[U-Boot] Query on CONFIG_SYS_THUMB_BUILD

[Albert ARIBAUD <albert.u.boot@aribaud.net>]

Hello Simon,

On Sat, 15 Nov 2014 15:10:47 -0700, Simon Glass <sjg@chromium.org>
wrote:
> Hi Albert,
> 
> On 15 November 2014 05:30, Albert ARIBAUD <albert.u.boot@aribaud.net> wrote:
> > Hello Simon,
> >
> > On Fri, 14 Nov 2014 18:56:07 -0700, Simon Glass <sjg@chromium.org>
> > wrote:
> >
> >> > I believe you've built crt0.S for ARM, not Thumb.
> >>
> >> Yes, but I suspect that is a function of the build system. I checked
> >> the rest of U-Boot and most of it (including SPL) is Thumb 2. I
> >> suppose we could use Thumb 2 for crt0.S if all the instructions are
> >> supported.
> >
> > Ok. Just in case, I'll run a check on whether crt0.S can be assembled
> > for Thumb and still wrk as expected. :)
> >
> > Do you have a list of source files which still build for ARM under
> > CONFIG_SYS_THUMB_BUILD? I' would prefer all of the code to be thumb for
> > consistence, except probably... exception :) entry points -- and even
> > these should be able to run in full Thumb 2.
> 
> No I don't have a list, but it might be all assembler files. I don't
> see why cro0.S would be special.

Ok, so after some research, .S files voluntarily not assembled in Thumb
mode when -mthumb is defined in gcc because of this:

Answer: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=27237

(summary: -mthumb for gcc means 'use thumb2', while it means 'use
dumb, 16-bit, thumb1' for GNU as, so this option is voluntarily not
passed on to GNU as. You have to use .thumb in the .S file instead.) 

Second: getting a successful, though quick'n'dirty, build with vectors.S
assembled in Thumb-2 mode needed surprisingly little change in
vectors.S. I tried this with mx53loco, and it only required:

	- adding '.syntax unified';

	- adding a .thumb directive -- *after* the vectors per se, which
	  must still be assembled in ARM mode because current hardware
	  always executes exceptions vectors in ARM mode (1);

	- using '.balign' instead of '.balignl' which causes the
	  assembler to complain that it cannot fit an integer
	  number of '0xdeadbeef' in the filling space;

	- making macro get_bad_stack use lr instead of r13, which
	  Thumb does not allow in 'msr spsr,' instructions;

	- adding '.thumb_func' to all routines so that the linker makes
	  all references to them odd and therefore, cause the CPU to
	  enforce Thumb mode when branching to them.

	(1) although you *could* produce an ARM-based SoC that runs in
	Thumb mode by default. In this case, you'd have to make the
	vectors themselves Thumb too.

Third: getting a successful *run* of the resulting file will require
some work which I'm not going to do without a good incentive :) -- and
so does producing a clean vectors.S, i.e. one which will assemble
correctly for both ARM and Thumb.

> Regards,
> Simon

Amicalement,
-- 
Albert.