ztimer/periodic: stop timer before reinit, remove aquire/release

    - remove ztimer_now calls -> remove aquire

sys/ztimer/periodic.c

@@ -40,12 +40,18 @@ static void _ztimer_periodic_reset(ztimer_periodic_t *timer, ztimer_now_t now)
     ztimer_set(timer->clock, &timer->timer, offset);
 }
 
+/* this function returns the current list offset (now at the time of interrupt)
+   it only valid to call this while handling a ztimer callback of this clock */
+static ztimer_now_t ztimer_clock_list_now(ztimer_clock_t *clock){
+    return clock->list.offset;
+}
+
 static void _ztimer_periodic_callback(void *arg)
 {
     ztimer_periodic_t *timer = arg;
 
-    if (timer->callback(timer->arg) == ZTIMER_PERIODIC_KEEP_GOING) {
-        ztimer_now_t now = ztimer_now(timer->clock);
+    if (timer->callback(timer->arg) == true) {
+        ztimer_now_t now = ztimer_clock_list_now(timer->clock);
         _ztimer_periodic_reset(timer, now);
     }
     else {
@@ -57,29 +63,25 @@ void ztimer_periodic_init(ztimer_clock_t *clock, ztimer_periodic_t *timer,
                           bool (*callback)(
                               void *), void *arg, uint32_t interval)
 {
-    ztimer_remove(clock, &timer->timer);
+    /* check if this is a reinit, ensure timer is stopped in case */
+    if (timer->timer.callback == _ztimer_periodic_callback) {
+        ztimer_periodic_stop(timer);
+    }
     *timer =
         (ztimer_periodic_t){ .clock = clock, .interval = interval,
                              .callback = callback, .arg = arg,
                              .timer = {
-                                 .callback = _ztimer_periodic_callback,
-                                 .arg = timer
+                                .callback = _ztimer_periodic_callback,
+                                .arg = timer
                              } };
 }
 
 void ztimer_periodic_start(ztimer_periodic_t *timer)
 {
-    ztimer_acquire(timer->clock);
-
-    uint32_t now = ztimer_now(timer->clock);
-
-    timer->last = now;
-    _ztimer_periodic_reset(timer, now);
+    timer->last = ztimer_set(timer->clock, &timer->timer, timer->interval) + timer->interval;
 }
 
 void ztimer_periodic_stop(ztimer_periodic_t *timer)
 {
     ztimer_remove(timer->clock, &timer->timer);
-
-    ztimer_release(timer->clock);
 }