[csl] add OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_LOCAL_TIME_SYNC option (#9568) (#9568)

Allow platforms to reduce the periodic access to `otPlatRadioGetNow`
to calculate CSL synchronization elapsed time in the case when those
radio API calls are costly.

etc/cmake/options.cmake

@@ -188,6 +188,7 @@ ot_option(OT_COMMISSIONER OPENTHREAD_CONFIG_COMMISSIONER_ENABLE "commissioner")
 ot_option(OT_CSL_AUTO_SYNC OPENTHREAD_CONFIG_MAC_CSL_AUTO_SYNC_ENABLE "data polling based on csl")
 ot_option(OT_CSL_DEBUG OPENTHREAD_CONFIG_MAC_CSL_DEBUG_ENABLE "csl debug")
 ot_option(OT_CSL_RECEIVER OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE "csl receiver")
+ot_option(OT_CSL_RECEIVER_LOCAL_TIME_SYNC OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_LOCAL_TIME_SYNC "use local time for csl elapsed sync time")
 ot_option(OT_DATASET_UPDATER OPENTHREAD_CONFIG_DATASET_UPDATER_ENABLE "dataset updater")
 ot_option(OT_DEVICE_PROP_LEADER_WEIGHT OPENTHREAD_CONFIG_MLE_DEVICE_PROPERTY_LEADER_WEIGHT_ENABLE "device prop for leader weight")
 ot_option(OT_DHCP6_CLIENT OPENTHREAD_CONFIG_DHCP6_CLIENT_ENABLE "DHCP6 client")

src/core/config/mac.h

@@ -425,6 +425,28 @@
 #define OPENTHREAD_CONFIG_MAC_CSL_AUTO_SYNC_ENABLE OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
 #endif
 
+/**
+ * @def OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_LOCAL_TIME_SYNC
+ *
+ * This setting configures the usage of local time rather than radio time for calculating the
+ * elapsed time since last CSL synchronization event in order to schedule the duration of the
+ * CSL receive window.
+ *
+ * This is done at expense of too short or too long receive windows depending on the drift
+ * between the two clocks within the CSL timeout period. In order to compensate for a too
+ * short receive window, CSL uncertainty can be increased.
+ *
+ * This setting can be useful for platforms in which is important to reduce the number of
+ * radio API calls, for instance when they are costly. One typical situation is a multicore
+ * chip architecture in which different instances of current time are being kept in host and
+ * radio cores. In this case, accessing the radio core current time API requires serialization
+ * and it is more costly than just accessing local host time.
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_LOCAL_TIME_SYNC
+#define OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_LOCAL_TIME_SYNC 0
+#endif
+
 /**
  * @def OPENTHREAD_CONFIG_MAC_CSL_MIN_PERIOD
  *

src/core/mac/sub_mac.cpp

@@ -352,7 +352,11 @@ void SubMac::HandleReceiveDone(RxFrame *aFrame, Error aError)
         // Assuming the risk of the parent missing the Enh-ACK in favor of smaller CSL receive window
         if ((mCslPeriod > 0) && aFrame->mInfo.mRxInfo.mAckedWithSecEnhAck)
         {
+#if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_LOCAL_TIME_SYNC
+            mCslLastSync = TimerMicro::GetNow();
+#else
             mCslLastSync = TimeMicro(static_cast<uint32_t>(aFrame->mInfo.mRxInfo.mTimestamp));
+#endif
         }
     }
 #endif // OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
@@ -613,9 +617,13 @@ void SubMac::HandleTransmitDone(TxFrame &aFrame, RxFrame *aAckFrame, Error aErro
 #if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
         // Actual synchronization timestamp should be from the sent frame instead of the current time.
         // Assuming the error here since it is bounded and has very small effect on the final window duration.
-        if (mCslPeriod > 0)
+        if (aAckFrame != nullptr && aFrame.GetHeaderIe(CslIe::kHeaderIeId) != nullptr)
         {
+#if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_LOCAL_TIME_SYNC
+            mCslLastSync = TimerMicro::GetNow();
+#else
             mCslLastSync = TimeMicro(static_cast<uint32_t>(otPlatRadioGetNow(&GetInstance())));
+#endif
         }
 #endif
         break;
@@ -1232,9 +1240,13 @@ void SubMac::HandleCslTimer(void)
 void SubMac::GetCslWindowEdges(uint32_t &aAhead, uint32_t &aAfter)
 {
     uint32_t semiPeriod = mCslPeriod * kUsPerTenSymbols / 2;
-    uint32_t curTime    = static_cast<uint32_t>(otPlatRadioGetNow(&GetInstance()));
-    uint32_t elapsed;
-    uint32_t semiWindow;
+    uint32_t curTime, elapsed, semiWindow;
+
+#if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_LOCAL_TIME_SYNC
+    curTime = TimerMicro::GetNow();
+#else
+    curTime = static_cast<uint32_t>(otPlatRadioGetNow(&GetInstance()));
+#endif
 
     elapsed = curTime - mCslLastSync.GetValue();