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

[Matt Porter]

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.

Yes.

> > 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
> 
> 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 ?

We have no ARM state on this core so that's not possible.

> 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.

Correct. The last statement is why this patch exists. There is no ARM
mode on M3, we immediately fault. Having bit[0]=0 for BX/BLX is not
permitted on V7M...something not covered in the generic description
of these instructions.

Incidentally, I forgot to update this with Kamil's comment that it
should be implemented as |1 and will address that now.

-Matt
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