Fix cumulative drift in periodic timers

Currently, periodic timers allow errors to accumulate because the next
deadline is calculated relative to the current system tick (now +
period). Any latency in the tick handler execution—whether due to
interrupt jitter or system load—permanently shifts the phase of all
future expirations.

Fix the drift by anchoring the timer schedule to an absolute baseline.
A new field, last_expected_fire_tick, is introduced to track the
theoretical expiration time.

When the timer fires, the next deadline is now computed by advancing
this expected time by the period, rather than relying on the actual
execution time. This ensures the timer maintains its long-term
frequency accuracy regardless of short-term scheduling delays.

Author: visitorckw

include/sys/timer.h

@@ -26,7 +26,10 @@ typedef enum {
 typedef struct {
     /* Timing Parameters */
     uint32_t deadline_ticks; /* Expiration time in absolute system ticks */
-    uint32_t period_ms;      /* Reload period in milliseconds */
+    uint32_t last_expected_fire_tick; /* Last calculated expected fire time for
+                                       * periodic timer
+                                       */
+    uint32_t period_ms;               /* Reload period in milliseconds */
 
     /* Timer Identification and State */
     uint16_t id;       /* Unique handle assigned by the kernel */

kernel/timer.c

@@ -251,7 +251,9 @@ void _timer_tick_handler(void)
 
         /* Handle auto-reload timers */
         if (t->mode == TIMER_AUTORELOAD) {
-            t->deadline_ticks = now + MS_TO_TICKS(t->period_ms);
+            /* Calculate next expected fire tick to prevent cumulative error */
+            t->last_expected_fire_tick += MS_TO_TICKS(t->period_ms);
+            t->deadline_ticks = t->last_expected_fire_tick;
             timer_sorted_insert(t); /* Re-insert for next expiration */
         } else {
             t->mode = TIMER_DISABLED; /* One-shot timers are done */
@@ -305,6 +307,7 @@ int32_t mo_timer_create(void *(*callback)(void *arg),
     t->arg = arg;
     t->period_ms = period_ms;
     t->deadline_ticks = 0;
+    t->last_expected_fire_tick = 0;
     t->mode = TIMER_DISABLED;
     t->_reserved = 0;
 
@@ -387,7 +390,8 @@ int32_t mo_timer_start(uint16_t id, uint8_t mode)
 
     /* Configure and start timer */
     t->mode = mode;
-    t->deadline_ticks = mo_ticks() + MS_TO_TICKS(t->period_ms);
+    t->last_expected_fire_tick = mo_ticks() + MS_TO_TICKS(t->period_ms);
+    t->deadline_ticks = t->last_expected_fire_tick;
 
     if (unlikely(timer_sorted_insert(t) != ERR_OK)) {
         t->mode = TIMER_DISABLED;