[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 01:05:24PM -0500, Felipe Balbi wrote:
> On Tue, Apr 21, 2015 at 02:01:31PM -0400, Matt Porter 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.
> > 
> > Yes.
> > 
> > > > 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 ?
> > 
> > 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.
> 
> it just seems weird that bit0 wouldn't just be assume 1 by the core
> itself. I suppose as a consequence we can't use blx label with v7m
> either ? :-s

It also seems weird to me that it wouldn't just be ignored on anything
armv7-m. Yes, blx label would be "very bad" which is why it is not
supported on ARMv7-m :)

And to clarify for those listening at home... From The ARMv7-M
Instruction Set Appendix A4.1.1:

"
Thumb interworking is held as bit [0] of an interworking address.
Interworking addresses are used in the following instructions:
? BX or BLX
? an LDR or LDM that loads the PC.
ARMv7-M only supports the Thumb instruction execution state, therefore
the value of address bit [0] must be 1 in interworking instructions,
otherwise a fault occurs. All instructions ignore bit [0] and write bits
[31:1]:?0? when updating the PC.
"

Also:

"
A7.7.19 BLX (register)
Branch with Link and Exchange calls a subroutine at an address and
instruction set specified by a register.
ARMv7-M only supports the Thumb instruction set. An attempt to change
the instruction execution state
causes the processor to take an exception on the instruction at the
target address.
"

-Matt
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