arch: arm: Fixup stack end calculations

This patch fixes calculations for the top of the interrupt and main
stacks.  Due to power of two alignment requirements for certain MPUs,
the guard size must be taken into account due to the guard being
counted against the initial stack size.

Signed-off-by: Andy Gross <andy.gross@linaro.org>

Author: Andy Gross

arch/arm/include/cortex_m/stack.h

@@ -40,8 +40,13 @@ extern K_THREAD_STACK_DEFINE(_interrupt_stack, CONFIG_ISR_STACK_SIZE);
  */
 static ALWAYS_INLINE void _InterruptStackSetup(void)
 {
+#ifdef CONFIG_MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT
+	u32_t msp = (u32_t)(K_THREAD_STACK_BUFFER(_interrupt_stack) +
+			    CONFIG_ISR_STACK_SIZE - MPU_GUARD_ALIGN_AND_SIZE);
+#else
 	u32_t msp = (u32_t)(K_THREAD_STACK_BUFFER(_interrupt_stack) +
 			    CONFIG_ISR_STACK_SIZE);
+#endif
 
 	_MspSet(msp);
 }

arch/arm/include/kernel_arch_func.h

@@ -45,8 +45,14 @@ _arch_switch_to_main_thread(struct k_thread *main_thread,
 	/* get high address of the stack, i.e. its start (stack grows down) */
 	char *start_of_main_stack;
 
+#ifdef CONFIG_MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT
+	start_of_main_stack =
+		K_THREAD_STACK_BUFFER(main_stack) + main_stack_size -
+		MPU_GUARD_ALIGN_AND_SIZE;
+#else
 	start_of_main_stack =
 		K_THREAD_STACK_BUFFER(main_stack) + main_stack_size;
+#endif
 	start_of_main_stack = (void *)STACK_ROUND_DOWN(start_of_main_stack);
 
 	_current = main_thread;