[U-Boot] Query on CONFIG_SYS_THUMB_BUILD

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

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.