diff --git a/include/nuttx/hrtimer.h b/include/nuttx/hrtimer.h
index e6c617b6b29..c59064a9e59 100644
--- a/include/nuttx/hrtimer.h
+++ b/include/nuttx/hrtimer.h
@@ -118,12 +118,7 @@ extern "C"
  *   None
  ****************************************************************************/
 
-static inline_function
-void hrtimer_init(FAR hrtimer_t *hrtimer, hrtimer_cb func)
-{
-  memset(hrtimer, 0, sizeof(hrtimer_t));
-  hrtimer->func = func;
-}
+#define hrtimer_init(hrtimer) memset(hrtimer, 0, sizeof(hrtimer_t))
 
 /****************************************************************************
  * Name: hrtimer_cancel
@@ -184,6 +179,7 @@ int hrtimer_cancel_sync(FAR hrtimer_t *hrtimer);
  *
  * Input Parameters:
  *   hrtimer - Timer instance to start
+ *   func    - Expiration callback function
  *   expired - Expiration time in nanoseconds
  *   mode    - HRTIMER_MODE_ABS or HRTIMER_MODE_REL
  *
@@ -191,7 +187,7 @@ int hrtimer_cancel_sync(FAR hrtimer_t *hrtimer);
  *   OK on success; a negated errno value on failure.
  ****************************************************************************/
 
-int hrtimer_start(FAR hrtimer_t *hrtimer,
+int hrtimer_start(FAR hrtimer_t *hrtimer, hrtimer_entry_t func,
                   uint64_t expired,
                   enum hrtimer_mode_e mode);
 
diff --git a/sched/hrtimer/hrtimer.h b/sched/hrtimer/hrtimer.h
index 99858415d29..92e932ac947 100644
--- a/sched/hrtimer/hrtimer.h
+++ b/sched/hrtimer/hrtimer.h
@@ -198,10 +198,7 @@ RB_PROTOTYPE(hrtimer_tree_s, hrtimer_node_s, entry, hrtimer_compare);
 
 static inline_function bool hrtimer_is_armed(FAR hrtimer_t *hrtimer)
 {
-  /* RB-tree root has NULL parent, so root must be checked explicitly */
-
-  return RB_PARENT(&hrtimer->node, entry) != NULL ||
-         RB_ROOT(&g_hrtimer_tree) == &hrtimer->node;
+  return hrtimer->func != NULL;
 }
 
 /****************************************************************************
@@ -217,7 +214,7 @@ static inline_function void hrtimer_remove(FAR hrtimer_t *hrtimer)
 
   /* Explicitly clear parent to mark the timer as unarmed */
 
-  RB_PARENT(&hrtimer->node, entry) = NULL;
+  hrtimer->func = NULL;
 }
 
 /****************************************************************************
diff --git a/sched/hrtimer/hrtimer_process.c b/sched/hrtimer/hrtimer_process.c
index f94f699aa9e..c7a40fd2511 100644
--- a/sched/hrtimer/hrtimer_process.c
+++ b/sched/hrtimer/hrtimer_process.c
@@ -139,6 +139,7 @@ void hrtimer_process(uint64_t now)
 
           DEBUGASSERT(hrtimer->expired > period);
 
+          hrtimer->func = func;
           hrtimer_insert(hrtimer);
         }
 
diff --git a/sched/hrtimer/hrtimer_start.c b/sched/hrtimer/hrtimer_start.c
index caba6fc50f8..0f24e0952c9 100644
--- a/sched/hrtimer/hrtimer_start.c
+++ b/sched/hrtimer/hrtimer_start.c
@@ -44,6 +44,7 @@
  *
  * Input Parameters:
  *   hrtimer - Pointer to the hrtimer structure.
+ *   func    - Expiration callback function.
  *   expired - Expiration time in nanoseconds. Interpretation
  *             depends on mode.
  *   mode    - Timer mode (HRTIMER_MODE_ABS or HRTIMER_MODE_REL).
@@ -59,7 +60,7 @@
  *     nanoseconds from the current time.
  ****************************************************************************/
 
-int hrtimer_start(FAR hrtimer_t *hrtimer,
+int hrtimer_start(FAR hrtimer_t *hrtimer, hrtimer_entry_t func,
                   uint64_t expired,
                   enum hrtimer_mode_e mode)
 {
@@ -77,6 +78,8 @@ int hrtimer_start(FAR hrtimer_t *hrtimer,
       hrtimer_remove(hrtimer);
     }
 
+  hrtimer->func = func;
+
   /* Compute absolute expiration time */
 
   if (mode == HRTIMER_MODE_ABS)