[Ozone-DRI] Fix waiting for page flips in mirror mode

ui/ozone/platform/dri/hardware_display_controller.cc

 // Copyright 2014 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 "ui/ozone/platform/dri/hardware_display_controller.h"
 
 #include <drm.h>
 #include <errno.h>
 #include <string.h>
 #include <xf86drm.h>
@@ skipping 101 matching lines @@
   TRACE_EVENT0("dri", "HDC::WaitForPageFlipEvent");
 
   drmEventContext drm_event;
   drm_event.version = DRM_EVENT_CONTEXT_VERSION;
   drm_event.page_flip_handler = HandlePageFlipEvent;
   drm_event.vblank_handler = NULL;
 
   bool has_pending_page_flips = false;
   // Wait for the page-flips to complete.
   for (size_t i = 0; i < crtc_controllers_.size(); ++i) {
-    if (crtc_controllers_[i]->page_flip_pending()) {
+    // Messages do not come in any particular order so we need to make sure the

[alexst (slow to review), 2014/10/09 20:54:40]

This deals with messages from multiple monitors on a crtc in mirrored mode if I
understand it correctly. I think it's a crucial thing to add in a comment here.

[dnicoara, 2014/10/10 13:54:00]

Actually, it can be multiple CRTCs in mirror mode. HandleEvent can process an
event for a different CRTC, so without the while-loop it is possible that it'd
process an event for the second CRTC. Moving to the second CRTC this condition
would fail and it'd end up waiting for 1 event rather than 2.

[alexst (slow to review), 2014/10/10 15:50:55]

Right, I actually like your response better than the comment you left in the
code.

I am just being extra nitty about comments. I know my way around this code, but
it's got subtitles and I still have to open my editor and browse around to
convince myself things do what I understand them to do, which is why I keep
asking for more comments.

+    // flip for the current controller has been processed.
+    while (crtc_controllers_[i]->page_flip_pending()) {
       has_pending_page_flips = true;
       crtc_controllers_[i]->drm()->HandleEvent(drm_event);
     }
   }
 
   // In case there are no pending pageflips do not replace the current planes
   // since they are still being used.
   if (has_pending_page_flips)
     current_planes_.swap(pending_planes_);
 
@@ skipping 74 matching lines @@
 uint64_t HardwareDisplayController::GetTimeOfLastFlip() const {
   uint64_t time = 0;
   for (size_t i = 0; i < crtc_controllers_.size(); ++i)
     if (time < crtc_controllers_[i]->time_of_last_flip())
       time = crtc_controllers_[i]->time_of_last_flip();
 
   return time;
 }
 
 }  // namespace ui