Store audio device's active state in preference

chromeos/audio/audio_device.h

 // Copyright (c) 2013 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 CHROMEOS_AUDIO_AUDIO_DEVICE_H_
 #define CHROMEOS_AUDIO_AUDIO_DEVICE_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 39 matching lines @@
     return (type == AUDIO_TYPE_HEADPHONE ||
             type == AUDIO_TYPE_INTERNAL_MIC ||
             type == AUDIO_TYPE_MIC ||
             type == AUDIO_TYPE_USB ||
             type == AUDIO_TYPE_BLUETOOTH ||
             type == AUDIO_TYPE_HDMI ||
             type == AUDIO_TYPE_INTERNAL_SPEAKER);
   }
 
   bool is_input;
+
+  // Id of this audio device. The legacy |id| is assigned to be unique everytime
+  // when each device got plugged, so that the same physical device will have
+  // a different id after unplug then re-plug.
+  // The |stable_device_id| is designed to be persistent across system reboot
+  // and plug/unplug for the same physical device. It is guaranteed that
+  // different type of hardware has different |stable_device_id|, but not
+  // guaranteed to be different between the same kind of audio device, e.g
+  // USB headset. |id| and |stable_device_id| can be used together to achieve
+  // various goals.
   uint64_t id;
+  uint64_t stable_device_id;
   std::string display_name;
   std::string device_name;
   std::string mic_positions;
   AudioDeviceType type;
   uint8_t priority;
   bool active;
   uint64_t plugged_time;
 };
 
 typedef std::vector<AudioDevice> AudioDeviceList;
@@ skipping 16 matching lines @@
       return true;
     } else {
       return false;
     }
   }
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_AUDIO_AUDIO_DEVICE_H_