[U-Boot] [PATCH v2 1/2] arm: make arm926ejs startup code thumb compatible

[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 r10 instead of r12 should be save.

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

---

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 | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arm/cpu/arm926ejs/start.S b/arch/arm/cpu/arm926ejs/start.S
index 959d1ed86d..317df5c401 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	sl, lr		/* perserve link reg across call */
 	bl	lowlevel_init	/* go setup pll,mux,memory */
-	mov	lr, ip		/* restore link */
+	mov	lr, sl		/* restore link */
 #endif
-	mov	pc, lr		/* back to my caller */
+	bx	lr		/* back to my caller */
 #endif /* CONFIG_SKIP_LOWLEVEL_INIT */
-- 
2.11.0