Scott Wood <scottw...@freescale.com> wrote on 2012/07/20 23:11:33:
>
> On 07/20/2012 04:20 AM, Joakim Tjernlund wrote:
> > PowerPC mandates SP to be 16 bytes aligned.
> > Furthermore, a stack frame is added, pointing to the reset vector
> > which is just get in the way when gdb is walking the stack because
> > the reset vector may not accessible depending on emulator settings.
> > Also use a temp register so gdb doesn't pick up intermediate values.
> >
> > Signed-off-by: Joakim Tjernlund <joakim.tjernl...@transmode.se>
> > ---
> > arch/powerpc/cpu/mpc85xx/start.S | 16 +++++-----------
> > 1 files changed, 5 insertions(+), 11 deletions(-)
> >
> > diff --git arch/powerpc/cpu/mpc85xx/start.S arch/powerpc/cpu/mpc85xx/start.S
> > index 8d66cf1..8c75af9 100644
> > --- arch/powerpc/cpu/mpc85xx/start.S
> > +++ arch/powerpc/cpu/mpc85xx/start.S
> > @@ -848,18 +848,12 @@ version_string:
> > .globl _start_cont
> > _start_cont:
> > /* Setup the stack in initial RAM,could be L2-as-SRAM or L1 dcache*/
> > - lis r1,CONFIG_SYS_INIT_RAM_ADDR@h
> > - ori r1,r1,CONFIG_SYS_INIT_SP_OFFSET@l
> > -
> > + lis r2,(CONFIG_SYS_INIT_RAM_ADDR)@h
> > + ori r2,r2,((CONFIG_SYS_INIT_SP_OFFSET-16)&~0xf)@l /* Align to 16 */
>
> Please don't use r2 as a general purpose register -- even if it's early
> enough that we haven't started using it for its fixed purpose yet.
I guess that would be a tiny improvement, but it is nothing compared
with the r1 abuse in this file :)
>
> > + stw r0,-4(r2) /* Store a NULL LR */
> > + stw r0,0(r2) /* Store a NULL Back Chain */
> > + mr r1,r2 /* Transfer to SP(r1) */
>
> Why are you storing anything below the stack pointer? LR goes above the
> backchain, not below -- and it's not valid anyway for the current stack
> frame. If you want to have a terminating stack frame with a NULL LR,
> you need to create a second stack frame, like the code currently does.
Right, it should be +4 where the NULL LR should be. As you said, it doesn't
matter
because it is not valid for this stack frame and could be left out.
I do not want to create a frame as the current code does, it
might create an memory access to 0xfffffffc which I don't want.
How about this then:
lis r3,(CONFIG_SYS_INIT_RAM_ADDR)@h
ori r3,r3,((CONFIG_SYS_INIT_SP_OFFSET-16)&~0xf)@l /* Align to 16 */
li r0,0
stw r0,0(r3) /* Terminate Back Chain */
stw r0,+4(r3) /* NULL LR, to be nice. */
mr r1,r3 /* Transfer to SP(r1) */
or if that reset address is wanted:
lis r3,(CONFIG_SYS_INIT_RAM_ADDR)@h
ori r3,r3,((CONFIG_SYS_INIT_SP_OFFSET-16)&~0xf)@l /* Align to 16 */
li r0,0
stw r0,0(r3) /* Terminate Back Chain */
stw r0,+4(r3) /* NULL LR, to be nice. */
stwu r3,-16(r3) /* save back chain and move SP */
lis r0,RESET_VECTOR@h
ori r0,RESET_VECTOR@l
stw r0,+20(r3) /* save return address */
mr r1,r3 /* Transfer to SP(r1) */
I would have to test with the emulator if both are OK or if there will
be an access to RESET_VECTOR(I don't have that connected ATM).
Jocke
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