Hi Albert,
On 16 November 2014 07:50, Albert ARIBAUD albert.u.boot@aribaud.net wrote:
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.
That doesn't sound too trciky, but I agree it's not a huge deal. I suppose the main benefit is consistency, since the code size improvement would be small.
Regards, Simon