arch: arm: add Cortex-M MPU context save/restore API

Add struct z_mpu_context_retained and parameterized functions
z_arm_save_mpu_context() and z_arm_restore_mpu_context() to allow
saving and restoring MPU configuration to/from retained RAM.

This enables preserving MPU state across deep sleep or power-down
modes on Cortex-M devices.

The API is exposed under include/zephyr/arch/arm/mpu/arm_mpu.h
and implemented in arch/arm/core/mpu/arm_mpu.c.

Signed-off-by: Michele Sardo <msmttchr@gmail.com>

Author: msmttchr

arch/arm/core/mpu/arm_mpu.c

@@ -14,11 +14,18 @@
 #include <kernel_arch_data.h>
 #include <zephyr/mem_mgmt/mem_attr.h>
 #include <zephyr/dt-bindings/memory-attr/memory-attr-arm.h>
+#include <zephyr/arch/arm/mpu/arm_mpu.h>
 
 #define LOG_LEVEL CONFIG_MPU_LOG_LEVEL
 #include <zephyr/logging/log.h>
 LOG_MODULE_DECLARE(mpu);
 
+#if Z_ARM_CPU_HAS_PMSAV8_MPU
+#define ATTRIBUTE_AND_SIZE_REG_NAME RLAR
+#else
+#define ATTRIBUTE_AND_SIZE_REG_NAME RASR
+#endif
+
 #if defined(CONFIG_ARMV8_M_BASELINE) || defined(CONFIG_ARMV8_M_MAINLINE)
 /* The order here is on purpose since ARMv8-M SoCs may define
  * CONFIG_ARMV6_M_ARMV8_M_BASELINE or CONFIG_ARMV7_M_ARMV8_M_MAINLINE
@@ -407,6 +414,72 @@ void arm_core_mpu_configure_dynamic_mpu_regions(const struct z_arm_mpu_partition
 	}
 }
 
+#if defined(CONFIG_CPU_CORTEX_M)
+/**
+ * @brief Save the current MPU configuration into the provided context struct.
+ */
+void z_arm_save_mpu_context(struct z_mpu_context_retained *ctx)
+{
+	uint32_t regions = get_num_regions();
+
+	__ASSERT_NO_MSG(ctx != NULL);
+
+	if (regions == 0 || regions > Z_ARM_MPU_MAX_REGIONS) {
+		LOG_DBG("Invalid MPU region count: %u", regions);
+		ctx->num_valid_regions = 0;
+		return;
+	}
+
+	ctx->num_valid_regions = regions;
+
+	for (uint32_t i = 0; i < regions; i++) {
+		MPU->RNR = i;
+		__DSB(); /* Ensure MPU->RNR write completes before reading registers */
+		__ISB();
+		ctx->rbar[i] = MPU->RBAR;
+		ctx->rasr_rlar[i] = MPU->ATTRIBUTE_AND_SIZE_REG_NAME;
+	}
+#if Z_ARM_CPU_HAS_PMSAV8_MPU
+	ctx->mair[0] = MPU->MAIR0;
+	ctx->mair[1] = MPU->MAIR1;
+#endif
+	ctx->ctrl = MPU->CTRL;
+}
+
+/**
+ * @brief Restore the MPU configuration from the provided context struct.
+ */
+void z_arm_restore_mpu_context(const struct z_mpu_context_retained *ctx)
+{
+	__ASSERT_NO_MSG(ctx != NULL);
+
+	if (ctx->num_valid_regions == 0 || ctx->num_valid_regions > Z_ARM_MPU_MAX_REGIONS) {
+		LOG_DBG("Invalid MPU context num_valid_regions: %u", ctx->num_valid_regions);
+		return;
+	}
+
+	/* Disable MPU before reprogramming */
+	arm_core_mpu_disable();
+
+	for (uint32_t i = 0; i < ctx->num_valid_regions; i++) {
+		MPU->RNR = i;
+		MPU->RBAR = ctx->rbar[i];
+		MPU->ATTRIBUTE_AND_SIZE_REG_NAME = ctx->rasr_rlar[i];
+	}
+
+#if Z_ARM_CPU_HAS_PMSAV8_MPU
+	MPU->MAIR0 = ctx->mair[0];
+	MPU->MAIR1 = ctx->mair[1];
+#endif
+	/* Restore MPU control register (including enable bit if set) */
+	MPU->CTRL = ctx->ctrl;
+
+	/* Ensure MPU settings take effect before continuing */
+	__DSB();
+	__ISB();
+}
+#endif /* CONFIG_CPU_CORTEX_M */
+
 /* ARM MPU Driver Initial Setup */
 
 /*

include/zephyr/arch/arm/mpu/arm_mpu.h

@@ -17,7 +17,23 @@
 #error "Unsupported ARM CPU"
 #endif
 
+#if defined(CONFIG_ARMV8_M_MAINLINE) || defined(CONFIG_ARMV8_M_BASELINE)
+/* PMSAv8 MPU */
+#define Z_ARM_CPU_HAS_PMSAV8_MPU 1
+#else
+/* PMSAv6 / PMSAv7 (MPU is identical) */
+#define Z_ARM_CPU_HAS_PMSAV8_MPU 0
+#endif
+
+#if defined(CONFIG_ARMV8_M_MAINLINE)
+#define Z_ARM_MPU_MAX_REGIONS 16U
+#else
+#define Z_ARM_MPU_MAX_REGIONS 8U
+#endif
+
+
 #ifndef _ASMLANGUAGE
+#include <stdint.h>
 
 /* Region definition data structure */
 struct arm_mpu_region {
@@ -68,6 +84,42 @@ struct arm_mpu_config {
  */
 extern const struct arm_mpu_config mpu_config;
 
+#if defined(CONFIG_CPU_CORTEX_M)
+/**
+ * @brief MPU context structure to retain MPU register state across deep sleep.
+ *
+ * This structure holds the MPU region base and attribute registers,
+ * as well as the MPU control register and a valid region count.
+ *
+ * The implemented architecture dictates which MPU registers exist:
+ * - ARMv8-M has per-region RBAR+RLAR, and global MAIR0~1
+ * - ARMv6/v7-M have per-region RBAR+RASR
+ */
+struct z_mpu_context_retained {
+	uint32_t rbar[Z_ARM_MPU_MAX_REGIONS];
+	uint32_t rasr_rlar[Z_ARM_MPU_MAX_REGIONS];
+#if Z_ARM_CPU_HAS_PMSAV8_MPU
+	uint32_t mair[2];
+#endif
+	uint32_t ctrl;
+	uint32_t num_valid_regions;
+};
+
+/**
+ * @brief Save the current MPU configuration into the provided context struct.
+ *
+ * @param ctx Pointer to the MPU context structure to save into.
+ */
+void z_arm_save_mpu_context(struct z_mpu_context_retained *ctx);
+
+/**
+ * @brief Restore the MPU configuration from the provided context struct.
+ *
+ * @param ctx Pointer to the MPU context structure to restore from.
+ */
+void z_arm_restore_mpu_context(const struct z_mpu_context_retained *ctx);
+
+#endif /* CONFIG_CPU_CORTEX_M */
 #endif /* _ASMLANGUAGE */
 
 #endif /* ZEPHYR_INCLUDE_ARCH_ARM_MPU_ARM_MPU_H_ */