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

[Bin Meng]

Hi Albert,

On Tue, Feb 2, 2016 at 5:53 PM, Albert ARIBAUD
<albert.u.boot at aribaud.net> wrote:
> Hello Bin and Simon,
>
> On Tue, 2 Feb 2016 15:25:48 +0800, Bin Meng <bmeng.cn at gmail.com> wrote:
>> Hi Simon,
>>
>> On Tue, Feb 2, 2016 at 11:58 AM, Simon Glass <sjg at chromium.org> wrote:
>> > +Bin (sorry, meant to copy you before)
>
>> >>> 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 believe we can do dram_init() in 64-bit mode as well if MRC is
>> written in pure C.
>
> Bin: not sure what you mean by "if MRC is written in pure C" -- there
> is no C construct that can even approach the mrc instruction, which can
> only be emitted through an asm statement.

You are exposed as an ARM guy :-) I was talking about an Intel term
Memory Reference Code which are a amount of magic codes to initialize
system RAM.

>
>> > 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.
>
> Simon: seems like a sensible approach, as it does not mix 32 and 64
> bits in one "build artefact", plus it seems logical in that SPL's
> role is to get the platform ready for U-Boot; switching from
> power-on32-bit mode to 64-bit mode belongs quite "naturally" in SPL.
>
>> What's the benefit of doing a 64-bit bootloader? Intel's UEFI BIOS has
>> a 32-bit and 64-bit version, and has caused some troubles for the next
>> stage loader (bootia32.efi vs. bootx64.efi). I know for PowerPC, a
>> 64-bit U-Boot does not exist as 32-bit U-Boot can load 32-bit and
>> 64-bit kernel, just like what we have for x86. 64-bit U-Boot was only
>> seen on ARMv8, but that's the architecture limitation I believe, and
>> we have to do that.
>
> Some U-Boot users who might want to get rid of x86 32-bit code in
> x86 64-bit platforms just like in the past some people must have wanted
> to get rid of real-mode 16-bit x86 code in order to run pure 32-bit; the

Yep, but unfortunately we still cannot get rid of real-mode 16-bit x86
code even today :(

> idea is that if you can do with as well as without a feature, then that
> feature is potential dead code, and is candidate for removal, all the
> more when that feature partly collides with another feature, as here
> where 32-bit and 64-bit support sort of overlap partially.
>

I wonder if some day these processors (arm, x86, whatever else?) will
come out of reset in the 64-bit mode directly. No more any legacy
modes. At that time, 64-bit mode bootloader is definitely a must.

> Now, we can wait until x86 32-bit is really dead (as in "not used
> except in a few legacy projects whose engineers' children are about to
> retire") and then scrape dead code parts which no one really understands
> any more, or we can try and anticipate and replace code while we still
> have a grasp of what it does. I personally like the idea of anticipating
> better.
>
> Just in case, note that I do not mean x86 32-bit support should be
> removed from U-Boot now or later. I mean that if we can make x86 64-bit
> support in U-Boot less and less dependent on x86 32-bit support, then I
> think we should, so that the day we completely drop x86 32-bit support,
> x86 64-bit support will be (as) unaffected (as possible).
>

I agree with the philosophy here. But I sense this might be too
anticipating as there are some other tasks to do for U-Boot 32-bit
like ACPI and SMM. 32-bit is enough for now, unless we want to access
>4GB memory in U-Boot shell?

Regards,
Bin