STM32 Bootloader support with retained ram boot mode infrastructure

boards/st/nucleo_wb55rg/nucleo_wb55rg.dts

@@ -10,12 +10,18 @@
 #include "arduino_r3_connector.dtsi"
 #include <zephyr/dt-bindings/input/input-event-codes.h>
 
+
+&sram0 {
+	reg = <0x20000000 (DT_SIZE_K(192) - 4)>;
+};
+
 / {
 	model = "STMicroelectronics STM32WB55RG-NUCLEO board";
 	compatible = "st,stm32wb55rg-nucleo";
 
 	chosen {
 		zephyr,console = &usart1;
+		zephyr,boot-mode = &boot_mode;
 		zephyr,shell-uart = &usart1;
 		zephyr,bt-mon-uart = &lpuart1;
 		zephyr,bt-c2h-uart = &lpuart1;
@@ -24,6 +30,27 @@
 		zephyr,code-partition = &slot0_partition;
 	};
 
+	sram_ret@2002fffc {
+		compatible = "zephyr,memory-region", "mmio-sram";
+		reg = <0x2002fffc 0x4>;
+		zephyr,memory-region = "RetainedMem";
+		status = "okay";
+
+		retainedmem {
+			compatible = "zephyr,retained-ram";
+			status = "okay";
+			#address-cells = <1>;
+			#size-cells = <1>;
+
+			boot_mode: retention@0 {
+				compatible = "zephyr,retention";
+				status = "okay";
+				reg = <0x0 0x1>;
+			};
+		};
+	};
+
+
 	leds: leds {
 		compatible = "gpio-leds";
 

drivers/retained_mem/retained_mem_zephyr_ram.c

@@ -33,7 +33,9 @@ static inline void zephyr_retained_mem_ram_lock_take(const struct device *dev)
 #ifdef CONFIG_RETAINED_MEM_MUTEXES
 	struct zephyr_retained_mem_ram_data *data = dev->data;
 
-	k_mutex_lock(&data->lock, K_FOREVER);
+	if (!k_is_pre_kernel()) {
+		k_mutex_lock(&data->lock, K_FOREVER);
+	}
 #else
 	ARG_UNUSED(dev);
 #endif
@@ -44,7 +46,9 @@ static inline void zephyr_retained_mem_ram_lock_release(const struct device *dev
 #ifdef CONFIG_RETAINED_MEM_MUTEXES
 	struct zephyr_retained_mem_ram_data *data = dev->data;
 
-	k_mutex_unlock(&data->lock);
+	if (!k_is_pre_kernel()) {
+		k_mutex_unlock(&data->lock);
+	}
 #else
 	ARG_UNUSED(dev);
 #endif

dts/arm/st/f0/stm32f072.dtsi

@@ -10,6 +10,12 @@
 	soc {
 		compatible = "st,stm32f072", "st,stm32f0", "simple-bus";
 
+		rom_bootloader: rom_bootloader@1fffc800 {
+			compatible = "st,stm32-bootloader";
+			reg = <0x1fffc800 DT_SIZE_K(12)>;
+			status = "okay";
+		};
+
 		can1: can@40006400 {
 			compatible = "st,stm32-bxcan";
 			reg = <0x40006400 0x400>;

dts/arm/st/f4/stm32f411.dtsi

@@ -18,6 +18,12 @@
 	soc {
 		compatible = "st,stm32f411", "st,stm32f4", "simple-bus";
 
+		rom_bootloader: rom_bootloader@1fff0000 {
+			compatible = "st,stm32-bootloader";
+			reg = <0x1fff0000 DT_SIZE_K(29)>;
+			status = "okay";
+		};
+
 		spi5: spi@40015000 {
 			compatible = "st,stm32-spi";
 			#address-cells = <1>;

dts/arm/st/wb/stm32wb.dtsi

@@ -144,6 +144,12 @@
 	};
 
 	soc {
+		rom_bootloader: rom_bootloader@1fff0000 {
+			compatible = "st,stm32-bootloader";
+			reg = <0x1fff0000 DT_SIZE_K(28)>;
+			status = "okay";
+		};
+
 		flash: flash-controller@58004000 {
 			compatible = "st,stm32-flash-controller", "st,stm32wb-flash-controller";
 			reg = <0x58004000 0x400>;

dts/bindings/arm/st,stm32-bootloader.yaml

@@ -0,0 +1,12 @@
+# Copyright (c) 2025 Peter Johanson
+# SPDX-License-Identifier: Apache-2.0
+
+description: ST STM32 ROM Bootloader Details
+
+compatible: "st,stm32-bootloader"
+
+include: base.yaml
+
+properties:
+  reg:
+    required: true

soc/st/stm32/Kconfig

@@ -38,6 +38,20 @@ config STM32_BACKUP_SRAM_INIT_PRIORITY
 	help
 	  STM32 Backup SRAM device initialization priority.
 
