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

[Tom Rini]

On Wed, Apr 15, 2015 at 12:33:43PM +0200, Kamil Lulko wrote:
> 2015-04-14 20:07 GMT+02:00 Matt Porter <mporter at konsulko.com>:
> >
> > 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
> > to 1 to stay in thumb mode.
> >
> > Signed-off-by: Matt Porter <mporter at 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
> >         rc = do_go_exec ((void *)addr, argc - 1, argv + 1);
> >         if (rc != 0) rcode = 1;
> 
> I think addr |= 1 would be better - there is always a possibility that
> kernel image has the zero bit already set (this is the case in my own
> Buildroot build setup I am using for STM32F4 builds). Anyways -
> keeping this bit set should be the responsibility of kernel image
> build process so such machine specific quirk can be kept out of the
> common code.

I'd agree about |='ing in 1.  But it's not a machine quirk, it's a
requirement of the Cortex-M family that you not exit Thumb-mode (since
it's Thumb-only) and given how we end up trying to jump to the address
(here or in 'go' which Matt didn't post the patch for, but same logic)
we can / will have problems if we don't do this.  You can work around it
for images we throw a header into but 'go' is where this gets really
annoying.

-- 
Tom
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