On Wed, Apr 15, 2015 at 12:33:43PM +0200, Kamil Lulko wrote: > 2015-04-14 20:07 GMT+02:00 Matt Porter <mpor...@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 <mpor...@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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