rp2: Refactor soft timer to use hardware timer alarm.

Progress towards removing pico-sdk alarm pool, due to a known issue.

This work was funded through GitHub Sponsors.

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

ports/rp2/main.c

@@ -76,6 +76,8 @@ int main(int argc, char **argv) {
     // This is a tickless port, interrupts should always trigger SEV.
     SCB->SCR |= SCB_SCR_SEVONPEND_Msk;
 
+    soft_timer_init();
+
     #if MICROPY_HW_ENABLE_UART_REPL
     bi_decl(bi_program_feature("UART REPL"))
     setup_default_uart();

ports/rp2/mpconfigport.h

@@ -154,6 +154,10 @@
 #define MICROPY_SSL_MBEDTLS                     (1)
 #define MICROPY_PY_LWIP_SOCK_RAW                (MICROPY_PY_LWIP)
 
+// Hardware timer alarm index. Available range 0-3.
+// Number 3 is currently used by pico-sdk (PICO_TIME_DEFAULT_ALARM_POOL_HARDWARE_ALARM_NUM)
+#define MICROPY_HW_SOFT_TIMER_ALARM_NUM         (2)
+
 // fatfs configuration
 #define MICROPY_FATFS_ENABLE_LFN                (1)
 #define MICROPY_FATFS_LFN_CODE_PAGE             437 /* 1=SFN/ANSI 437=LFN/U.S.(OEM) */

ports/rp2/mphalport.c

@@ -35,6 +35,7 @@
 #include "pendsv.h"
 #include "tusb.h"
 #include "uart.h"
+#include "hardware/irq.h"
 #include "hardware/rtc.h"
 #include "pico/unique_id.h"
 
@@ -46,8 +47,6 @@
 // microseconds since the Epoch.
 static uint64_t time_us_64_offset_from_epoch;
 
-static alarm_id_t soft_timer_alarm_id = 0;
-
 #if MICROPY_HW_ENABLE_UART_REPL || MICROPY_HW_USB_CDC
 
 #ifndef MICROPY_HW_STDIN_BUFFER_LEN
@@ -273,21 +272,26 @@ uint32_t storage_read_blocks(uint8_t *dest, uint32_t block_num, uint32_t num_blo
     panic_unsupported();
 }
 
-static int64_t soft_timer_callback(alarm_id_t id, void *user_data) {
-    soft_timer_alarm_id = 0;
-    pendsv_schedule_dispatch(PENDSV_DISPATCH_SOFT_TIMER, soft_timer_handler);
-    return 0; // don't reschedule this alarm
-}
-
 uint32_t soft_timer_get_ms(void) {
     return mp_hal_ticks_ms();
 }
 
 void soft_timer_schedule_at_ms(uint32_t ticks_ms) {
-    if (soft_timer_alarm_id != 0) {
-        cancel_alarm(soft_timer_alarm_id);
-    }
     int32_t ms = soft_timer_ticks_diff(ticks_ms, mp_hal_ticks_ms());
     ms = MAX(0, ms);
-    soft_timer_alarm_id = add_alarm_in_ms(ms, soft_timer_callback, NULL, true);
+    if (hardware_alarm_set_target(MICROPY_HW_SOFT_TIMER_ALARM_NUM, delayed_by_ms(get_absolute_time(), ms))) {
+        // "missed" hardware alarm target
+        hardware_alarm_force_irq(MICROPY_HW_SOFT_TIMER_ALARM_NUM);
+    }
+}
+
+static void soft_timer_hardware_callback(unsigned int alarm_num) {
+    // The timer alarm ISR needs to call here and trigger PendSV dispatch via
+    // a second ISR, as PendSV may be currently suspended by the other CPU.
+    pendsv_schedule_dispatch(PENDSV_DISPATCH_SOFT_TIMER, soft_timer_handler);
+}
+
+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);
 }

ports/rp2/mpnetworkport.c

@@ -66,7 +66,7 @@ static void gpio_irq_handler(void) {
 void cyw43_irq_init(void) {
     gpio_add_raw_irq_handler_with_order_priority(CYW43_PIN_WL_HOST_WAKE, gpio_irq_handler, CYW43_SHARED_IRQ_HANDLER_PRIORITY);
     irq_set_enabled(IO_IRQ_BANK0, true);
-    NVIC_SetPriority(PendSV_IRQn, PICO_LOWEST_IRQ_PRIORITY);
+    NVIC_SetPriority(PendSV_IRQn, IRQ_PRI_PENDSV);
 }
 
 void cyw43_post_poll_hook(void) {

ports/rp2/pendsv.h

@@ -42,6 +42,9 @@ enum {
 
 #define PENDSV_DISPATCH_NUM_SLOTS PENDSV_DISPATCH_MAX
 
+// PendSV IRQ priority, to run system-level tasks that preempt the main thread.
+#define IRQ_PRI_PENDSV PICO_LOWEST_IRQ_PRIORITY
+
 typedef void (*pendsv_dispatch_t)(void);
 
 void pendsv_suspend(void);

shared/runtime/softtimer.h

@@ -81,6 +81,9 @@ static inline void soft_timer_reinsert(soft_timer_entry_t *entry, uint32_t initi
 //   pend-SV IRQ level, or equivalent.
 uint32_t soft_timer_get_ms(void);
 void soft_timer_schedule_at_ms(uint32_t ticks_ms);
+
+// Optional port-specific initialisation function (provided and called by the port if needed).
+void soft_timer_init(void);
 #endif
 
 #endif // MICROPY_INCLUDED_SHARED_RUNTIME_SOFTTIMER_H