chromeos: Turn off displays on suspend

ui/display/chromeos/display_configurator.h

 // 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.
 
 #ifndef UI_DISPLAY_CHROMEOS_DISPLAY_CONFIGURATOR_H_
 #define UI_DISPLAY_CHROMEOS_DISPLAY_CONFIGURATOR_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 378 matching lines @@
 
   // Pending requests. These values are used when triggering the next display
   // configuration.
   //
   // Stores the user requested state or INVALID if nothing was requested.
   MultipleDisplayState requested_display_state_;
 
   // Stores the requested power state.
   chromeos::DisplayPowerState requested_power_state_;
 
+  // The display power state before suspend.
+  chromeos::DisplayPowerState saved_power_state_;

[Daniel Erat, 2016/04/08 20:26:51]

here's a possibly-naive question: why is this needed? it looks like
requested_power_state_ gets copied to saved_power_state_ just before the system
suspends, and then both of them get checked on resume. is this present to handle
the case where powerd sends another request during dark resume? if so, could you
document that here?

i *think* that what's going on is that SetDisplayPower() (which is usually
called by powerd) updates requested_power_state_, and you want to save the
actually-requested-by-powerd state before you call SetDisplayPower() manually so
you can restore the state later. if that's correct, please just create a new
SetDisplayPowerInternal() method. powerd can call SetDisplayPower(), which will
update the requested state and call SetDisplayPowerInternal(), while your change
can just call SetDisplayPowerInternal() directly.

[dbasehore, 2016/04/08 20:38:11]

This is to handle the display state changing out from under the display
configurator (whether from powerd or some other place). I'll document this.

I assume SetDisplayPowerInternal will be called from the display configurator to
turn off the displays (and not save the off state in requested_power_state_)?

[Daniel Erat, 2016/04/08 20:42:11]

yeah, that's the idea. then we don't need to track an extra state --
requested_power_state_ contains the state that powerd asked for, and that's it.
the display can be configured differently for suspend, but we know what we want
to go back to after resuming.

+
   // True if |requested_power_state_| has been changed due to a user request.
   bool requested_power_state_change_;
 
   // Bitwise-or value of the |kSetDisplayPower*| flags defined above.
   int requested_power_flags_;
 
   // List of callbacks from callers waiting for the display configuration to
   // start/finish. Note these callbacks belong to the pending request, not a
   // request currently active.
   std::vector<ConfigurationCallback> queued_configuration_callbacks_;
@@ skipping 51 matching lines @@
 
   // This must be the last variable.
   base::WeakPtrFactory<DisplayConfigurator> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(DisplayConfigurator);
 };
 
 }  // namespace ui
 
 #endif  // UI_DISPLAY_CHROMEOS_DISPLAY_CONFIGURATOR_H_