtualScreen only for compositor

ui/gl/gl_context_cgl.cc

Index: ui/gl/gl_context_cgl.cc
diff --git a/ui/gl/gl_context_cgl.cc b/ui/gl/gl_context_cgl.cc
index 7d5a92317cd91d87baaa6bcd59a57709f3c0d666..f88f9c940951b9ed3c8f9147e9d79e0d15c4c313 100644
--- a/ui/gl/gl_context_cgl.cc
+++ b/ui/gl/gl_context_cgl.cc
@@ -19,6 +19,8 @@ namespace gfx {
 
 bool g_support_renderer_switching;
 
+bool g_force_cgl_set_virtual_screen = false;
+
 static CGLPixelFormatObj GetPixelFormat() {
   static CGLPixelFormatObj format;
   if (format)
@@ -149,7 +151,10 @@ bool GLContextCGL::MakeCurrent(GLSurface* surface) {
 
       screen_renderer_id &= kCGLRendererIDMatchingMask;
       if (screen_renderer_id == renderer_id) {
-        CGLSetVirtualScreen(static_cast<CGLContextObj>(context_), i);
+        // This call can hang on some AMD drivers
+        // http://crbug.com/227228
+        if (g_force_cgl_set_virtual_screen)

[jbauman, 2013/04/06 01:52:02]

I'd recommend moving this check up to the beginning of the block (next to
g_support_renderer_switching), to avoid doing unnecessary
CGLDescribePixelFormats if it's disabled.

[ccameron, 2013/04/08 17:25:14]

I put the whole thing in an if-block. My initial inclination was to maintain
valid values for screen_ and renderer_id_ in case they were used elsewhere, but
this is indeed the only place where they are used.

+          CGLSetVirtualScreen(static_cast<CGLContextObj>(context_), i);
         screen_ = i;
         break;
       }
@@ -267,6 +272,11 @@ bool GLContextCGL::GetTotalGpuMemory(size_t* bytes) {
   return false;
 }
 
+// static
+void GLContextCGL::ForceCGLSetVirtualScreen() {
+  g_force_cgl_set_virtual_screen = true;
+}
+
 GLContextCGL::~GLContextCGL() {
   Destroy();
 }