[U-Boot] Query on CONFIG_SYS_THUMB_BUILD

[Simon Glass]

Hi Victor,

On 13 November 2014 09:29, Victor Ascroft <victorascroft at 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?

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.

Regards,
Simon