xtimer: xtimer_periodic_wakeup

1. interrupt save.
2. Change targed passed criteria.
3. Recalculate last_wakeup to be a multiple of periods after now.
   This skips missed targets and avoids a race condition.

sys/xtimer/xtimer.c

@@ -1,7 +1,8 @@
 /*
  * Copyright (C) 2015 Kaspar Schleiser <kaspar@schleiser.de>
- * Copyright (C) 2016 Eistec AB
- *
+ *               2016 Eistec AB
+ *               2018 Josua Arndt
+ * 
  * This file is subject to the terms and conditions of the GNU Lesser
  * General Public License v2.1. See the file LICENSE in the top level
  * directory for more details.
@@ -15,6 +16,7 @@
  * @brief xtimer convenience functionality
  * @author Kaspar Schleiser <kaspar@schleiser.de>
  * @author Joakim Nohlgård <joakim.nohlgard@eistec.se>
+ * @author Josua Arndt <jarndt@ias.rwth-aachen.de>
  * @}
  */
 
@@ -71,33 +73,53 @@ void _xtimer_tsleep(uint32_t offset, uint32_t long_offset)
 }
 
 void _xtimer_periodic_wakeup(uint32_t *last_wakeup, uint32_t period) {
+
     xtimer_t timer;
     mutex_t mutex = MUTEX_INIT;
+    uint32_t mult;
 
     timer.callback = _callback_unlock_mutex;
     timer.arg = (void*) &mutex;
 
     uint32_t target = (*last_wakeup) + period;
+
+
+    /* wait for hardware timer overflow */
+    uint32_t max = _xtimer_lltimer_mask(0xFFFFFFFF) - XTIMER_BACKOFF;
+    /* make sure the timer counter arrives in the next timer period */
+    while( _xtimer_lltimer_now() >= max ){}
+
+    unsigned state = irq_disable();
     uint32_t now = _xtimer_now();
+
     /* make sure we're not setting a value in the past */
     if (now < (*last_wakeup)) {
-        /* base timer overflowed between last_wakeup and now */
-        if (!((now < target) && (target < (*last_wakeup)))) {
+        /* last_wakeup < target, base overflowed but target not, target passed. */
+        /* target <= now , both overflowed, target passed. */
+        if ( (*last_wakeup < target) || (target <= now) ) {
             /* target time has already passed */
-            *last_wakeup = now;
+            now = _xtimer_now();
+            /* now - target, will always be the difference. (modulo power of two) */
+            mult = (now - target)/ period;
+            *last_wakeup = (mult * period) + target;
+            irq_restore(state);
             return;
         }
     }
     else {
-        /* base timer did not overflow */
-        if ((((*last_wakeup) <= target) && (target <= now))) {
+        /* last_wakeup < now, base timer did not overflow */
+        /* target <= now AND target did not overflow, target passed */
+        if ( (target <= now) && ((*last_wakeup) <= target) ) {
             /* target time has already passed */
-            *last_wakeup = now;
+            now = _xtimer_now();
+            mult = (now - target)/ period;
+            *last_wakeup = (mult * period) + target;
+            irq_restore(state);
             return;
         }
     }
 
-    /*
+    /* For very small offsets, spin.
      * For large offsets, set an absolute target time.
      * As that might cause an underflow, for small offsets, set a relative
      * target time.
@@ -115,11 +137,15 @@ void _xtimer_periodic_wakeup(uint32_t *last_wakeup, uint32_t period) {
      * tl;dr Don't return too early!
      */
     uint32_t offset = target - now;
-    DEBUG("xps, now: %9" PRIu32 ", tgt: %9" PRIu32 ", off: %9" PRIu32 "\n", now, target, offset);
+    DEBUG("xps, now: %9" PRIu32 ", tgt: %9" PRIu32 ", off: %9" PRIu32 ", backoff: %9" PRIu32 "\n",
+          now, target, offset, (uint32_t)XTIMER_PERIODIC_SPIN);
+
     if (offset < XTIMER_PERIODIC_SPIN) {
+        irq_restore(state);
         _xtimer_spin(offset);
     }
-    else {
+    else
+    {
         if (offset < XTIMER_PERIODIC_RELATIVE) {
             /* NB: This will overshoot the target by the amount of time it took
              * to get here from the beginning of xtimer_periodic_wakeup()
@@ -130,9 +156,24 @@ void _xtimer_periodic_wakeup(uint32_t *last_wakeup, uint32_t period) {
         }
         mutex_lock(&mutex);
         DEBUG("xps, abs: %" PRIu32 "\n", target);
+        irq_restore(state);
         _xtimer_set_absolute(&timer, target);
         mutex_lock(&mutex);
     }
+    /*
+     * Note: last_wakeup _must never_ specify a time in the future after
+     * _xtimer_periodic_sleep returns.
+     * If this happens, last_wakeup may specify a time in the future when the
+     * next call to _xtimer_periodic_sleep is made, which in turn will trigger
+     * the overflow logic above and make the next timer fire too early, causing
+     * last_wakeup to point even further into the future, leading to a chain
+     * reaction.
+     *
+     * tl;dr Don't return too early!
+     *
+     * This is ensured in xtimer_core _timer_callback with setting
+     * XTIMER_SHOOT_OVERHEAD
+     */
 
     *last_wakeup = target;
 }