Add workaround for multisampling color masks for the command buffer.

gpu/command_buffer/service/gles2_cmd_decoder.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 "gpu/command_buffer/service/gles2_cmd_decoder.h"
 
 #include <limits.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
@@ skipping 2352 matching lines @@
 ScopedResolvedFrameBufferBinder::ScopedResolvedFrameBufferBinder(
     GLES2DecoderImpl* decoder, bool enforce_internal_framebuffer, bool internal)
     : decoder_(decoder) {
   resolve_and_bind_ = (
       decoder_->offscreen_target_frame_buffer_.get() &&
       decoder_->IsOffscreenBufferMultisampled() &&
       (!decoder_->framebuffer_state_.bound_read_framebuffer.get() ||
        enforce_internal_framebuffer));
   if (!resolve_and_bind_)
     return;
-
-  // TODO(erikchen): On old AMD GPUs on macOS, glColorMask doesn't work
-  // correctly for multisampled renderbuffers and the alpha channel can be
-  // overwritten. Add a workaround to clear the alpha channel before resolving.
-  // https://crbug.com/602484.
   ScopedGLErrorSuppressor suppressor(
       "ScopedResolvedFrameBufferBinder::ctor", decoder_->GetErrorState());
+
+  // On old AMD GPUs on macOS, glColorMask doesn't work correctly for
+  // multisampled renderbuffers and the alpha channel can be overwritten. This
+  // workaround clears the alpha channel before resolving.
+  bool alpha_channel_needs_clear =
+      decoder_->should_use_native_gmb_for_backbuffer_ &&
+      !decoder_->offscreen_buffer_should_have_alpha_ &&
+      decoder_->ChromiumImageNeedsRGBEmulation() &&
+      decoder_->feature_info_->workarounds()
+          .disable_multisampling_color_mask_usage;
+  if (alpha_channel_needs_clear) {
+    glBindFramebufferEXT(GL_DRAW_FRAMEBUFFER_EXT,
+                         decoder_->offscreen_target_frame_buffer_->id());
+    decoder_->state_.SetDeviceColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_TRUE);
+    decoder->state_.SetDeviceCapabilityState(GL_SCISSOR_TEST, false);
+    glClearColor(0, 0, 0, 1);
+    glClear(GL_COLOR_BUFFER_BIT);
+    decoder_->RestoreClearState();
+  }
+
   glBindFramebufferEXT(GL_READ_FRAMEBUFFER_EXT,
                        decoder_->offscreen_target_frame_buffer_->id());
   GLuint targetid;
   if (internal) {
     if (!decoder_->offscreen_resolved_frame_buffer_.get()) {
       decoder_->offscreen_resolved_frame_buffer_.reset(
           new BackFramebuffer(decoder_));
       decoder_->offscreen_resolved_frame_buffer_->Create();
       decoder_->offscreen_resolved_color_texture_.reset(
           new BackTexture(decoder));
@@ skipping 14698 matching lines @@
 }
 
 // Include the auto-generated part of this file. We split this because it means
 // we can easily edit the non-auto generated parts right here in this file
 // instead of having to edit some template or the code generator.
 #include "base/macros.h"
 #include "gpu/command_buffer/service/gles2_cmd_decoder_autogen.h"
 
 }  // namespace gles2
 }  // namespace gpu