Complete disable_gl_rgb_format workaround for Mali-400 GPU

gpu/command_buffer/service/gles2_cmd_decoder.cc

Index: gpu/command_buffer/service/gles2_cmd_decoder.cc
diff --git a/gpu/command_buffer/service/gles2_cmd_decoder.cc b/gpu/command_buffer/service/gles2_cmd_decoder.cc
index ae331e5a4a080d4cf275e8cb35d2caf30011a28a..cd2fb56c2e387210bcbaca64abeb5c275bcfe64b 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder.cc
+++ b/gpu/command_buffer/service/gles2_cmd_decoder.cc
@@ -1851,6 +1851,8 @@ class GLES2DecoderImpl : public GLES2Decoder, public ErrorStateClient {
   }
 
   bool BackBufferHasAlpha() const {
+    if (state_.emulating_rgb_with_rgba)
+      return false;
     if (back_buffer_draw_buffer_ == GL_NONE)
       return false;
     if (offscreen_target_frame_buffer_.get()) {
@@ -2753,9 +2755,11 @@ bool GLES2DecoderImpl::Initialize(const scoped_refptr<gfx::GLSurface>& surface,
       } else {
         offscreen_target_samples_ = 1;
         offscreen_target_color_format_ =
-            attrib_parser.alpha_size > 0 || workarounds().disable_gl_rgb_format
-                ? GL_RGBA
-                : GL_RGB;
+            (attrib_parser.alpha_size > 0) ? GL_RGBA : GL_RGB;
+        if (workarounds().disable_gl_rgb_format) {

[bajones, 2016/02/12 21:07:41]

This branch and the one below are wrong, because they'll trigger even when an
RGBA buffer was requested, incorrectly marking it as emulated RGB. The condition
should be:

if (offscreen_target_color_format_ == GL_RGB &&
workarounds().disable_gl_rgb_format) { ... }

[halliwell, 2016/02/12 22:52:56]

Doh.  Thank you.

+          offscreen_target_color_format_ = GL_RGBA;
+          state_.emulating_rgb_with_rgba = true;

[Zhenyao Mo, 2016/02/12 21:06:06]

You should only set this if attrib_parser.alpha_size == 0.  Otherwise
BackBufferHasAlpha() will return false even if we request RGBA.

[halliwell, 2016/02/12 22:52:56]

And thank you :)

+        }
       }
 
       // ANGLE only supports packed depth/stencil formats, so use it if it is
@@ -2778,9 +2782,11 @@ bool GLES2DecoderImpl::Initialize(const scoped_refptr<gfx::GLSurface>& surface,
       }
     } else {
       offscreen_target_color_format_ =
-          attrib_parser.alpha_size > 0 || workarounds().disable_gl_rgb_format
-              ? GL_RGBA
-              : GL_RGB;
+          (attrib_parser.alpha_size > 0) ? GL_RGBA : GL_RGB;
+      if (workarounds().disable_gl_rgb_format) {

[bajones, 2016/02/12 21:07:41]

Ditto

[halliwell, 2016/02/12 22:52:56]

Done.

+        offscreen_target_color_format_ = GL_RGBA;
+        state_.emulating_rgb_with_rgba = true;

[Zhenyao Mo, 2016/02/12 21:06:06]

Same here.

[halliwell, 2016/02/12 22:52:56]

Done.

+      }
 
       // If depth is requested at all, use the packed depth stencil format if
       // it's available, as some desktop GL drivers don't support any non-packed
@@ -6145,7 +6151,12 @@ void GLES2DecoderImpl::ClearUnclearedAttachments(
     // We should always use alpha == 0 here, because 1) some draw buffers may
     // have alpha and some may not; 2) we won't have the same situation as the
     // back buffer where alpha channel exists but is not requested.
-    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
+    glClearColor(0.0f, 0.0f, 0.0f,

[Zhenyao Mo, 2016/02/12 21:06:06]

This is problematic, because we could have some attachments as RGBA, and others
as RGB but emulated by RGBA.  To do it right, we will need to clear differently.

If you are time limited, for a quick hack, you can set it to 1 if the workaround
is active and the following condition is true. but please add a TODO here for
following up.

[halliwell, 2016/02/12 22:52:56]

Agreed.  I updated this to check for the workaround.  I will submit in this form
in our downstream branch.

However, there is no time pressure to submit to Chromium, so I'll come back to
this next week and change how clearing is done to handle this case properly.

+                 (GLES2Util::GetChannelsForFormat(
+                      framebuffer->GetColorAttachmentFormat()) &
+                  0x0008) != 0
+                     ? 0.0f
+                     : 1.0f);
     state_.SetDeviceColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
     clear_bits |= GL_COLOR_BUFFER_BIT;
     if (feature_info_->feature_flags().ext_draw_buffers ||