ARM: 8451/1: v7-M: Set an early stack for __v7m_setup

On ARM v7-M, when PROCINFO_INITFUNC (__v7m_setup) is called,
a stack is needed before calling the supervisor call (SVC),
which is used by the supervisor call to save the context.

Currently, __v7m_setup() prepares a temporary stack in the .text.init
section, which is is broken if the kernel is executing directly from
read-only memory.

In particular, this is the case for LPC43xx, which allows
to execute the kernel in-place from a serial flash through its SPIFI
controller.

This commit fixes the issue by seting an early stack to its usual location.

Also, __v7m_setup() is currently saving and restoring the previous
stack. That was bogus, because there's no stack previously set,
so this commit removes it.

Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 5 insertions, 9 deletions

diff --git a/arch/arm/mm/proc-v7m.S b/arch/arm/mm/proc-v7m.S
index 67d9209077c6..7229d8d0be1a 100644
--- a/arch/arm/mm/proc-v7m.S
+++ b/arch/arm/mm/proc-v7m.S
@@ -12,6 +12,7 @@
  */
 #include <linux/linkage.h>
 #include <asm/assembler.h>
+#include <asm/memory.h>
 #include <asm/v7m.h>
 #include "proc-macros.S"
 
@@ -97,19 +98,19 @@ __v7m_setup:
         mov     r5, #0x00800000
         str     r5, [r0, V7M_SCB_SHPR3] @ set PendSV priority
 
-        @ SVC to run the kernel in this mode
+        @ SVC to switch to handler mode. Notice that this requires sp to
+        @ point to writeable memory because the processor saves
+        @ some registers to the stack.
         badr    r1, 1f
         ldr     r5, [r12, #11 * 4]      @ read the SVC vector entry
         str     r1, [r12, #11 * 4]      @ write the temporary SVC vector entry
         mov     r6, lr                  @ save LR
-        mov     r7, sp                  @ save SP
-        ldr     sp, =__v7m_setup_stack_top
+        ldr     sp, =init_thread_union + THREAD_START_SP
         cpsie   i
         svc     #0
 1:      cpsid   i
         str     r5, [r12, #11 * 4]      @ restore the original SVC vector entry
         mov     lr, r6                  @ restore LR
-        mov     sp, r7                  @ restore SP
 
         @ Special-purpose control register
         mov     r1, #1
@@ -123,11 +124,6 @@ __v7m_setup:
         ret     lr
 ENDPROC(__v7m_setup)
 
-        .align 2
-__v7m_setup_stack:
-        .space  4 * 8                           @ 8 registers
-__v7m_setup_stack_top:
-
         define_processor_functions v7m, dabort=nommu_early_abort, pabort=legacy_pabort, nommu=1
 
         .section ".rodata"