[U-Boot] [PATCH 12/26] ARM: add relocation support

[Wolfgang Denk <wd@denx.de>]

Dear Graeme Russ,

In message <AANLkTimDxzWm9-sekLv60-zNDFmBKE3dc=kxiO+ZqZZn@mail.gmail.com> you wrote:
>
> > I agree that it would be nice to have U-Boot completely position-
> > independent and completelky restartable, but this would be a new
> > feature, which newer existed before.
> >
> 
> x86 has a nice feature whereby I can build two images that differ only by
> TEXT_BASE. I can build using TEXT_BASE somewhere in low memory, tftp the
> image and the 'go' directly to the base address of the image

You can build such images for any architecture, but usually they will
not work.

The code assumes to be started at the reset vector, and to find a CPU
with all peripherals and registers in virgine state which is only
present after a reset.

On many processors there are certain steps that are irreversiable by
software - there are write-once registers, there are memory controller
registers which change the behaviour of the memory controller when the
are written for the first time, etc.

> This works because the x86 reset vector is at top of memory (0xfffffff0),
> not bottom (0x00000000) so the very first bytes of u-boot.bin are not the
> reset vector, but rather where the reset vector jumps to after performing
> some very low-level hardware init

The actual position of the reset vector in the address space has
nothing to do with that isse. It's other things like register states
or mode of operation of certain units (memory controller, MMU etc.)
that are different when coming frash out of reset versus when coming
from a running U-Boot systems.

> As long as I do not mess with the low-level init code, I can be confident
> that an image that 'boots' using the 'go' command will boot from Flash

This _may_ work on certain boards and processors, but it does not
(and I think it cannot) work in general.


Best regards,

Wolfgang Denk

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