ztimer_periodic: make callback function return bool

The callback function of `ztimer_periodic` is only expected to have
two states.
So let it return `true` if the timer should keep repeating, `false`
otherwise.

sys/event/periodic.c

@@ -22,7 +22,7 @@
 #include "ztimer/periodic.h"
 #include "event/periodic.h"
 
-static int _event_periodic_callback(void *arg)
+static bool _event_periodic_callback(void *arg)
 {
     event_periodic_t *event_periodic = (event_periodic_t *)arg;
 

sys/include/ztimer/periodic.h

@@ -66,6 +66,7 @@
 #ifndef ZTIMER_PERIODIC_H
 #define ZTIMER_PERIODIC_H
 
+#include <stdbool.h>
 #include <stdint.h>
 
 #include "ztimer.h"
@@ -77,7 +78,7 @@ extern "C" {
 /**
  * @brief Periodic timer stop unless it returns this value
  */
-#define ZTIMER_PERIODIC_KEEP_GOING 0
+#define ZTIMER_PERIODIC_KEEP_GOING true
 
 /**
  * @brief   ztimer periodic structure
@@ -87,7 +88,7 @@ typedef struct {
     ztimer_clock_t *clock;      /**< clock for this timer               */
     uint32_t interval;          /**< interval of this timer             */
     ztimer_now_t last;          /**< last trigger time                  */
-    int (*callback)(void *);    /**< called on each trigger             */
+    bool (*callback)(void *);   /**< called on each trigger             */
     void *arg;                  /**< argument for callback              */
 } ztimer_periodic_t;
 
@@ -100,11 +101,12 @@ typedef struct {
  * @param[in]       clock       the clock to configure this timer on
  * @param[inout]    timer       periodic timer object to initialize
  * @param[in]       callback    function to call on each trigger
+ *                              returns `true` if the timer should keep going
  * @param[in]       arg         argument to pass to callback function
  * @param[in]       interval    period length of this timer instance
  */
 void ztimer_periodic_init(ztimer_clock_t *clock, ztimer_periodic_t *timer,
-                          int (*callback)(void *),
+                          bool (*callback)(void *),
                           void *arg, uint32_t interval);
 
 /**

sys/ztimer/periodic.c

@@ -54,7 +54,7 @@ static void _ztimer_periodic_callback(void *arg)
 }
 
 void ztimer_periodic_init(ztimer_clock_t *clock, ztimer_periodic_t *timer,
-                          int (*callback)(
+                          bool (*callback)(
                               void *), void *arg, uint32_t interval)
 {
     *timer =

tests/ztimer_periodic/main.c

@@ -41,7 +41,7 @@ static const char *_names[] = { "ZTIMER_MSEC", "ZTIMER_USEC" };
 static uint32_t _intervals[] = { 100, 10000 };
 static uint32_t _max_offsets[] = { 2, 100 };
 
-static int callback(void *arg)
+static bool callback(void *arg)
 {
     _times[_count] = ztimer_now(arg);
 
@@ -58,7 +58,7 @@ static int callback(void *arg)
         mutex_unlock(&_mutex);
     }
 
-    return (_count == REPEAT);
+    return _count < REPEAT;
 }
 
 int main(void)