[U-Boot] [RFC] arm926ejs: fix jump to RAM nand_boot

[Reinhard Meyer <u-boot@emk-elektronik.de>]

Dear Albert ARIBAUD,
> Le 02/11/2010 09:57, Reinhard Meyer a ?crit :
>> Dear Heiko Schocher,
>>> But there is a possibility to prevent one copy, if TEXT_BASE =
>>> relocation address = CONFIG_SYS_NAND_U_BOOT_DST
>>>
>>> In this case nand_spl code copies u-boot from nand to
>>> CONFIG_SYS_NAND_U_BOOT_DST. As this is equal to the relocation address,
>>> no need to copy code in relocate_code().
>>>
>>> But as codesize changes (and with it relocation address) this
>>> is not a perfect solution.
>>
>> Worse: it would certainly crash since the original and relocated memory
>> areas must not overlap - they sure would with even the slightest size
>> change.
> 
> I'm not sure I get you there. If u-boot was linked for a given address
> and happends to be loaded at that same address, then there is no need to
> relocate, right?

*if* the code size changes, "need" for relocation would arise...

>> I would recommend that we add code to check for overlapping relocation
>> into
>> board.c and print a panic message if an overlap is detected.
> 
> I *think* overlap would be correctly handled if we modify the copy code
> to copy from top to bottom, because we know that the destination is
> higher than the source.

No, in above case the (new) destination could be lower than the loaded
address (e.g. code size increased).

In any case it is not worth the effort to plan for overlapping relocations.
We can always load u-boot to a very low address of SDRAM and let it relocate.

> The only tricky case would be when the target address is right in the
> middle of the source relocation code, because then the last iteratons of
> the copy code would corrupt this code; that can be avoided by making
> sure the SPL loads u-boot low enough in RAM.

Exactly.

My original message was: "please to not give people the idea that they can
avoid relocation by loading u-boot at the exact address the relocation would
calculate. That is bound to *really* break at the slightest change to u-boot."

Best Regards,
Reinhard