[PATCH] board: dragonboard410c: Load U-Boot directly without LK

[Tom Rini]

On Wed, Jul 07, 2021 at 11:06:02AM +0200, Stephan Gerhold wrote:

> At the moment the U-Boot port for the DragonBoard 410c is designed
> to be loaded as an Android boot image after Qualcomm's Little Kernel (LK)
> bootloader. This is simple to set up but LK is redundant in this case,
> since everything done by LK can be also done directly by U-Boot.
> 
> Dropping LK entirely has at least the following advantages:
>   - Easier installation/board code (no need for Android boot images)
>   - (Slightly) faster boot
>   - Boot directly in 64-bit without a round trip to 32-bit for LK
> 
> So far this was not possible yet because of unsolved problems:
> 
>   1. Signing tool: The firmware expects a "signed" ELF image with extra
>      (Qualcomm-specific) ELF headers, usually used for secure boot.
>      The DragonBoard 410c does not have secure boot by default but the
>      extra ELF headers are still required.
> 
>   2. PSCI bug: There seems to be a bug in the PSCI implementation
>      (part of the TrustZone/tz firmware) that causes all other CPU cores
>      to be started in 32-bit mode if LK is missing in the boot chain.
>      This causes Linux to hang early during boot.
> 
> There is a solution for both problems now:
> 
>   1. qtestsign (https://github.com/msm8916-mainline/qtestsign)
>      can be used as a "signing" tool for U-Boot and other firmware.
> 
>   2. A workaround for the "PSCI bug" is to execute the TZ syscall when
>      entering U-Boot. That way PSCI is made aware of the 64-bit switch
>      and starts all other CPU cores in 64-bit mode as well.
> 
> Simplify the dragonboard410c board by removing all the extra code that
> is only used to build an Android boot image that can be loaded by LK.
> This allows dropping the custom linker script, special image magic,
> as well as most of the special build/installation instructions.
> 
> CONFIG_REMAKE_ELF is used to build a new ELF image that has both U-Boot
> and the appended DTB combined. The resulting u-boot.elf can then be
> passed to the "signing" tool (e.g. qtestsign).
> 
> The PSCI workaround is placed in the "boot0" hook that is enabled
> with CONFIG_ENABLE_ARM_SOC_BOOT0_HOOK. The extra check for EL1 allows
> compatibility with custom firmware that enters U-Boot in EL2 or EL3,
> e.g. qhypstub (https://github.com/msm8916-mainline/qhypstub).
> 
> As a first step these changes apply only to DragonBoard410c.
> Similar changes could likely also work for the DragonBoard 820c.
> 
> Note that removing LK wouldn't be possible that easily without a lot of
> work already done three years ago by Ramon Fried. A lot of missing
> initialization, pinctrl etc was already added back then even though
> it was not strictly needed yet.
> 
> Cc: Ramon Fried <rfried.dev at gmail.com>
> Signed-off-by: Stephan Gerhold <stephan at gerhold.net>

Applied to u-boot/master, thanks!

-- 
Tom
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