[U-Boot] 64-bit x86 U-Boot?

[Simon Glass]

+Bin (sorry, meant to copy you before)

Hi Albert,

On 1 February 2016 at 00:14, Albert ARIBAUD <albert.u.boot at aribaud.net> wrote:
> Hello Simon,
>
> On Sun, 31 Jan 2016 19:20:31 -0700, Simon Glass <sjg at chromium.org>
> wrote:
>> Hi Bin,
>>
>> At present U-Boot supports booting a 64-bit kernel directly. It can
>> also be loaded as a 64-bit payload from EFI. But it cannot be built as
>> a 64-bit boot loader.
>>
>> I took a bit of a look at this. It looks like we need to stay in
>> 32-bit mode until the FSP is loaded. Also, to get to 64-bit mode I'm
>> pretty sure we need page tables, which means we need somewhere to put
>> them!
>>
>> Looking at the board_f init sequence, it seems that arch_cpu_init() is
>> the earlist we could switch to 64-bit. We'd need to grab some memory
>> from somewhere to do this - I wonder if this can be CAR? There is no
>> SRAM on x86 chips I think.
>>
>> For non-FSP devices we don't init the RAM until much later -
>> dram_init(). That means that a significant portion of the init
>> sequence would be 32-bit code. I'm not sure that will work.
>>
>> I suppose one option is to only go to 64-bit mode when relocating. But
>> then we end up with lots of code that needs to run in 32-bit and
>> 64-bit.
>>
>> Do you have any ideas on this?
>
> How about starting with implementing the last option, i.e. switch to 64
> bits when DDR is available, mainline that, then progressively work your
> way toward an earlier switch?

The tricky thing is that I'm not sure we can run 32-bit code in 64-bit
mode or vice-versa. The instruction set is similar but there are data
size settings that probably make it incompatible.

I wonder whether we might need to resort to SPL for the 32-bit
portion, and jump to a 64-bit U-Boot from there? Tegra does something
similar to that.

Regards,
Simon