[U-Boot] Skipping relocation RAM to RAM, esp. on i.MX6?

[Simon Glass]

Hi,

On Sat, Feb 4, 2012 at 1:15 AM, Aneesh V <aneesh at ti.com> wrote:
> Hi Dirk,
>
>
> On Friday 03 February 2012 12:55 PM, Dirk Behme wrote:
>>
>> Hi,
>>
>> on i.MX6 devices, e.g. ARM2 or SabreLite, the ROM boot loader copies the
>> U-Boot image from the boot device, e.g. the SD card, to the main memory.
>> This does mean that U-Boot is started in RAM.
>>
>> With this, one might wonder why any relocation RAM -> RAM is done anyway
>> and if this could be skipped?
>>
>> Looking into the details shows that board_init_f() in
>> arch/arm/lib/board.c and relocate_code() in arch/arm/cpu/armv7/start.S
>> [1] are involved in this.
>>
>> In board_init_f() the relocation destination address 'addr' is
>> calculated. This is basically at the end of the available RAM (- some
>> space for various stuff like TLB tables etc.). At SabreLite this results
>> in 0x4FF8D000.
>>
>> By the boot loader, the U-Boot is loaded to
>>
>> CONFIG_SYS_TEXT_BASE 0x17800000
>>
>> This results in relocate_code() copying U-Boot from RAM 0x17800000 to
>> RAM 0x4FF8D000.
>>
>> Setting CONFIG_SYS_TEXT_BASE to the relocation destination address
>> 0x4FF8D000 does avoid the (unnecessary?) copy by
>>
>> cmp r0, r6
>> moveq r9, #0 /* no relocation. relocation offset(r9) = 0 */
>> beq clear_bss /* skip relocation */
>>
>> in relocate_code().
>>
>> But:
>>
>> 1) The resulting image still runs without the relocation
>> (CONFIG_SYS_TEXT_BASE 0x4FF8D000). But e.g. the U-Boot command line
>> doesn't work properly any more. Most probably this is because not only
>> the copy is skipped by the 'beq clear_bss', but the whole 'fix .rel.dyn
>> relocations' is skipped too.
>>
>> 2) It's hard to set CONFIG_SYS_TEXT_BASE at compile time to the
>> relocation address calculated at runtime in board_init_f() due to the
>> amount of #ifdef and runtime calculation done there. So finding a
>> generic approach which could easily defined in the config files to avoid
>> the relocation seems difficult.
>
>
> I haven't really completely read your mail. But here is an
> implementation I had provided long time back for ARM. But Wolfgang
> didn't want to take it. You can see the patch and the following
> discussion in this thread:
>
> http://thread.gmane.org/gmane.comp.boot-loaders.u-boot/96352