rp2/pendsv: Account for PendSV running on both cores, and without CYW43.

Changes:
- Move setting of PendSV priority to pendsv_init().
- Call pendsv_init() from CPU1 as well, to ensure priority is the same.

This work was funded through GitHub Sponsors.

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

ports/rp2/mpnetworkport.c

@@ -42,7 +42,6 @@ static soft_timer_entry_t mp_network_soft_timer;
 #if MICROPY_PY_NETWORK_CYW43
 #include "lib/cyw43-driver/src/cyw43.h"
 #include "lib/cyw43-driver/src/cyw43_stats.h"
-#include "hardware/irq.h"
 
 #if !defined(__riscv)
 #if PICO_RP2040
@@ -97,9 +96,6 @@ 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);
-    #if !defined(__riscv)
-    NVIC_SetPriority(PendSV_IRQn, IRQ_PRI_PENDSV);
-    #endif
 }
 
 // This hook will run on whichever CPU serviced the PendSV interrupt

ports/rp2/mpthreadport.c

@@ -106,6 +106,9 @@ static void core1_entry_wrapper(void) {
     // Allow MICROPY_BEGIN_ATOMIC_SECTION to be invoked from core0.
     multicore_lockout_victim_init();
 
+    // Set PendSV interrupt priority correctly for CPU1
+    pendsv_init();
+
     if (core1_entry) {
         core1_entry(core1_arg);
     }

ports/rp2/pendsv.c

@@ -28,6 +28,7 @@
 #include "py/mpconfig.h"
 #include "py/mpthread.h"
 #include "pendsv.h"
+#include "hardware/irq.h"
 
 #if PICO_RP2040
 #include "RP2040.h"
@@ -45,6 +46,9 @@ static pendsv_dispatch_t pendsv_dispatch_table[PENDSV_DISPATCH_NUM_SLOTS];
 
 static inline void pendsv_resume_run_dispatch(void);
 
+// PendSV IRQ priority, to run system-level tasks that preempt the main thread.
+#define IRQ_PRI_PENDSV PICO_LOWEST_IRQ_PRIORITY
+
 void PendSV_Handler(void);
 
 #if MICROPY_PY_THREAD
@@ -53,8 +57,14 @@ void PendSV_Handler(void);
 // loop of mp_wfe_or_timeout(), where we don't want the CPU event bit to be set.
 static mp_thread_recursive_mutex_t pendsv_mutex;
 
+// Called from CPU0 during boot, but may be called later when CPU1 wakes up
 void pendsv_init(void) {
-    mp_thread_recursive_mutex_init(&pendsv_mutex);
+    if (get_core_num() == 0) {
+        mp_thread_recursive_mutex_init(&pendsv_mutex);
+    }
+    #if !defined(__riscv)
+    NVIC_SetPriority(PendSV_IRQn, IRQ_PRI_PENDSV);
+    #endif
 }
 
 void pendsv_suspend(void) {
@@ -117,11 +127,13 @@ static inline void pendsv_resume_run_dispatch(void) {
 
 void pendsv_schedule_dispatch(size_t slot, pendsv_dispatch_t f) {
     pendsv_dispatch_table[slot] = f;
+    // There is a race here where other core calls pendsv_suspend() before ISR
+    // can execute so this check fails, but dispatch will happen later when
+    // other core calls pendsv_resume().
     if (pendsv_suspend_count() == 0) {
         #if PICO_ARM
-        // There is a race here where other core calls pendsv_suspend() before
+        // Note this register is part of each CPU core, so setting it on CPUx
-        // ISR can execute, but dispatch will happen later when other core
+        // will set the IRQ and run PendSV_Handler on CPUx only.
-        // calls pendsv_resume().
         SCB->ICSR = SCB_ICSR_PENDSVSET_Msk;
         #elif PICO_RISCV
         struct timespec ts;
@@ -136,15 +148,19 @@ void pendsv_schedule_dispatch(size_t slot, pendsv_dispatch_t f) {
 }
 
 // PendSV interrupt handler to perform background processing.
+//
+// Handler can execute on either CPU if MICROPY_PY_THREAD is set (no code on
+// CPU1 calls pendsv_schedule_dispatch(), but CPU1 can call pendsv_resume()
+// which will trigger it).
 void PendSV_Handler(void) {
 
     #if MICROPY_PY_THREAD
     if (!mp_thread_recursive_mutex_lock(&pendsv_mutex, 0)) {
-        // Failure here means core 1 holds pendsv_mutex. ISR will
+        // Failure here means other core holds pendsv_mutex. ISR will
-        // run again after core 1 calls pendsv_resume().
+        // run again after that core calls pendsv_resume().
         return;
     }
-    // Core 0 should not already have locked pendsv_mutex
+    // This core should not already have locked pendsv_mutex
     assert(pendsv_mutex.mutex.enter_count == 1);
     #else
     assert(pendsv_suspend_count() == 0);

ports/rp2/pendsv.h

@@ -42,9 +42,6 @@ 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_init(void);