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

[Aneesh V <aneesh@ti.com>]

On Sunday 05 February 2012 11:49 AM, Simon Glass wrote:
> Hi,
>
> On Sat, Feb 4, 2012 at 1:15 AM, Aneesh V<aneesh@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
>
>  From your patch Aneesh I evolved something that I still use - it deals
> with the case where malloc cannot fit below the text area.
>
> I find any sort of messing with the ICE startup a pain - although I
> have often been able to script it. But for me I need to attach the
> device tree to the binary and a few other things so I might as well
> disable relocation at the same time. It also allows me to debug
> seamlessly in board_init_f() as well as afterwards.
>
> I will send a patch.

Great!

>
> It would be good to get something in mainline despite the
> protestations, if only to avoid all the work that people have to do to
> figure out this problem.

I am always in favor of that:)

best regards,
Aneesh