shared/runtime/softtimer: Fix ticks range when computing ticks diff.

The previous computation incorrectly assumed that the uint32_t ticks
counter MICROPY_SOFT_TIMER_TICKS_MS was in the range [0,0x80000000) where
its actually [0,0xffffffff].  This means the diff calculation can be
simplified compared to the original implementation copied from
utime_mphal.c, which has to deal with a ticks range constrained by the
small int range.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

shared/runtime/softtimer.c

@@ -30,9 +30,6 @@
 #include "py/runtime.h"
 #include "softtimer.h"
 
-#define TICKS_PERIOD 0x80000000
-#define TICKS_DIFF(t1, t0) ((int32_t)(((t1 - t0 + TICKS_PERIOD / 2) & (TICKS_PERIOD - 1)) - TICKS_PERIOD / 2))
-
 extern __IO uint32_t MICROPY_SOFT_TIMER_TICKS_MS;
 
 volatile uint32_t soft_timer_next;
@@ -42,16 +39,23 @@ volatile uint32_t soft_timer_next;
 // and is explicitly GC traced by soft_timer_gc_mark_all().
 STATIC soft_timer_entry_t *soft_timer_heap;
 
+static inline int32_t ticks_diff(uint32_t t1, uint32_t t0) {
+    // t1 is after t0 (i.e. positive result) if there exists a uint32_t X <= INT_MAX
+    // such that t0 + X = t1. Otherwise t1 is interepreted to be earlier than
+    // t0 (negative result).
+    return t1 - t0;
+}
+
 STATIC int soft_timer_lt(mp_pairheap_t *n1, mp_pairheap_t *n2) {
     soft_timer_entry_t *e1 = (soft_timer_entry_t *)n1;
     soft_timer_entry_t *e2 = (soft_timer_entry_t *)n2;
-    return TICKS_DIFF(e1->expiry_ms, e2->expiry_ms) < 0;
+    return ticks_diff(e1->expiry_ms, e2->expiry_ms) < 0;
 }
 
 STATIC void soft_timer_schedule_systick(uint32_t ticks_ms) {
     uint32_t irq_state = disable_irq();
     uint32_t uw_tick = MICROPY_SOFT_TIMER_TICKS_MS;
-    if (TICKS_DIFF(ticks_ms, uw_tick) <= 0) {
+    if (ticks_diff(ticks_ms, uw_tick) <= 0) {
         soft_timer_next = uw_tick + 1;
     } else {
         soft_timer_next = ticks_ms;
@@ -79,7 +83,7 @@ void soft_timer_deinit(void) {
 void soft_timer_handler(void) {
     uint32_t ticks_ms = MICROPY_SOFT_TIMER_TICKS_MS;
     soft_timer_entry_t *heap = soft_timer_heap;
-    while (heap != NULL && TICKS_DIFF(heap->expiry_ms, ticks_ms) <= 0) {
+    while (heap != NULL && ticks_diff(heap->expiry_ms, ticks_ms) <= 0) {
         soft_timer_entry_t *entry = heap;
         heap = (soft_timer_entry_t *)mp_pairheap_pop(soft_timer_lt, &heap->pairheap);
         if (entry->flags & SOFT_TIMER_FLAG_PY_CALLBACK) {