arch: arm: refactor pm_s2ram

What is the change?
 - APIs `pm_s2ram_mark_set` and `pm_s2ram_mark_check_and_clear`,
   used to set/check pm_s2ram magic marker,
   no longer have Arm specific limitations.

Why is this needed?
 - These APIs are generic and should not have arch specific dependency.

Signed-off-by: Sudan Landge <[email protected]>

Author: wearyzen

arch/arm/core/cortex_m/reset.S

@@ -21,6 +21,7 @@ _ASM_FILE_PROLOGUE
 GTEXT(z_arm_reset)
 GTEXT(z_early_memset)
 GDATA(z_interrupt_stacks)
+GDATA(z_main_stack)
 #if defined(CONFIG_DEBUG_THREAD_INFO)
 GDATA(z_sys_post_kernel)
 #endif
@@ -29,7 +30,6 @@ GTEXT(soc_reset_hook)
 #endif
 #if defined(CONFIG_INIT_ARCH_HW_AT_BOOT)
 GTEXT(z_arm_init_arch_hw_at_boot)
-GDATA(z_main_stack)
 #endif
 #if defined(CONFIG_PM_S2RAM)
 GTEXT(arch_pm_s2ram_resume)
@@ -83,9 +83,44 @@ SECTION_SUBSEC_FUNC(TEXT,_reset_section,__start)
 #endif /* CONFIG_INIT_ARCH_HW_AT_BOOT */
 
 #if defined(CONFIG_PM_S2RAM)
+    /*
+     * Temporarily set MSP to interrupt stack so that arch_pm_s2ram_resume can
+     * use stack for calling pm_s2ram_mark_check_and_clear.
+     * This is safe because suspend never being called from an ISR ensures that
+     * interrupt stack was not live during suspend.
+     * Note:
+     * if resuming from suspend, MSP is restored from cpu context
+     * if not resuming, MSP will be set back to z_main_stack for proper init
+     * And, apart from arch_pm_s2ram_resume being able to use the stack for
+     * a short while, there is no change in behavior in either of the paths.
+     */
+    ldr r0, =z_interrupt_stacks + CONFIG_ISR_STACK_SIZE + MPU_GUARD_ALIGN_AND_SIZE
+    msr msp, r0
+
     bl arch_pm_s2ram_resume
 #endif /* CONFIG_PM_S2RAM */
 
+    /*
+     * MSP needs to be set to main stack for following scenarios:
+     * - If CONFIG_PM_S2RAM is enabled, MSP is set to z_interrupt_stacks
+     *   for resume functions to make use of a stack. However,
+     *   if the device was not suspended and if resume functions return,
+     *   MSP needs to be set back to z_main_stack to ensure proper
+     *   initialization.
+     * - If CONFIG_PM_S2RAM is not enabled but CONFIG_INIT_ARCH_HW_AT_BOOT is,
+     *   MSP needs to be set to z_main_stack for proper initialization in case
+     *   device was loaded through chain loading or a debugger, as the initial
+     *   value of MSP would be anything that the previous image loaded.
+     * - If CONFIG_INIT_STACKS is enabled, we need to ensure MSP is not set
+     *   to z_interrupt_stacks, so we set it to z_main_stack.
+     *
+     * Since these scenarios cover most of the cases, we set MSP to
+     * z_main_stack here.
+     *
+     */
+    ldr r0, =z_main_stack + CONFIG_MAIN_STACK_SIZE
+    msr msp, r0
+
     /* Note: Make sure that variables like z_sys_post_kernel
      *       are set after the call to arch_pm_s2ram_resume
      *       to avoid any issues with suspend/resume path.
@@ -110,8 +145,6 @@ SECTION_SUBSEC_FUNC(TEXT,_reset_section,__start)
     str r0, [r1]
     dsb
 #endif /* CONFIG_CPU_HAS_ARM_MPU */
-    ldr r0, =z_main_stack + CONFIG_MAIN_STACK_SIZE
-    msr msp, r0
 
     /* Initialize core architecture registers and system blocks */
     bl z_arm_init_arch_hw_at_boot