[U-Boot] RFC: Alternative boot_jump_linux() function

[Ramon Fried]

On Fri, May 25, 2018 at 3:11 PM, Tom Rini <trini at konsulko.com> wrote:
> On Thu, May 17, 2018 at 07:48:45PM +0300, Ramon Fried wrote:
>> On Thu, May 17, 2018 at 5:22 PM, Tom Rini <trini at konsulko.com> wrote:
>> > On Thu, May 17, 2018 at 02:01:55PM +0300, Ramon Fried wrote:
>> >
>> >> Hi.
>> >> I'm currently working on snapdragon bootloader support and in the
>> >> particular case where U-boot is running in Aarch32 and the kernel is
>> >> Aarch64 the specific implementation is to jump to Linux through SCM
>> >> call.
>> >>
>> >> I try to find the best possible way to provide an alternative boot function.
>> >> Adding #ifdef ARCH_SNAPDRAGON will just be too specific in
>> >> arm/lib/bootm.c in my opinion and I'm thinking of introducing kind of
>> >> a callback function in gd. that if exists will jump there instead of
>> >> executing boot_jump_linux().
>> >>
>> >> What do you think ?
>> >
>> > So, to be clear, we're on an aarch64 SoC, but U-Boot has been entered in
>> > 32bit mode.  And we need to boot a 64bit Linux Kernel.  What else, if
>> > anything, is also loaded/still in residence/etc?  Can you explain the
>> > overall usecase a bit more and why we're in 32bit mode?  Thanks!
>> >
>> OK.
>> Basically, Qualcomm has the following boot flow:
>>
>> PBL -> SBL -> LK -> Linux Kernel
>>
>> PBL - 32bit Primary boot loader(ROM) loads the SBL from eMMC.
>>
>> SBL - 32bit Secondary boot loader(eMMC/UFS), proprietary closed source,
>> Initializes the DDR, starts the trustzone and can load either a 32bit
>> or a 64bit (1)
>> ELF bootloader, given that it's signed.
>>
>> LK - Little kernel, 32bit. Qualcomm maintains a downstream version, open source.
>> Loads 64bit Android boot image from eMMC and jumps to it using SMC
>> call made to Trustzone.
>>
>> This was used primarily for Android, but when Dragonboards boards came
>> out, Qualcomm and Linaro started
>> distributing 64bit Linux OE & Debian builds that were booted by LK.
>> Because U-boot is much more useful than LK, the first U-boot
>> contribution was made by Mateusz Kulikowski.
>>
>> Basically, Mateusz port is wrapping U-boot to appear as a kernel so LK
>> boots it, then it boots Linux.
>> So basically, LK did the 32 -> 64 transition.
>>
>> Now, here where I appear in the story... :)
>
> Thanks for the detailed background.
>
>> I'm trying to get rid of LK and use U-boot as the real bootloader for Linux.
>> I succeeded in that, and I have all the patches ready for submission,
>> I just need to add a dedicated boot function
>> that does the SMC call, hence this discussion.
>>
>> (1) - It implies that we can in practice just use 64bit U-boot for Snapdragon.
>> The only problem I have right now is that there's isn't any
>> open-source tool to sign the Bootloader that supports ELF64
>> executable.
>
> Is it possible to just write that tool, or update an existing tool to do
> that signature?
Yes. It should be possible.

Actually, when I did have access to qcom internal tool for doing so I
was able to run U-boot
on 64bit mode. I had problems booting the kernel, as after the boot
weird things were happening,
like other cpu weren't responding, eMMC driver wasn't working etc.
I suspect it's related to low level initialization that was lacking,
or something similar.

I ended up working on the 32bit mode because I no longer have access
to the internal tool.
I believe that in the end we will end up with a working 64bit version.
I'm just thinking that meanwhile It maybe worthwhile to also have the
32bit version.

What do you think ?
>
> --
> Tom