ports/stm32: Add Zephyr threading support.

Enable threading on STM32 ports using the Zephyr kernel integration layer.
Tested on NUCLEO_F429ZI with 34/34 thread tests passing.

Key changes:
- Makefile: Add MICROPY_ZEPHYR_THREADING option and Zephyr kernel build rules
- main.c: Initialize Zephyr kernel and configure static heap for threading
- mpconfigport.h: Thread configuration with GIL and EVENT_POLL_HOOK
- stm32_it.c: Fault handler instrumentation for debugging
- zephyr_arch_stm32.c: STM32-specific Zephyr architecture implementation
- SysTick/PendSV: Integrate with Zephyr scheduler for context switching

Enable with: make BOARD=NUCLEO_F429ZI MICROPY_ZEPHYR_THREADING=1

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

Author: pi-anl

ports/stm32/Makefile

@@ -60,6 +60,72 @@ include $(TOP)/extmod/extmod.mk
 
 GIT_SUBMODULES += lib/libhydrogen lib/stm32lib lib/tinyusb
 
+# Zephyr threading integration (optional)
+MICROPY_ZEPHYR_THREADING ?= 0
+
+ifeq ($(MICROPY_ZEPHYR_THREADING),1)
+# Add Zephyr submodule
+GIT_SUBMODULES += lib/zephyr
+
+# Remove -Werror for Zephyr headers
+CFLAGS := $(filter-out -Werror,$(CFLAGS))
+
+# Set architecture for Zephyr
+ZEPHYR_ARCH := arm
+
+# Enable idle thread for k_msleep() support in EVENT_POLL_HOOK
+# MUST be set BEFORE including zephyr_kernel.mk so idle.c is compiled
+MICROPY_ZEPHYR_USE_IDLE_THREAD = 1
+
+# Include Zephyr kernel build
+include $(TOP)/extmod/zephyr_kernel/zephyr_kernel.mk
+
+# Add Zephyr includes and flags
+CFLAGS += $(ZEPHYR_INC)
+CFLAGS += -Wno-error -Wno-macro-redefined
+# Define MICROPY_ZEPHYR_THREADING as a preprocessor symbol for C code
+CFLAGS += -DMICROPY_ZEPHYR_THREADING=1
+CFLAGS += -DMICROPY_ZEPHYR_USE_IDLE_THREAD=1
+
+# CRITICAL: Explicitly add optimization flags for Zephyr builds
+# Without this, code is compiled with no optimization, causing 50-100x code size bloat
+CFLAGS += -Os -DNDEBUG
+
+# Add STM32-specific Zephyr architecture implementation
+SRC_C += zephyr_arch_stm32.c
+
+# Ensure Zephyr objects depend on generated headers
+$(addprefix $(BUILD)/, $(SRC_THIRDPARTY_C:.c=.o)): $(ZEPHYR_GEN_HEADERS)
+
+# Pattern-specific CFLAGS for Zephyr objects
+# Zephyr kernel files: Use STM32 CMSIS headers + Zephyr config (for CONFIG_* symbols)
+$(BUILD)/$(TOP)/lib/zephyr/%.o: CFLAGS += -include $(TOP)/extmod/zephyr_kernel/zephyr_config_cortex_m.h -Wno-unused-parameter -Wno-sign-compare -Wno-unused-function -Wno-return-type
+$(BUILD)/$(TOP)/extmod/zephyr_kernel/%.o: CFLAGS += -Wno-unused-parameter -Wno-unused-function
+
+# Build Zephyr assembly files (context switching, PSP initialization)
+ZEPHYR_SWAP_HELPER_O = $(BUILD)/swap_helper.o
+ZEPHYR_PSP_SWITCH_O = $(BUILD)/zephyr_psp_switch.o
+OBJ += $(ZEPHYR_SWAP_HELPER_O)
+OBJ += $(ZEPHYR_PSP_SWITCH_O)
+OBJ += $(BUILD)/zephyr_offsets.o
+
+# Common assembler flags for Zephyr .S files
+ZEPHYR_AS_FLAGS = -x assembler-with-cpp -D_ASMLANGUAGE \
+	$(filter-out -std=% -Wdouble-promotion -Wfloat-conversion -Werror,$(CFLAGS)) \
+	$(CFLAGS_MCU_$(MCU_SERIES)) \
+	-include $(TOP)/extmod/zephyr_kernel/zephyr_config_cortex_m.h \
+	-Wa,-mimplicit-it=thumb
+
+$(ZEPHYR_SWAP_HELPER_O): $(TOP)/lib/zephyr/arch/arm/core/cortex_m/swap_helper.S $(ZEPHYR_GEN_HEADERS)
+	$(ECHO) "CC $<"
+	$(Q)$(CC) $(ZEPHYR_AS_FLAGS) -c -o $@ $<
+
+$(ZEPHYR_PSP_SWITCH_O): $(TOP)/extmod/zephyr_kernel/zephyr_psp_switch.S
+	$(ECHO) "CC $<"
+	$(Q)$(CC) $(ZEPHYR_AS_FLAGS) -c -o $@ $<
+
+endif
+
 CROSS_COMPILE ?= arm-none-eabi-
 LD_DIR=boards
 USBDEV_DIR=usbdev
@@ -114,7 +180,11 @@ INC += -I$(TOP)/lib/tinyusb/src
 INC += -I$(TOP)/shared/tinyusb/
 INC += -Ilwip_inc
 
