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@theobroma-systems.com Signed-off-by: Christoph Muellner christoph.muellner@theobroma-systems.com
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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 */