core/hwtimer: add HWTIMER_WAIT_OVERHEAD

core/hwtimer.c

@@ -129,9 +129,7 @@ void hwtimer_wait(unsigned long ticks)
 
     mutex_t mutex = MUTEX_INIT;
     mutex_lock(&mutex);
-    /* TODO: introduce HWTIMER_INSTRUCTIONS_PER_TICK?
-     * -2 is to adjust the real value */
-    int res = hwtimer_set(ticks - 2, hwtimer_releasemutex, &mutex);
+    int res = hwtimer_set(ticks - (HWTIMER_WAIT_OVERHEAD), hwtimer_releasemutex, &mutex);
     if (res == -1) {
         mutex_unlock(&mutex);
         hwtimer_spin(ticks);

core/include/hwtimer.h

@@ -60,6 +60,23 @@ per second for the current architecture."
 #define HWTIMER_SPIN_BARRIER (6)
 #endif
 
+/**
+ * @brief       Overhead of the `hwtimer_wait` function
+ *
+ * @verbatim    This value is used to decrease the number of ticks that
+ *              `hwtimer_wait` uses to set the actual hardware timer.
+ *
+ *              The goal is to make sure the number of ticks spent in the
+ *              function corresponds to the ticks argument it was given.
+ *
+ *              Boards should override this.
+ *
+ * @def         HWTIMER_WAIT_OVERHEAD
+ */
+#ifndef HWTIMER_WAIT_OVERHEAD
+#define HWTIMER_WAIT_OVERHEAD (2)
+#endif
+
 /**
  * @brief       Convert microseconds to kernel timer ticks
  * @param[in]   us number of microseconds