rp2: Refactor to not use pico-sdk alarm pool functions for sleeping.

The best_effort_wfe_or_timeout() and sleep_us() pico-sdk functions use the
pico-sdk alarm pool internally, and that has a bug.

Some usages inside pico-sdk (notably multicore_lockout_start_blocking())
will still end up calling best_effort_wfe_or_timeout(), although usually
with "end_of_time" as the timeout value so it should avoid any alarm pool
race conditions.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

ports/rp2/mpconfigport.h

@@ -263,6 +263,7 @@ typedef intptr_t mp_off_t;
 #define BINARY_INFO_ID_MP_FROZEN 0x4a99d719
 #define MICROPY_FROZEN_LIST_ITEM(name, file) bi_decl(bi_string(BINARY_INFO_TAG_MICROPYTHON, BINARY_INFO_ID_MP_FROZEN, name))
 
+
 extern uint32_t rosc_random_u32(void);
 extern void lwip_lock_acquire(void);
 extern void lwip_lock_release(void);

ports/rp2/mphalport.c

@@ -198,17 +198,31 @@ mp_uint_t mp_hal_stdout_tx_strn(const char *str, mp_uint_t len) {
     return did_write ? ret : 0;
 }
 
+void mp_hal_delay_us(mp_uint_t us) {
+    // Avoid calling sleep_us() and invoking the alarm pool by splitting long
+    // sleeps into an optional longer sleep and a shorter busy-wait
+    uint64_t end = time_us_64() + us;
+    if (us > 1000) {
+        mp_hal_delay_ms(us / 1000);
+    }
+    while (time_us_64() < end) {
+        // Tight loop busy-wait for accurate timing
+    }
+}
+
 void mp_hal_delay_ms(mp_uint_t ms) {
-    absolute_time_t t = make_timeout_time_ms(ms);
+    mp_uint_t start = mp_hal_ticks_ms();
+    mp_uint_t elapsed = 0;
     do {
-        mp_event_handle_nowait();
-    } while (!best_effort_wfe_or_timeout(t));
+        mp_event_wait_ms(ms - elapsed);
+        elapsed = mp_hal_ticks_ms() - start;
+    } while (elapsed < ms);
 }
 
 void mp_hal_time_ns_set_from_rtc(void) {
-    // Delay at least one RTC clock cycle so it's registers have updated with the most
-    // recent time settings.
-    sleep_us(23);
+    // Outstanding RTC register writes need at least two RTC clock cycles to
+    // update. (See RP2040 datasheet section 4.8.4 "Reference clock").
+    mp_hal_delay_us(44);
 
     // Sample RTC and time_us_64() as close together as possible, so the offset
     // calculated for the latter can be as accurate as possible.
@@ -295,3 +309,17 @@ void soft_timer_init(void) {
     hardware_alarm_claim(MICROPY_HW_SOFT_TIMER_ALARM_NUM);
     hardware_alarm_set_callback(MICROPY_HW_SOFT_TIMER_ALARM_NUM, soft_timer_hardware_callback);
 }
+
+void mp_wfe_or_timeout(uint32_t timeout_ms) {
+    soft_timer_entry_t timer;
+
+    // Note the timer doesn't have an associated callback, it just exists to create a
+    // hardware interrupt to wake the CPU
+    soft_timer_static_init(&timer, SOFT_TIMER_MODE_ONE_SHOT, 0, NULL);
+    soft_timer_insert(&timer, timeout_ms);
+
+    __wfe();
+
+    // Clean up the timer node if it's not already
+    soft_timer_remove(&timer);
+}

ports/rp2/mphalport.h

@@ -57,23 +57,22 @@
         if ((TIMEOUT_MS) < 0) { \
             __wfe(); \
         } else { \
-            best_effort_wfe_or_timeout(make_timeout_time_ms(TIMEOUT_MS)); \
+            mp_wfe_or_timeout(TIMEOUT_MS); \
         } \
     } while (0)
 
 extern int mp_interrupt_char;
 extern ringbuf_t stdin_ringbuf;
 
+// Port-specific function to create a wakeup interrupt after timeout_ms and enter WFE
+void mp_wfe_or_timeout(uint32_t timeout_ms);
+
 uint32_t mp_thread_begin_atomic_section(void);
 void mp_thread_end_atomic_section(uint32_t);
 
 void mp_hal_set_interrupt_char(int c);
 void mp_hal_time_ns_set_from_rtc(void);
 
-static inline void mp_hal_delay_us(mp_uint_t us) {
-    sleep_us(us);
-}
-
 static inline void mp_hal_delay_us_fast(mp_uint_t us) {
     busy_wait_us(us);
 }

shared/runtime/softtimer.c

@@ -89,7 +89,7 @@ void soft_timer_handler(void) {
         heap = (soft_timer_entry_t *)mp_pairheap_pop(soft_timer_lt, &heap->pairheap);
         if (entry->flags & SOFT_TIMER_FLAG_PY_CALLBACK) {
             mp_sched_schedule(entry->py_callback, MP_OBJ_FROM_PTR(entry));
-        } else {
+        } else if (entry->c_callback) {
             entry->c_callback(entry);
         }
         if (entry->mode == SOFT_TIMER_MODE_PERIODIC) {