From 6ccd4ffd78fa565c182427fa2fc8296f963db792 Mon Sep 17 00:00:00 2001
From: Abtin Keshavarzian <abtink@google.com>
Date: Mon, 16 Aug 2021 09:48:19 -0700
Subject: [PATCH] [mac] tx beacon on the same radio as the request in
 multi-radio config (#6921)

This commit updates `Mac` under `MULTI_RADIO` config to limit the
radio links used when sending a MAC Beacon frame (which uses
broadcast destination and therefore by default would be sent on all
radios) to the same radio links from which the triggering Beacon
Request frame(s) were received. This ensures that when a multi-radio
node scans another multi-radio node we get one Beacon response on
each radio link.
---
 src/core/mac/mac.cpp | 28 ++++++++++++++++++++--------
 src/core/mac/mac.hpp |  1 +
 2 files changed, 21 insertions(+), 8 deletions(-)

diff --git a/src/core/mac/mac.cpp b/src/core/mac/mac.cpp
index 5a74c3ad3..1299fd752 100644
--- a/src/core/mac/mac.cpp
+++ b/src/core/mac/mac.cpp
@@ -856,18 +856,26 @@ TxFrame *Mac::PrepareBeaconRequest(void)
 
 TxFrame *Mac::PrepareBeacon(void)
 {
-    TxFrame &      frame = mLinks.GetTxFrames().GetBroadcastTxFrame();
+    TxFrame *      frame;
     uint8_t        beaconLength;
     uint16_t       fcf;
     Beacon *       beacon        = nullptr;
     BeaconPayload *beaconPayload = nullptr;
 
-    fcf = Frame::kFcfFrameBeacon | Frame::kFcfDstAddrNone | Frame::kFcfSrcAddrExt;
-    frame.InitMacHeader(fcf, Frame::kSecNone);
-    IgnoreError(frame.SetSrcPanId(mPanId));
-    frame.SetSrcAddr(GetExtAddress());
+#if OPENTHREAD_CONFIG_MULTI_RADIO
+    OT_ASSERT(!mTxBeaconRadioLinks.IsEmpty());
+    frame = &mLinks.GetTxFrames().GetTxFrame(mTxBeaconRadioLinks);
+    mTxBeaconRadioLinks.Clear();
+#else
+    frame = &mLinks.GetTxFrames().GetBroadcastTxFrame();
+#endif
 
-    beacon = reinterpret_cast<Beacon *>(frame.GetPayload());
+    fcf = Frame::kFcfFrameBeacon | Frame::kFcfDstAddrNone | Frame::kFcfSrcAddrExt;
+    frame->InitMacHeader(fcf, Frame::kSecNone);
+    IgnoreError(frame->SetSrcPanId(mPanId));
+    frame->SetSrcAddr(GetExtAddress());
+
+    beacon = reinterpret_cast<Beacon *>(frame->GetPayload());
     beacon->Init();
     beaconLength = sizeof(*beacon);
 
@@ -892,11 +900,11 @@ TxFrame *Mac::PrepareBeacon(void)
         beaconLength += sizeof(*beaconPayload);
     }
 
-    frame.SetPayloadLength(beaconLength);
+    frame->SetPayloadLength(beaconLength);
 
     LogBeacon("Sending", *beaconPayload);
 
-    return &frame;
+    return frame;
 }
 
 bool Mac::ShouldSendBeacon(void) const
@@ -2164,8 +2172,12 @@ bool Mac::HandleMacCommand(RxFrame &aFrame)
 
         if (ShouldSendBeacon())
         {
+#if OPENTHREAD_CONFIG_MULTI_RADIO
+            mTxBeaconRadioLinks.Add(aFrame.GetRadioType());
+#endif
             StartOperation(kOperationTransmitBeacon);
         }
+
         didHandle = true;
         break;
 
diff --git a/src/core/mac/mac.hpp b/src/core/mac/mac.hpp
index f97222f1c..89a2642a5 100644
--- a/src/core/mac/mac.hpp
+++ b/src/core/mac/mac.hpp
@@ -916,6 +916,7 @@ private:
 
 #if OPENTHREAD_CONFIG_MULTI_RADIO
     RadioTypes mTxPendingRadioLinks;
+    RadioTypes mTxBeaconRadioLinks;
     Error      mTxError;
 #endif