[U-Boot] Query on CONFIG_SYS_THUMB_BUILD

Stefan Agner
2014-11-18T16:07:03Z

On 2014-11-14 15:01, Simon Glass 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?
>
> 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.

Ok, turns out the problem with Thumb2 only appear when using
CONFIG_USE_ARCH_MEMCPY (hence added Matthias to CC). Does peach_pi still
works with that config enabled? On the Vybrid board we use that to speed
up NAND access, with the current NAND driver, data get copied by the
CPU.

In setup_reloc, common/board_f.c, probably the first use of memcpy,
things started to get weird. The code after memcpy doesn't get executed,
I think something with the stack goes wrong, but not really sure what
happens.

--
Stefan

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