Migrate WebGL contexts on backgrounded tabs to the integrated GPU.

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 2a105c4983d0fb1ee86e2e75ccf5336afbde3b44..4b05c552a17d92bdb6962406c05308f216e7b764 100644
--- a/ui/gl/gl_context_cgl.cc
+++ b/ui/gl/gl_context_cgl.cc
@@ -114,17 +114,8 @@ bool GLContextCGL::Initialize(GLSurface* compatible_surface,
   // create the context.
   if (!ui::GpuSwitchingManager::GetInstance()->SupportsDualGpus() ||
       gpu_preference == PreferDiscreteGpu) {
-    std::vector<CGLPixelFormatAttribute> discrete_attribs;
-    discrete_attribs.push_back((CGLPixelFormatAttribute) 0);
-    GLint num_pixel_formats;
-    if (CGLChoosePixelFormat(&discrete_attribs.front(),
-                             &discrete_pixelformat_,
-                             &num_pixel_formats) != kCGLNoError) {
-      LOG(ERROR) << "Error choosing pixel format.";
+    if (!AllocateDiscretePixelFormat())
       return false;
-    }
-    // The renderer might be switched after this, so ignore the saved ID.
-    share_group()->SetRendererID(-1);
   }
 
   CGLError res = CGLCreateContext(
@@ -147,6 +138,26 @@ bool GLContextCGL::Initialize(GLSurface* compatible_surface,
   return true;
 }
 
+bool GLContextCGL::AllocateDiscretePixelFormat() {
+  std::vector<CGLPixelFormatAttribute> discrete_attribs;
+  discrete_attribs.push_back((CGLPixelFormatAttribute) 0);
+  GLint num_pixel_formats;
+  if (CGLChoosePixelFormat(&discrete_attribs.front(),
+                           &discrete_pixelformat_,
+                           &num_pixel_formats) != kCGLNoError) {
+    LOG(ERROR) << "Error choosing discrete pixel format.";
+    return false;
+  }
+  // The renderer might be switched after this, so ignore the saved ID.
+  share_group()->SetRendererID(-1);
+  return true;
+}
+
+void GLContextCGL::ReleaseDiscretePixelFormat() {
+  CGLReleasePixelFormat(discrete_pixelformat_);
+  discrete_pixelformat_ = nullptr;
+}
+
 void GLContextCGL::Destroy() {
   if (yuv_to_rgb_converter_) {
     ScopedCGLSetCurrentContext(static_cast<CGLContextObj>(context_));
@@ -293,4 +304,14 @@ GpuPreference GLContextCGL::GetGpuPreference() {
   return gpu_preference_;
 }
 
+void GLContextCGL::SetPerformancePreference(GpuPerformancePreference pref) {
+  if (pref == PreferLowPower) {
+    if (discrete_pixelformat_)
+      ReleaseDiscretePixelFormat();
+  } else {
+    if (gpu_preference_ == PreferDiscreteGpu && !discrete_pixelformat_)
+      AllocateDiscretePixelFormat();

[jbauman, 2017/01/19 01:13:14]

I'm not sure this would actually switch the context onto the discrete GPU. Once
an offscreen context is created it seems like it's stuck to a GPU until
GLContextCGL::ForceGpuSwitchIfNeeded does CGLSetVirtualScreen, and it won't do
that call if discrete_pixelformat_ exists.

See also
https://chromium.googlesource.com/chromium/src/+/97419c0f3a44da99c730fc572011...
for the issues we've had in the past with switching contexts from the integrated
GPU to the discrete GPU.

[Ken Russell (switch to Gerrit), 2017/01/20 04:34:52]

Thanks for tracking down that change. I'd remembered it vaguely but not done the
code archaeology.

This CL definitely causes gfxCardStatus to think that the machine has switched
back to the integrated GPU, regardless of whether we're calling
CGLSetVirtualScreen. I'm not really sure exactly what effect it has and what
resources are being migrated between GPUs. We are hoping to get some more
details from Apple's OpenGL team about how integrated/discrete GPU transitions
work and exactly what is allowed, and needed, in order to save power in this
area.

+  }
+}
+
 }  // namespace gl