New radio capability to indicate CSMA backoff support by radio layer (#1395)

- This commit adds a new radio capability `kRadioCapsCsmaBackOff` to
  indicate that the radio supports CSMA backoff logic for frame
  transmission (but no frame retry).

- The existing capability `kRadioCapsTransmitRetries` is not changed
  and still indicates that the radio supports transmission retry logic
  along with collision avoidance (CSMA).

- `Mac` implementation is updated accordingly.

include/platform/radio.h

@@ -91,7 +91,8 @@ typedef enum otRadioCaps
     kRadioCapsNone              = 0,  ///< None
     kRadioCapsAckTimeout        = 1,  ///< Radio supports AckTime event
     kRadioCapsEnergyScan        = 2,  ///< Radio supports Energy Scans
-    kRadioCapsTransmitRetries   = 4,  ///< Radio supports transmission retry logic with collision avoidance
+    kRadioCapsTransmitRetries   = 4,  ///< Radio supports transmission retry logic with collision avoidance (CSMA).
+    kRadioCapsCsmaBackOff       = 8,  ///< Radio supports CSMA backoff for frame transmission (but no retry).
 } otRadioCaps;
 
 /**

src/core/mac/mac.cpp

@@ -81,10 +81,9 @@ static_assert(kMinBackoffSum > 0, "The min backoff value should be greater than
 
 void Mac::StartCsmaBackoff(void)
 {
-    if (RadioSupportsRetriesAndCsmaBackoff())
+    if (RadioSupportsCsmaBackoff())
     {
-        // If the radio supports the retry and back off logic, immediately schedule the send,
-        // and the radio will take care of everything.
+        // If the radio supports CSMA back off logic, immediately schedule the send.
         HandleBeginTransmit();
     }
     else
@@ -856,7 +855,7 @@ void Mac::TransmitDoneTask(RadioPacket *aPacket, bool aRxPending, ThreadError aE
         mCounters.mTxErrAbort++;
     }
 
-    if (!RadioSupportsRetriesAndCsmaBackoff() &&
+    if (!RadioSupportsCsmaBackoff() &&
         aError == kThreadError_ChannelAccessFailure &&
         mCsmaAttempts < kMaxCSMABackoffs)
     {
@@ -989,7 +988,7 @@ void Mac::SentFrame(ThreadError aError)
     case kThreadError_NoAck:
         otDumpDebgMac("NO ACK", sendFrame.GetHeader(), 16);
 
-        if (!RadioSupportsRetriesAndCsmaBackoff() &&
+        if (!RadioSupportsRetries() &&
             mTransmitAttempts < kMaxFrameAttempts)
         {
             mTransmitAttempts++;
@@ -1540,7 +1539,16 @@ void Mac::SetPromiscuous(bool aPromiscuous)
     }
 }
 
-bool Mac::RadioSupportsRetriesAndCsmaBackoff(void)
+bool Mac::RadioSupportsCsmaBackoff(void)
+{
+    /* Check either of the following conditions:
+     *   1) Radio provides the CSMA backoff capability (i.e., `kRadioCapsCsmaBackOff` bit is set) or;
+     *   2) It provides `kRadioCapsTransmitRetries` which indicates support for MAC retries along with CSMA backoff.
+     */
+    return (otPlatRadioGetCaps(mNetif.GetInstance()) & (kRadioCapsTransmitRetries | kRadioCapsCsmaBackOff)) != 0;
+}
+
+bool Mac::RadioSupportsRetries(void)
 {
     return (otPlatRadioGetCaps(mNetif.GetInstance()) & kRadioCapsTransmitRetries) != 0;
 }

src/core/mac/mac.hpp

@@ -560,13 +560,22 @@ public:
     void ClearSrcMatchEntries(void);
 
     /**
-     * This method indicates whether or not transmit retries and CSMA backoff logic is supported by the radio layer.
+     * This method indicates whether or not CSMA backoff is supported by the radio layer.
      *
-     * @retval true   Retries and CSMA are supported by the radio.
-     * @retval false  Retries and CSMA are not supported by the radio.
+     * @retval true   CSMA backoff is supported by the radio.
+     * @retval false  CSMA backoff is not supported by the radio.
      *
      */
-    bool RadioSupportsRetriesAndCsmaBackoff(void);
+    bool RadioSupportsCsmaBackoff(void);
+
+    /**
+     * This method indicates whether or not transmit retries is supported by the radio layer.
+     *
+     * @retval true   Retries (and CSMA) are supported by the radio.
+     * @retval false  Retries (and CSMA) are not supported by the radio.
+     *
+     */
+    bool RadioSupportsRetries(void);
 
 private:
     enum ScanType