[U-Boot] 'Relocation' for sc520 (i386) based boards

[Graeme Russ]

Hi All,

I've been thinking about where U-Boot runs in memory on my sc520
based board and it appears that it is always executing from ROM and
I started pondering if I should attempt to relocate U-Boot into RAM.

Note: Please correct me if I am mistaken and U-Boot does indeed run
from RAM on i386 / sc520

This, of course, would give me the benefit of having unfettered access
to the flash that U-Boot is located on for things like environment
storage, upgrade in place etc.

My thought is to not actually 'Relocate' U-Boot but to use a very
handy feature of the sc520 - The ability to map the BOOTCS (Boot ROM
Chip Select) to anywhere in memory using a PAR (Programmable Address
Region).

Currently, BOOTCS is mapped from 0x38000000. My board has 512k Boot
Flash, so the BOOTCS PAR is 0x38000000 - 0x3807ffff. With U-Boot in
the upper 256k (puts the reset vector at fff0 for cpu startup), this
puts u-boot code at 0x38040000 (TEXT_BASE in config.mk).

The SDRAM for my board is 128MB starting at 0x00000000 (ending at
0x08000000)

My thought is to change TEXT_BASE to 0x08040000, keep BOOTCS at
0x38000000, copy 0x38040000 - 0x3807ffff to 0x08040000 - 0x0807ffff
as soon as I enter protected mode (and have access to all memory) and
have set up the SDRAM controller etc, then jump directly to the copy
in RAM. I would then have two copies of u-boot in memory - One in RAM
at 0x08040000 and one in ROM at 0x38040000.

One problem I can think of is that the linker will point all the jump
labels into RAM - not an issue for the 16-bit code which uses relative
jumps, but I think I may run into issues during the real to protected
mode transition (far jump) which, by definition, has to happen before
I get access to the full memory space. How do I tell the linker that
these jumps should target 0x38040000+ and not 0x08040000+ ?

Does all this sound logical? viable? sensible? practical?

Regards,

Graeme