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 <[email protected]> wrote: > > Hi Bin, > > > > On 2 February 2016 at 08:02, Bin Meng <[email protected]> wrote: > >> Hi Albert, > >> > >> On Tue, Feb 2, 2016 at 5:53 PM, Albert ARIBAUD > >> <[email protected]> wrote: > >>> Hello Bin and Simon, > >>> > >>> On Tue, 2 Feb 2016 15:25:48 +0800, Bin Meng <[email protected]> wrote: > >>>> Hi Simon, > >>>> > >>>> On Tue, Feb 2, 2016 at 11:58 AM, Simon Glass <[email protected]> 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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