[U-Boot] [PATCH v2 2/6] common/cmd_boot: keep ARM v7M in thumb mode during do_go_exec()

Felipe Balbi
2015-04-21T17:54:26Z

On Tue, Apr 21, 2015 at 12:47:24PM -0500, Felipe Balbi wrote:
> On Tue, Apr 21, 2015 at 01:36:54PM -0400, Matt Porter wrote:
> > On ARM v7M, the processor will return to ARM mode when executing
> > a blx instruction with bit 0 of the address == 0. Always set it
>
> but that's what the 'x' is for, right ? eXchange the CPU mode.
>
> > to 1 to stay in thumb mode.
> >
> > Signed-off-by: Matt Porter <mporter@konsulko.com>
> > ---
> > common/cmd_boot.c | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > diff --git a/common/cmd_boot.c b/common/cmd_boot.c
> > index 8f2e070..20ce652 100644
> > --- a/common/cmd_boot.c
> > +++ b/common/cmd_boot.c
> > @@ -38,6 +38,10 @@ static int do_go(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
> > * pass address parameter as argv[0] (aka command name),
> > * and all remaining args
> > */
> > +#ifdef CONFIG_CPU_V7M
> > + /* For ARM V7M, set bit zero to stay in Thumb mode */
> > + addr++;
> > +#endif
>
> what if we were in ARM state when we reached this point ? You're now
> telling CPU to always switch to Thumb. Is this really what we want ?
>
> From ARM's instruction manual:
>
>
>
> "
> The BX and BLX instructions can change the processor state from ARM to
> Thumb, or from Thumb to ARM.
>
> BLX label always changes the state.
>
> BX Rm and BLX Rm derive the target state from bit[0] of Rm:
>
> if bit[0] of Rm is 0, the processor changes to, or remains in, ARM
> state
>
> if bit[0] of Rm is 1, the processor changes to, or remains in, Thumb
> state.
> "

oh wait, this is cortex-m, it's supposed to be thumb2 only, why do we
even need that bit ?

--
balbi
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