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

[Tom Rini]

On Wed, Feb 03, 2016 at 12:42:44PM +0800, Bin Meng wrote:
> Hi Simon,
> 
> On Wed, Feb 3, 2016 at 12:31 PM, Simon Glass <sjg at chromium.org> wrote:
> > Hi Bin,
> >
> > On 2 February 2016 at 08:02, Bin Meng <bmeng.cn at gmail.com> wrote:
> >> 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.
> >
> > ARM does. Not sure if Intel will, but they should IMO!
> >
> 
> Agreed. But guess Intel won't do that due to whatever backward
> compatible reasons..
> 
> >>
> >>> 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?
> >
> > Yes, I suppose there are more important things. The 32/64-bit split
> > bothers me. For example with the EFI loader series, U-Boot runs in
> > 32-bit mode so can only run a 32-bit EFI application (e.g. grub). That
> > seems like an annoying limitation. We don't have that limitation when
> > booting a kernel.
> 
> Isn't the limitation coming from UEFI itself? My understanding is that
> we cannot boot a 64-bit EFI application from a 32-bit UEFI BIOS as
> well.

Well, how much are distros going to dislike having to provide 32bit EFI
grub, for the x86 case here?

-- 
Tom
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