[U-Boot] Query on CONFIG_SYS_THUMB_BUILD

Albert ARIBAUD
2014-11-14T15:26:43Z

Hello Simon,

On Fri, 14 Nov 2014 07:01:59 -0700, Simon Glass <sjg@chromium.org>
wrote:
> Hi Victor,
>
> On 13 November 2014 09:29, Victor Ascroft <victorascroft@gmail.com> wrote:
> > Hello,
> >
> > I am working with a Cortex A5 Freescale Vybrid Processor. Since a thumb build leads to a saving of almost 1 MB for my u-boot image and consequently to faster serial downloads I have been looking at this. Currently enabling this option leads to a hang.
> >
> > After some debugging I have narrowed the place of hang to "ldr pc, =board_init_r" in arch/arm/lib/crt0.S. My debugging procedure was to put a branch to a small function which just printed a small message with puts, just before the ldr instruction and then a printing a small message with puts just at the start of board_init_r in common/board_r.c . For a non thumb build, the two messages get printed and I can boot to the u-boot prompt. For a thumb build, only the first message before the ldr instruction gets printed.
> >
> > In crt0.S
> > bl debug_print
> > ldr pc, =board_init_r
> >
> > In board_init_r
> > puts("In board_init_r\n"); // Right at start
> >
> > void debug_print(void)
> > {
> > // Defined in board file
> > puts("Debug print\n");
> > }
> >
> > My assembly knowledge is limited and after some consultation with a senior colleague, he told me things to check.
> >
> > An object dump of the crt0.o shows a branch to an even address. For thumb, this is expected to be odd. To just try out, I did a change as below
> > ldr r3, =board_init_r
> > add r3, #1
> > bx r3
> >
> > No change with this. My expectation was the compiler/linker/assembler would take care of the requirements, with the CONFIG_SYS_THUMB_BUILD. Frankly speaking I am not sure if this is the complete issue or only a part of it. I have seen patches with regards to OMAP send in by Aneesh V, which made changes of the form .type fn_name, %function to all the low level assembly functions, but, I couldn't dig up much more or variants thereof. Basically, from what I understand, this takes care of specifying .thumb_func for a thumb function or so to speak.
> >
> > Any pointers?

Victor,

In addition to Simon's question below on the toolchain, I would like to
know which commit you're trying to build.

> I tried this on a peach_pi (Samsung Chromebook 2 13") and it worked OK
> for me. The code sequence you refer to came out as below for me.
>
> 23e01e10 <clbss_l>:
> 23e01e10: e1500001 cmp r0, r1
> 23e01e14: 35802000 strcc r2, [r0]
> 23e01e18: 32800004 addcc r0, r0, #4
> 23e01e1c: 3afffffb bcc 23e01e10 <clbss_l>
> 23e01e20: fa000dec blx 23e055d8 <coloured_LED_init>
> 23e01e24: fb000deb blx 23e055da <red_led_on>
> 23e01e28: e1a00009 mov r0, r9
> 23e01e2c: e5991030 ldr r1, [r9, #48] ; 0x30
> 23e01e30: e59ff008 ldr pc, [pc, #8] ; 23e01e40
> <clbss_l+0x30>
> 23e01e34: 02073800 .word 0x02073800
> 23e01e38: 23e41eb0 .word 0x23e41eb0
> 23e01e3c: 23e77bf0 .word 0x23e77bf0
> 23e01e40: 23e057a9 .word 0x23e057a9
>
> The 'ldr pc' line is loading from 23e01e40 which does have an odd address.
>
> What toolchain are you using? I tried with gcc 4.8.2 - including
> linaro's 2013.10 release.
>
> In arch/arm/cpu/armv7/config.mk there is a fallback to armv5 from
> armv7-a, and this may cause it to generate Thumb code instead of Thumb
> 2. But you should get errors if that happens.
>
> It's hard to debug with such limited visibility. But if I put a puts()
> at the start of board_init_r(), I see it on the serial console.

Simon,

I believe you've built crt0.S for ARM, not Thumb.

> Regards,
> Simon
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Amicalement,
--
Albert.