+ifneq ($(MICROPY_ZEPHYR_THREADING),1)
 CFLAGS += $(INC) -Wall -Wpointer-arith -Werror -Wdouble-promotion -Wfloat-conversion -std=gnu99 -nostdlib $(CFLAGS_EXTRA)
+else
+CFLAGS += $(INC) -Wall -Wpointer-arith -Wdouble-promotion -Wfloat-conversion -std=gnu99 -nostdlib $(CFLAGS_EXTRA)
+endif
 CFLAGS += -D$(CMSIS_MCU) -DUSE_FULL_LL_DRIVER
 CFLAGS += $(CFLAGS_MCU_$(MCU_SERIES))
 CFLAGS += $(COPT)
@@ -507,13 +577,21 @@ $(BUILD)/mpremoteprocport.o: $(BUILD)/openamp/metal/config.h
 endif
 endif
 
+# Conditionally exclude legacy threading files when using Zephyr
+# Note: pendsv.c and systick.c are kept because they contain utility functions,
+# but their interrupt handlers are conditionally compiled out via #if !MICROPY_ZEPHYR_THREADING
+ifeq ($(MICROPY_ZEPHYR_THREADING),1)
+SRC_C := $(filter-out pybthread.c mpthreadport.c,$(SRC_C))
+endif
+
 # SRC_O should be placed first to work around this LTO bug with binutils <2.35:
 # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83967
 OBJ += $(addprefix $(BUILD)/, $(SRC_O))
 OBJ += $(PY_O)
 OBJ += $(addprefix $(BUILD)/, $(LIB_SRC_C:.c=.o))
 OBJ += $(addprefix $(BUILD)/, $(LIBM_SRC_C:.c=.o))
 OBJ += $(addprefix $(BUILD)/, $(SHARED_SRC_C:.c=.o))
+OBJ += $(addprefix $(BUILD)/, $(SRC_THIRDPARTY_C:.c=.o))
 OBJ += $(addprefix $(BUILD)/, $(DRIVERS_SRC_C:.c=.o))
 OBJ += $(addprefix $(BUILD)/, $(HAL_SRC_C:.c=.o))
 OBJ += $(addprefix $(BUILD)/, $(USBDEV_SRC_C:.c=.o))
@@ -682,6 +760,10 @@ $(BUILD)/firmware-trusted.bin: $(BUILD)/firmware.bin
 
