Hi Bin, On 2 February 2016 at 08:02, Bin Meng <bmeng...@gmail.com> wrote: > Hi Albert, > > On Tue, Feb 2, 2016 at 5:53 PM, Albert ARIBAUD > <albert.u.b...@aribaud.net> wrote: >> Hello Bin and Simon, >> >> On Tue, 2 Feb 2016 15:25:48 +0800, Bin Meng <bmeng...@gmail.com> wrote: >>> Hi Simon, >>> >>> On Tue, Feb 2, 2016 at 11:58 AM, Simon Glass <s...@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! > >> 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. But I'm not sure it is very important - just something I was thinking about. Regards, Simon _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de http://lists.denx.de/mailman/listinfo/u-boot