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. > > But I'm not sure it is very important - just something I was thinking about. > Yep, thanks for bringing this for discussion. It's very helpful. We can put the 64-bit port as a TODO list. Regards, Bin _______________________________________________ U-Boot mailing list [email protected] http://lists.denx.de/mailman/listinfo/u-boot
