ports/stm32: Fix SysTick priority for Zephyr threading.

When MICROPY_ZEPHYR_THREADING is enabled, SysTick must be maskable by
BASEPRI (priority >= 2) to allow Zephyr's arch_irq_lock() to protect
critical sections. The STM32 HAL in system_stm32.c was overwriting
the correct priority set in main.c with TICK_INT_PRIORITY (0x00),
making SysTick non-maskable.

This caused race conditions where SysTick could fire during critical
sections in the scheduler, corrupting thread state.

Make SysTick priority conditional on MICROPY_ZEPHYR_THREADING to
preserve the correct maskable priority (IRQ_PRI_SYSTICK = 2).

Signed-off-by: Andrew Leech <[email protected]>

Author: pi-anl

ports/stm32/system_stm32.c

@@ -585,8 +585,14 @@ MP_WEAK void SystemClock_Config(void) {
 
     HAL_SYSTICK_Config(HAL_RCC_GetHCLKFreq() / 1000);
     HAL_SYSTICK_CLKSourceConfig(SYSTICK_CLKSOURCE_HCLK);
+    // For Zephyr threading, maintain IRQ_PRI_SYSTICK priority set in main.c (priority 2, maskable)
+    // For normal mode, use TICK_INT_PRIORITY (priority 0, non-maskable)
+    #if MICROPY_ZEPHYR_THREADING
+    NVIC_SetPriority(SysTick_IRQn, IRQ_PRI_SYSTICK);
+    #else
     NVIC_SetPriority(SysTick_IRQn, NVIC_EncodePriority(NVIC_PRIORITYGROUP_4, TICK_INT_PRIORITY, 0));
     #endif
+    #endif
 
     #if defined(STM32H7) && !defined(NDEBUG)
     // Enable the Debug Module in low-power modes.

ports/stm32/zephyr_arch_stm32.c

@@ -180,7 +180,8 @@ void mp_zephyr_arch_init(void) {
 // Enable SysTick interrupt - must be called AFTER kernel is fully initialized
 // This should be called from micropython_main_thread_entry() after z_cstart() completes
 void mp_zephyr_arch_enable_systick_interrupt(void) {
-    // Set SysTick priority to maskable level (SystemClock_Config resets it to 0x00)
+    // Verify SysTick priority is still at maskable level
+    // (system_stm32.c now preserves IRQ_PRI_SYSTICK for Zephyr threading)
     NVIC_SetPriority(SysTick_IRQn, IRQ_PRI_SYSTICK);
     // Enable SysTick interrupt (add TICKINT bit to existing configuration)
     *SYST_CSR_ADDR = SYST_CSR_ENABLE | SYST_CSR_CLKSOURCE | SYST_CSR_TICKINT;