[U-Boot] [PATCH v3 1/2] arm: preserve lr correctly in arm926ejs startup code

[Klaus Goger]

When building the mxs platform in thumb mode gcc generates code using
the intra procedure call scratch register (ip/r12) for the calling the
lowlevel_init function. This modifies the lr in flush_dcache which
causes u-boot proper to end in an endless loop.

40002334:       e1a0c00e        mov     ip, lr
40002338:       eb00df4c        bl      4003a070
<__lowlevel_init_from_arm>
4000233c:       e1a0e00c        mov     lr, ip
40002340:       e1a0f00e        mov     pc, lr
[...]
4003a070 <__lowlevel_init_from_arm>:
4003a070:       e59fc004        ldr     ip, [pc, #4]    ; 4003a07c
<__lowlevel_init_from_arm+0xc>
4003a074:       e08fc00c        add     ip, pc, ip
4003a078:       e12fff1c        bx      ip
4003a07c:       fffc86cd        .word   0xfffc86cd

Instead of using the the ip/r12 register we use sl/r10 to preserve the
link register.

According to "Procedure Call Standard for the ARM Architecture" by ARM
subroutines have to preserve the contents of register r4-r8, r10, r11
and SP. So using r12 to store the link register will fail if the called
function is using r12 and not preserving it as allowed.
This can happen in ARM and thumb mode but will definitely be triggered
by building it in thumb.
Using r10 should be safe in any case as we are startup code and not
called by any C-function and no stack is set up.

Signed-off-by: Klaus Goger <klaus.goger at theobroma-systems.com>
Signed-off-by: Christoph Muellner <christoph.muellner at theobroma-systems.com>

---

Changes in v3:
- reword commit message as it isn't thumb specific
- use r10 instead of sl alias as we don't use it as stack limit pointer
- revert return to a pc mov as it is a unnecessary change for this patch

Changes in v2:
- use bl instead of blx to call lowlevel_init
- remove mxs tag as it apply to all arm926ejs platforms

 arch/arm/cpu/arm926ejs/start.S | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm/cpu/arm926ejs/start.S b/arch/arm/cpu/arm926ejs/start.S
index 959d1ed86d..a625b39787 100644
--- a/arch/arm/cpu/arm926ejs/start.S
+++ b/arch/arm/cpu/arm926ejs/start.S
@@ -105,9 +105,9 @@ flush_dcache:
 	/*
 	 * Go setup Memory and board specific bits prior to relocation.
 	 */
-	mov	ip, lr		/* perserve link reg across call */
+	mov	r10, lr		/* perserve link reg across call */
 	bl	lowlevel_init	/* go setup pll,mux,memory */
-	mov	lr, ip		/* restore link */
+	mov	lr, r10		/* restore link */
 #endif
 	mov	pc, lr		/* back to my caller */
 #endif /* CONFIG_SKIP_LOWLEVEL_INIT */
-- 
2.11.0