Add option for Radio layer to support retries and CSMA backoff logic. (#896)

* Add option for Radio layer to support tries and CSMA backoff logic.

examples/drivers/windows/otLwf/radio.c

@@ -131,6 +131,8 @@ otLwfRadioInit(
     pFilter->otRadioCapabilities = kRadioCapsEnergyScan;
     if ((pFilter->MiniportCapabilities.RadioCapabilities & OT_RADIO_CAP_ACK_TIMEOUT) != 0)
         pFilter->otRadioCapabilities |= kRadioCapsAckTimeout;
+    if ((pFilter->MiniportCapabilities.RadioCapabilities & OT_RADIO_CAP_MAC_RETRY_AND_COLLISION_AVOIDANCE) != 0)
+        pFilter->otRadioCapabilities |= kRadioCapsTransmitRetries;
 
     pFilter->otPhyState = kStateDisabled;
     pFilter->otCurrentListenChannel = 0xFF;

include/platform/radio.h

@@ -87,9 +87,10 @@ enum
 
 typedef enum otRadioCaps
 {
-    kRadioCapsNone          = 0,  ///< None
+    kRadioCapsNone              = 0,  ///< None
-    kRadioCapsAckTimeout    = 1,  ///< Radio supports AckTime event
+    kRadioCapsAckTimeout        = 1,  ///< Radio supports AckTime event
-    kRadioCapsEnergyScan    = 2,  ///< Radio supports Enegery Scans
+    kRadioCapsEnergyScan        = 2,  ///< Radio supports Enegery Scans
+    kRadioCapsTransmitRetries   = 4,  ///< Radio supports transmission retry logic with collission avoidance
 } otRadioCaps;
 
 /**

src/core/mac/mac.cpp

@@ -80,18 +80,27 @@ static_assert(kMinBackoffSum > 0, "The min backoff value should be greater than
 
 void Mac::StartCsmaBackoff(void)
 {
-    uint32_t backoffExponent = kMinBE + mTransmitAttempts + mCsmaAttempts;
+    if (RadioSupportsRetriesAndCsmaBackoff())
-    uint32_t backoff;
-
-    if (backoffExponent > kMaxBE)
     {
-        backoffExponent = kMaxBE;
+        // If the radio supports the retry and back off logic, immediately schedule the send,
+        // and the radio will take care of everything.
+        mBackoffTimer.Start(0);
     }
+    else
+    {
+        uint32_t backoffExponent = kMinBE + mTransmitAttempts + mCsmaAttempts;
+        uint32_t backoff;
 
-    backoff = kMinBackoff + (kUnitBackoffPeriod * kPhyUsPerSymbol * (1 << backoffExponent)) / 1000;
+        if (backoffExponent > kMaxBE)
-    backoff = (otPlatRandomGet() % backoff);
+        {
+            backoffExponent = kMaxBE;
+        }
 
-    mBackoffTimer.Start(backoff);
+        backoff = kMinBackoff + (kUnitBackoffPeriod * kPhyUsPerSymbol * (1 << backoffExponent)) / 1000;
+        backoff = (otPlatRandomGet() % backoff);
+
+        mBackoffTimer.Start(backoff);
+    }
 }
 
 Mac::Mac(ThreadNetif &aThreadNetif):
@@ -788,7 +797,8 @@ void Mac::TransmitDoneTask(bool aRxPending, ThreadError aError)
 
     mCounters.mTxTotal++;
 
-    if (aError == kThreadError_ChannelAccessFailure &&
+    if (!RadioSupportsRetriesAndCsmaBackoff() &&
+        aError == kThreadError_ChannelAccessFailure &&
         mCsmaAttempts < kMaxCSMABackoffs)
     {
         mCsmaAttempts++;
@@ -904,7 +914,8 @@ void Mac::SentFrame(ThreadError aError)
     case kThreadError_NoAck:
         otDumpDebgMac("NO ACK", sendFrame.GetHeader(), 16);
 
-        if (mTransmitAttempts < kMaxFrameAttempts)
+        if (!RadioSupportsRetriesAndCsmaBackoff() &&
+            mTransmitAttempts < kMaxFrameAttempts)
         {
             mTransmitAttempts++;
             StartCsmaBackoff();
@@ -1376,6 +1387,11 @@ void Mac::SetPromiscuous(bool aPromiscuous)
     }
 }
 
+bool Mac::RadioSupportsRetriesAndCsmaBackoff(void)
+{
+    return (otPlatRadioGetCaps(mNetif.GetInstance()) & kRadioCapsTransmitRetries) != 0;
+}
+
 Whitelist &Mac::GetWhitelist(void)
 {
     return mWhitelist;

src/core/mac/mac.hpp

@@ -545,6 +545,15 @@ public:
      */
     void ClearSrcMatchEntries();
 
+    /**
+     * This function indicates whether or not transmit retries and CSMA backoff logic 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 RadioSupportsRetriesAndCsmaBackoff(void);
+
 private:
     enum ScanType
     {