 # List of sources for qstr extraction
 SRC_QSTR += $(SRC_C) $(SRC_CXX) $(SHARED_SRC_C) $(GEN_PINS_SRC)
+# Add MicroPython-Zephyr integration files for root pointer extraction
+ifeq ($(MICROPY_ZEPHYR_THREADING),1)
+SRC_QSTR += $(ZEPHYR_MP_SRC_C)
+endif
 
 # Making OBJ use an order-only dependency on the generated pins.h file
 # has the side effect of making the pins.h file before we actually compile

ports/stm32/boards/NUCLEO_F429ZI/mpconfigboard.h

@@ -1,6 +1,9 @@
 #define MICROPY_HW_BOARD_NAME       "NUCLEO-F429ZI"
 #define MICROPY_HW_MCU_NAME         "STM32F429"
 
+// Enable Zephyr threading for development
+#define MICROPY_ZEPHYR_THREADING    1
+
 #define MICROPY_HW_HAS_SWITCH       (1)
 #define MICROPY_HW_HAS_FLASH        (1)
 #define MICROPY_HW_ENABLE_RNG       (1)

ports/stm32/boards/NUCLEO_F429ZI/mpconfigboard.mk

@@ -11,4 +11,7 @@ MICROPY_PY_SSL = 1
 MICROPY_SSL_MBEDTLS = 1
 MICROPY_HW_ENABLE_ISR_UART_FLASH_FUNCS_IN_RAM = 1
 
+# Enable Zephyr threading for development
+MICROPY_ZEPHYR_THREADING = 1
+
 FROZEN_MANIFEST = $(BOARD_DIR)/manifest.py

ports/stm32/gccollect.c

@@ -26,6 +26,7 @@
 
 #include "py/gc.h"
 #include "py/mpthread.h"
+#include "py/mphal.h"
 #include "shared/runtime/gchelper.h"
 #include "shared/runtime/softtimer.h"
 #include "gccollect.h"

ports/stm32/irq.h

@@ -59,6 +59,27 @@ extern uint32_t irq_stats[IRQ_STATS_MAX];
 // value of the state variable, but rather just pass the return
 // value from disable_irq back to enable_irq.
 
+#if MICROPY_ZEPHYR_THREADING
+// Zephyr IRQ management
+#include <zephyr/arch/cpu.h>
+
+static inline uint32_t query_irq(void) {
+    // For Zephyr, assume IRQs are enabled when not in atomic section
+    // This is a simplification - Zephyr manages IRQs via arch_irq_lock/unlock
+    return IRQ_STATE_ENABLED;
+}
+
+static inline void enable_irq(mp_uint_t state) {
+    arch_irq_unlock(state);
+}
+
+static inline mp_uint_t disable_irq(void) {
+    return arch_irq_lock();
+}
+
+#else
+// STM32 native IRQ management
+
 static inline uint32_t query_irq(void) {
     return __get_PRIMASK();
 }
@@ -73,6 +94,8 @@ static inline mp_uint_t disable_irq(void) {
     return state;
 }
 
+#endif // MICROPY_ZEPHYR_THREADING
+
 #if __CORTEX_M >= 0x03
 
 // irqs with a priority value greater or equal to "pri" will be disabled
@@ -146,7 +169,14 @@ static inline void restore_irq_pri(uint32_t state) {
 
 #else
 
+// For Zephyr threading, SysTick uses priority 2 (maskable) instead of priority 0
+// arch_irq_lock() uses PRIMASK to mask ALL interrupts (except NMI/HardFault)
+// Priority 2 is chosen to be safely maskable and allow timely context switching
+#if MICROPY_ZEPHYR_THREADING
+#define IRQ_PRI_SYSTICK         NVIC_EncodePriority(NVIC_PRIORITYGROUP_4, 2, 0)
+#else
 #define IRQ_PRI_SYSTICK         NVIC_EncodePriority(NVIC_PRIORITYGROUP_4, 0, 0)
+#endif
 
 // The UARTs have no FIFOs, so if they don't get serviced quickly then characters
 // get dropped. The handling for each character only consumes about 0.5 usec