arch/arm: introduce the pre-stack/RAM init hook

Introduce hook for customize reset.S code even before stack is
initialized or RAM is accessed. Hook can be enabled using
CONFIG_SOC_PRE_RAM_HOOK=y.
Hook implementation is by soc_start_hook() function which should
be provided by custom code.

Signed-off-by: Andrzej Puzdrowski <[email protected]>

Author: nvlsianpu

arch/arm/core/cortex_m/reset.S

@@ -35,6 +35,10 @@ GTEXT(z_arm_init_arch_hw_at_boot)
 #if defined(CONFIG_PM_S2RAM)
 GTEXT(arch_pm_s2ram_resume)
 #endif
+#if defined(CONFIG_SOC_PRE_RAM_HOOK)
+GTEXT(soc_pre_ram_hook)
+#endif
+
 
 /*
  * PACBTI Mask for CONTROL register:
@@ -100,6 +104,10 @@ SECTION_SUBSEC_FUNC(TEXT,_reset_section,__start)
 
 #endif /* CONFIG_INIT_ARCH_HW_AT_BOOT */
 
+#if defined(CONFIG_SOC_PRE_RAM_HOOK)
+    /* Call custom code that executes before any stack is set up or RAM memory is accessed */
+    bl soc_pre_ram_hook
+#endif
 #if defined(CONFIG_PM_S2RAM)
     /*
      * Temporarily set MSP to interrupt stack so that arch_pm_s2ram_resume can

include/zephyr/platform/hooks.h

@@ -19,6 +19,15 @@
  * directly from application code but may be freely used within the OS.
  */
 
+/**
+ * @brief SoC  hook executed before RAM initialization, at the beginning
+ * of the reset vector.
+ *
+ * This hook is implemented by the SoC and can be used to perform any
+ * SoC-specific initialization. Its execution takes place even before
+ * the stack is initialized or RAM is accessed.
+ */
+void soc_pre_ram_hook(void);
 
 #ifdef CONFIG_SOC_RESET_HOOK
 /**

kernel/Kconfig.init

@@ -6,6 +6,27 @@
 
 menu "SoC and Board Hooks"
 
+config SOC_PRE_RAM_HOOK
+	bool "Run SoC-specific early reset hook"
+	help
+	  Run a SoC-specific hook early in the reset/startup code (__start).
+	  A custom hook soc_pre_ram_hook() will be executed at the beginning
+	  of the architecture-specific startup code, after hardware has been
+	  configured as required by CONFIG_INIT_ARCH_HW_AT_BOOT.
+
+	  Returning from this hook is not mandatory: it can be used to implement
+	  special logic to resume execution in some scenarios (for example, when
+	  a bootloader is used).
+
+	  The stack pointer register is considered as scratch by the caller of
+	  this hook. In particular, this means that the hook is NOT allowed to
+	  "push" any data using this stack pointer. However, the hook may use
+	  an implementation-specific area as stack if desired; in such case,
+	  the original value of the stack pointer needs not to be "restored"
+	  before returning control to the hook's caller.
+
+	  Additional constraints may be imposed on the hook by the architecture.
+
 config SOC_RESET_HOOK
 	bool "Run early SoC reset hook"
 	help