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

[Matt Porter <mporter@konsulko.com>]

On Tue, Apr 21, 2015 at 12:57:09PM -0500, Felipe Balbi wrote:
> On Tue, Apr 21, 2015 at 12:54:26PM -0500, Felipe Balbi wrote:
> > 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 ?
> 
> seems like it must be set for cortex-m, but then shouldn't this be done
> by GCC ? Are we, perhaps, using wrong GCC arguments when building for
> cortex-m ?

From "make V=1":

	... -march=armv7-m -mthumb ...

$ arm-none-eabi-gcc -v
...
gcc version 4.8.3 20140913 (release) (4.8.3-11ubuntu1+11)

-Matt
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