Re: [PATCH] ARM U-Boot BSS zeroing

[Goswin von Brederlow]

On Mon, May 13, 2013 at 03:37:32PM +0000, Leif Lindholm wrote:
> The import into the collaborative branch discarded the call to
> get_real_bss_start before starting the BSS zeroing operation.
> 
> While get_real_bss_start might have been a bit of an ugly hack,
> simply discarding it means that we now end up doing unaligned
> STR operations, and any platform with MMU disabled fails to boot.
> 
> The attached patch prepends a bytewise zeroing loop until
> word-aligned, and then continue as before.
> 
> /
>     Leif

> === modified file 'grub-core/kern/arm/uboot/startup.S'
> --- grub-core/kern/arm/uboot/startup.S        2013-05-03 13:07:39 +0000
> +++ grub-core/kern/arm/uboot/startup.S        2013-05-13 14:59:43 +0000
> @@ -100,7 +100,13 @@
>       @ Since we _are_ the C run-time, we need to manually zero the BSS
>       @ region before continuing
>       ldr     r0, =EXT_C(__bss_start) @ zero from here
> -     ldr     r1, =EXT_C(_end)                @ to here
> +     @ If unaligned, bytewise zero until base address aligned.
> +     mov     r1, #0
> +1:   tst     r0, #3
> +     beq     2f
> +     strb    r1, [r0], #1
> +     b       1b
> +2:   ldr     r1, =EXT_C(_end)                @ to here
>       mov     r2, #0
>  1:   str     r2, [r0], #4
>       cmp     r0, r1

What if I have a 1 byte BSS aligned at 1 byte? You would zero 3 bytes
and overshoot _end and then what happens after the cmp in the last line?

MfG
        Goswin