[mle] mParentChanges counter without INFORM_PREVIOUS_PARENT (#7134)

This commit allows `mParentChanges` counter which tracks the number
of times device (as a child) switches its parent to be used without
`INFORM_PREVIOUS_PARENT` feature.

include/openthread/instance.h

@@ -53,7 +53,7 @@ extern "C" {
  * @note This number versions both OpenThread platform and user APIs.
  *
  */
-#define OPENTHREAD_API_VERSION (173)
+#define OPENTHREAD_API_VERSION (174)
 
 /**
  * @addtogroup api-instance

include/openthread/thread.h

@@ -168,10 +168,7 @@ typedef struct otMleCounters
     uint16_t mBetterPartitionAttachAttempts; ///< Number of attempts to attach to a better partition.
 
     /**
-     * Number of times device changed its parents.
-     *
-     * Support for this counter requires the feature option OPENTHREAD_CONFIG_MLE_INFORM_PREVIOUS_PARENT_ON_REATTACH to
-     * be enabled.
+     * Number of times device changed its parent.
      *
      * A parent change can happen if device detaches from its current parent and attaches to a different one, or even
      * while device is attached when the periodic parent search feature is enabled  (please see option

src/core/thread/mle.cpp

@@ -97,9 +97,7 @@ Mle::Mle(Instance &aInstance)
 #if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
     , mCslTimeout(OPENTHREAD_CONFIG_CSL_TIMEOUT)
 #endif
-#if OPENTHREAD_CONFIG_MLE_INFORM_PREVIOUS_PARENT_ON_REATTACH
     , mPreviousParentRloc(Mac::kShortAddrInvalid)
-#endif
 #if OPENTHREAD_CONFIG_PARENT_SEARCH_ENABLE
     , mParentSearchIsInBackoff(false)
     , mParentSearchBackoffWasCanceled(false)
@@ -386,9 +384,7 @@ Error Mle::Restore(void)
         mParent.SetRloc16(Rloc16FromRouterId(RouterIdFromRloc16(networkInfo.GetRloc16())));
         mParent.SetState(Neighbor::kStateRestored);
 
-#if OPENTHREAD_CONFIG_MLE_INFORM_PREVIOUS_PARENT_ON_REATTACH
         mPreviousParentRloc = mParent.GetRloc16();
-#endif
     }
 #if OPENTHREAD_FTD
     else
@@ -693,10 +689,16 @@ void Mle::SetStateChild(uint16_t aRloc16)
     UpdateParentSearchState();
 #endif
 
+    if ((mPreviousParentRloc != Mac::kShortAddrInvalid) && (mPreviousParentRloc != mParent.GetRloc16()))
+    {
+        mCounters.mParentChanges++;
+
 #if OPENTHREAD_CONFIG_MLE_INFORM_PREVIOUS_PARENT_ON_REATTACH
-    InformPreviousParent();
-    mPreviousParentRloc = mParent.GetRloc16();
+        InformPreviousParent();
 #endif
+    }
+
+    mPreviousParentRloc = mParent.GetRloc16();
 
 #if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
     if (Get<Mac::Mac>().IsCslCapable())
@@ -4155,10 +4157,6 @@ void Mle::InformPreviousParent(void)
     Message *        message = nullptr;
     Ip6::MessageInfo messageInfo;
 
-    VerifyOrExit((mPreviousParentRloc != Mac::kShortAddrInvalid) && (mPreviousParentRloc != mParent.GetRloc16()));
-
-    mCounters.mParentChanges++;
-
     VerifyOrExit((message = Get<Ip6::Ip6>().NewMessage(0)) != nullptr, error = kErrorNoBufs);
     SuccessOrExit(error = message->SetLength(0));
 

src/core/thread/mle.hpp

@@ -1887,9 +1887,7 @@ private:
     uint32_t mCslTimeout;
 #endif
 
-#if OPENTHREAD_CONFIG_MLE_INFORM_PREVIOUS_PARENT_ON_REATTACH
     uint16_t mPreviousParentRloc;
-#endif
 
 #if OPENTHREAD_CONFIG_PARENT_SEARCH_ENABLE
     bool       mParentSearchIsInBackoff : 1;