> On 21 Apr 2018, at 15:10, Måns Rullgård <m...@mansr.com> wrote: > > Klaus Goger <klaus.go...@theobroma-systems.com > <mailto:klaus.go...@theobroma-systems.com>> writes: > >> 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.go...@theobroma-systems.com> >> Signed-off-by: Christoph Muellner <christoph.muell...@theobroma-systems.com> > > This problem isn't specific to Thumb mode. An ARM build would also > break if the lowlevel_init function happened to clobber r12, which it is > permitted to do. It's just dumb luck that this hasn't happened yet.
True. Since we are in low-level code and are not called by any C-functions (we are startup code), using sl/r10 is ok. > >> --- >> >> 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 */ > > I prefer to use plain register names (r10) rather than the aliases (sl) > when not using them for the special functions indicated by the latter. Ok, will change that. > >> #endif >> - mov pc, lr /* back to my caller */ >> + bx lr /* back to my caller */ > > This change seems unrelated. Yes, bx is the preferred instruction, but > using mov here isn't breaking anything. If it bothers you, feel free to > make a separate patch fixing all the instances of mov to the pc > register, not just this one. We’ll remove it from the patch. Thanks, Christoph
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