bluetooth: Return int8_t and use -128 for unknown tx power.

device/bluetooth/bluetooth_device.h

Index: device/bluetooth/bluetooth_device.h
diff --git a/device/bluetooth/bluetooth_device.h b/device/bluetooth/bluetooth_device.h
index 2d7f4b96d6991375155559beb44fb2ee72ea7b53..768543abe3c9644e9fbd2161a741207b7062e890 100644
--- a/device/bluetooth/bluetooth_device.h
+++ b/device/bluetooth/bluetooth_device.h
@@ -77,18 +77,27 @@ class DEVICE_BLUETOOTH_EXPORT BluetoothDevice {
     DEVICE_KEYBOARD_MOUSE_COMBO
   };
 
-  // The value returned if the RSSI or transmit power cannot be read.
-  static const int kUnknownPower = 127;
+  // The value returned if the RSSI cannot be read.
+  // We use 127 based on Bluetooth Spec Core Version 4.2 Vol 2, Part E 7.7.65.2
+  // LE Advertising Report Event.
+  enum : int8_t { kUnknownRSSI = 127 };
+  // The value returned if the TxPower cannot be read.
+  // We use -128 based on Core Specification Supplement (CSS) v6 Part A 1.5.2
+  // Format.
+  enum : int8_t { kUnknownTxPower = -128 };
+
   // The value returned if the appearance is not present.
   static const uint16_t kAppearanceNotPresent = 0xffc0;
 
   struct DEVICE_BLUETOOTH_EXPORT ConnectionInfo {
-    int rssi;
-    int transmit_power;
-    int max_transmit_power;
+    int8_t rssi;
+    int8_t transmit_power;
+    int8_t max_transmit_power;
 
     ConnectionInfo();
-    ConnectionInfo(int rssi, int transmit_power, int max_transmit_power);
+    ConnectionInfo(int8_t rssi,
+                   int8_t transmit_power,
+                   int8_t max_transmit_power);
     ~ConnectionInfo();
   };
 
@@ -284,14 +293,14 @@ class DEVICE_BLUETOOTH_EXPORT BluetoothDevice {
 
   // The received signal strength, in dBm. This field is avaliable and valid
   // only during discovery. If not during discovery, or RSSI wasn't reported,
-  // this method will return |kUnknownPower|.
-  virtual int16_t GetInquiryRSSI() const = 0;
+  // this method will return |kUnknownRSSI|.
+  virtual int8_t GetInquiryRSSI() const = 0;
 
   // The transmitted power level. This field is avaliable only for LE devices
   // that include this field in AD. It is avaliable and valid only during
   // discovery. If not during discovery, or TxPower wasn't reported, this
-  // method will return |kUnknownPower|.
-  virtual int16_t GetInquiryTxPower() const = 0;
+  // method will return |kUnknownTxPower|.
+  virtual int8_t GetInquiryTxPower() const = 0;
 
   // The ErrorCallback is used for methods that can fail in which case it
   // is called, in the success case the callback is simply not called.
@@ -328,7 +337,7 @@ class DEVICE_BLUETOOTH_EXPORT BluetoothDevice {
   // indicate a stronger signal.
   //
   // If the device isn't connected, then the ConnectionInfo struct passed into
-  // the callback will be populated with |kUnknownPower|.
+  // the callback will be populated with |kUnknownRSSI| and |kUnknownTxPower|.
   virtual void GetConnectionInfo(const ConnectionInfoCallback& callback) = 0;
 
   // Initiates a connection to the device, pairing first if necessary.