[U-Boot] How does u-boot know where to put its start code?

[Rogan Dawes <rogan@dawes.za.net>]

On 2011/04/20 7:42 AM, Albert ARIBAUD wrote:
> Le 20/04/2011 04:23, Hebbar, Gururaja a ?crit :
>> Hi,
>>
>> On Wed, Apr 20, 2011 at 02:43:23, Rogan Dawes wrote:
>>> Hi folks,
>>>
>>> I'm trying to understand a bit more about how u-boot creates the
>>> image, such that the CPU reset vector is pointing to the right piece
>>> of code when it is reset.
>>>
>>> i.e. my DNS323 (Orion5x) has a reset vector of 0xffff0000. But for
>>> the life of me, I can't find anywhere that actually references that
>>> value to place the start code at that point.
>>>
>>
>> Placing the final boot image is left to user who flashes/burns it
>> board. But it should be same as _TEXT_BASE (this is being removed now.
>> Orion5x is arm based). Also look
>> at<u-boot-src>\arch\arm\cpu\arm926ejs\start.S& 
>> <u-boot-src>\arch\arm\cpu\arm926ejs\u-boot.lds for more info on how
>> linker is instructed to place the starting code at predefined address.
>>
>>> I'm basically trying to make sure that my CONFIG_SYS_TEXT_BASE is
>>> correct (the address in the flash to which I write the whole
>>> u-boot.bin file, right?.
>>>
>> This is passed to linker as the entry point.
> 
> There is another point re: orion5x based boards: often, their designers
> preferred generating a linear image for U-Boot, but the fact that the
> vector address is at FFFF0000 makes it impossible to directly the image
> there because it is always greater than 64K. So the designers put some
> "pseudo-rom boot code" at FFFF0000 that will finally jump to an address
> lower in FLASH; for ED Mini V2 it is FFF90000, and that's where the
> U-Boot image is supposed to be flashed.

So, is that the address that you would use for CONFIG_SYS_TEXT_BASE ?

> Rogan, I bet in the DNS323 case, the same applies modulo your Flash
> size. Try tracing through the FFFF0000 code, it should not last more
> than a few tens of instructions before it jumps to some absolute address.

Do you think it would be possible to figure it out from the original
vendor u-boot?

Thanks

Rogan