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

[Ramon Fried]

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... :)

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.

Thanks,
Ramon.


> Tom