+config STM32_BOOTLOADER
+	bool "STM32 Bootloader Support"
+	depends on RETENTION_BOOT_MODE && DT_HAS_ST_STM32_BOOTLOADER_ENABLED
+	help
+	  Enable support for jumping into the STM32 bootloader when the
+	  bootmode is set.
+
+config STM32_BOOTLOADER_INIT_PRIORITY
+	int "STM32 bootloader check init priority"
+	default 3
+	help
+	  STM32 bootloader check initialization priority (must be higher than init
+	  priorities for retention/retained memory drivers).
+
 config STM32_ENABLE_DEBUG_SLEEP_STOP
 	bool "Allow debugger attach in stop/sleep Mode"
 	help

soc/st/stm32/Kconfig.defconfig

@@ -81,6 +81,16 @@ config CLOCK_CONTROL_INIT_PRIORITY
 	default 1
 	depends on CLOCK_CONTROL
 
+if STM32_BOOTLOADER
+
+config RETAINED_MEM_INIT_PRIORITY
+	default 1
+
+config RETENTION_INIT_PRIORITY
+	default 2
+
+endif
+
 # Get flash configuration for NS image from dts flash partition
 config USE_DT_CODE_PARTITION
 	default y if TRUSTED_EXECUTION_NONSECURE

soc/st/stm32/common/CMakeLists.txt

@@ -19,6 +19,8 @@ endif()
 zephyr_sources_ifdef(CONFIG_STM32_BACKUP_SRAM stm32_backup_sram.c)
 zephyr_linker_sources_ifdef(CONFIG_STM32_BACKUP_SRAM SECTIONS stm32_backup_sram.ld)
 
+zephyr_sources_ifdef(CONFIG_STM32_BOOTLOADER stm32_bootloader.c)
+
 if (NOT CONFIG_DEBUG AND CONFIG_PM)
   zephyr_sources_ifdef(CONFIG_DT_HAS_SWJ_CONNECTOR_ENABLED pm_debug_swj.c)
 endif()

soc/st/stm32/common/stm32_bootloader.c

@@ -0,0 +1,74 @@
+/*
+ * Copyright (c) 2020 Google LLC.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <soc.h>
+#include <zephyr/init.h>
+#include <zephyr/retention/retention.h>
+#include <zephyr/retention/bootmode.h>
+#include <stm32_ll_system.h>
+
+#if defined(CONFIG_ARM_MPU)
+extern void arm_core_mpu_disable(void);
+#endif
+
+static const uint32_t bootloader = DT_REG_ADDR(DT_INST(0, st_stm32_bootloader));
+static FUNC_NORETURN void jump_to_bootloader(void)
+{
+	int i;
+	void (*jmp)(void);
+
+	__disable_irq();
+
+	for (i = 0; i < ARRAY_SIZE(NVIC->ICER); i++) {
+		NVIC->ICER[i] = 0xFFFFFFFF;
+		NVIC->ICPR[i] = 0xFFFFFFFF;
+	}
+
+#if defined(CONFIG_ARM_MPU)
+	/*
+	 * Needed to allow the bootloader to erase/write to flash, and the MPU
+	 * is set up in the arch init phase.
+	 */
+	arm_core_mpu_disable();
+#endif
+
+	/*
+	 * Disable SysTick before jumping, to keep the bootloader happy
+	 */
+	SysTick->CTRL = 0;
+
+	LL_SYSCFG_SetRemapMemory(LL_SYSCFG_REMAP_SYSTEMFLASH);
+
+	jmp = (void (*)(void))(void (*)(void))(*((uint32_t *)((bootloader + 4))));
+
+	/*
+	 * We need to clear a few things set by the Zephyr early startup code
+	 */
+	__set_CONTROL(0);
+#if !defined(CONFIG_CPU_CORTEX_M0)
+	__set_BASEPRI(0);
+#endif
+
+	__set_MSP(*(uint32_t *)bootloader);
+
+	__enable_irq();
+
+	jmp();
+
+	CODE_UNREACHABLE;
+}
+
+static int bootloader_check_boot_init(void)
+{
+	if (bootmode_check(BOOT_MODE_TYPE_BOOTLOADER) > 0) {
+		bootmode_clear();
+		jump_to_bootloader();
+	}
+
+	return 0;
+}
+
+SYS_INIT(bootloader_check_boot_init, POST_KERNEL, CONFIG_STM32_BOOTLOADER_INIT_PRIORITY);

subsys/retention/retention.c

@@ -63,7 +63,9 @@ static inline void retention_lock_take(const struct device *dev)
 #ifdef CONFIG_RETENTION_MUTEXES
 	struct retention_data *data = dev->data;
 
-	k_mutex_lock(&data->lock, K_FOREVER);
+	if (!k_is_pre_kernel()) {
+		k_mutex_lock(&data->lock, K_FOREVER);
+	}
 #else
 	ARG_UNUSED(dev);
 #endif
@@ -74,7 +76,9 @@ static inline void retention_lock_release(const struct device *dev)
 #ifdef CONFIG_RETENTION_MUTEXES
 	struct retention_data *data = dev->data;
 
-	k_mutex_unlock(&data->lock);
+	if (!k_is_pre_kernel()) {
+		k_mutex_unlock(&data->lock);
+	}
 #else
 	ARG_UNUSED(dev);
 #endif