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

[Aneesh V <aneesh@ti.com>]

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

br,
Aneesh