[nrf fromtree] Revert "arch: arm: cortex_m: pm_s2ram: Rework S2RAM mark functions"

What is the change?
 - This reverts commit 474d4c3

Why do we need this change?
 - This commit was added because Cortex-M didn't have a valid
   stack to make required functionality work however,
   the previous commit fixes this and makes interrupt stack
   available for use. This removes Arm specific limitation from
   these generic APIs so revert the commit to reflect the same.

Signed-off-by: Sudan Landge <sudan.landge@arm.com>
(cherry picked from commit b4fb5d3)

Author: wearyzen

arch/arm/core/cortex_m/pm_s2ram.S

@@ -178,9 +178,7 @@ SECTION_FUNC(TEXT, arch_pm_s2ram_suspend)
 	/*
 	 * Mark entering suspend to RAM.
 	 */
-	mov 	r1, lr
-	bl 	pm_s2ram_mark_set
-	mov	lr, r1
+	bl pm_s2ram_mark_set
 
 	/*
 	 * Call the system_off function passed as parameter. This should never
@@ -199,9 +197,7 @@ SECTION_FUNC(TEXT, arch_pm_s2ram_suspend)
 	/*
 	 * Reset the marking of suspend to RAM, return is ignored.
 	 */
-	mov 	r1, lr
-	bl	pm_s2ram_mark_check_and_clear
-	mov 	lr, r1
+	bl pm_s2ram_mark_check_and_clear
 
 	/* Move the stored return value of system_off back to r0,
 	 * setting it as return value for this function.
@@ -216,13 +212,16 @@ GTEXT(arch_pm_s2ram_resume)
 SECTION_FUNC(TEXT, arch_pm_s2ram_resume)
 	/*
 	 * Check if reset occurred after suspending to RAM.
+	 * Store LR to ensure we can continue boot when we are not suspended
+	 * to RAM. In addition to LR, R0 is pushed too, to ensure "SP mod 8 = 0",
+	 * as stated by ARM rule 6.2.1.2 for AAPCS32.
 	 */
-	mov 	r1, lr
-	bl 	pm_s2ram_mark_check_and_clear
-	mov 	lr, r1
-	cmp     r0, #0x1
-	beq     .L_resume
-	bx      lr
+	push    {r0, lr}
+	bl      pm_s2ram_mark_check_and_clear
+	cmp	r0, #0x1
+	pop     {r0, lr}
+	beq	.L_resume
+	bx	lr
 
 .L_resume:
 	/*

arch/arm/core/cortex_m/pm_s2ram.c

@@ -22,44 +22,20 @@ __noinit _cpu_context_t _cpu_context;
  */
 static __noinit uint32_t marker;
 
-void __attribute__((naked)) pm_s2ram_mark_set(void)
+void pm_s2ram_mark_set(void)
 {
-	__asm__ volatile(
-		/* Set the marker to MAGIC value */
-		"str	%[_magic_val], [%[_marker]]\n"
-
-		"bx	lr\n"
-		:
-		: [_magic_val] "r"(MAGIC), [_marker] "r"(&marker)
-		: "r1", "r4", "memory");
+	marker = MAGIC;
 }
 
-bool __attribute__((naked)) pm_s2ram_mark_check_and_clear(void)
+bool pm_s2ram_mark_check_and_clear(void)
 {
-	__asm__ volatile(
-		/* Set return value to 0 */
-		"mov	r0, #0\n"
-
-		/* Check the marker */
-		"ldr	r3, [%[_marker]]\n"
-		"cmp	r3, %[_magic_val]\n"
-		"bne	exit\n"
-
-		/*
-		 * Reset the marker
-		 */
-		"str	r0, [%[_marker]]\n"
+	if (marker == MAGIC) {
+		marker = 0;
 
-		/*
-		 * Set return value to 1
-		 */
-		"mov	r0, #1\n"
+		return true;
+	}
 
-		"exit:\n"
-		"bx lr\n"
-		:
-		: [_magic_val] "r"(MAGIC), [_marker] "r"(&marker)
-		: "r0", "r1", "r3", "r4", "memory");
+	return false;
 }
 
 #endif /* CONFIG_PM_S2RAM_CUSTOM_MARKING */

include/zephyr/arch/common/pm_s2ram.h

@@ -65,11 +65,6 @@ int arch_pm_s2ram_suspend(pm_s2ram_system_off_fn_t system_off);
  *
  * Default implementation is setting a magic word in RAM. CONFIG_PM_S2RAM_CUSTOM_MARKING
  * allows custom implementation.
- * The following requirements must be fulfilled:
- * - the function cannot use stack (asm function or function with 'naked' attribute)
- * - the content of the R1 and R4 registers must remain unchanged
- * - returning from the function should be performed with the `bx lr` instruction
- *
  */
 void pm_s2ram_mark_set(void);
 
@@ -81,11 +76,6 @@ void pm_s2ram_mark_set(void);
  *
  * Default implementation is checking a magic word in RAM. CONFIG_PM_S2RAM_CUSTOM_MARKING
  * allows custom implementation.
- * The following requirements must be fulfilled:
- * - the function cannot use stack (most likely asm function)
- * - the content of the R1 and R4 registers must remain unchanged
- * - the function's return value is passed by R0 register
- * - returning from the function should be performed with the `bx lr` instruction
  *
  * @retval true if marking is found which indicates resuming after suspend-to-RAM.
  * @retval false if marking is not found which indicates standard boot.