> On 26 Apr 2018, at 20:18, Klaus Goger <klaus.go...@theobroma-systems.com> > wrote: > > 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.
Maybe you could rephrase this one, as the “not preserving it as allowed” made me read this twice until I realised that it is equivalent to “does not preserve (and is not required by the PCS to preserve)”? > 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.go...@theobroma-systems.com> > Signed-off-by: Christoph Muellner <christoph.muell...@theobroma-systems.com> Reviewed-by: Philipp Tomsich <philipp.toms...@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 > > _______________________________________________ > U-Boot mailing list > U-Boot@lists.denx.de > https://lists.denx.de/listinfo/u-boot
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