[PATCH] sunxi: Add arm64 FEL support

[Jagan Teki]

On Thu, Nov 19, 2020 at 4:24 PM Andre Przywara <andre.przywara at arm.com> wrote:
>
> So far we did not support the BootROM based FEL USB debug mode on the
> 64-bit builds for Allwinner SoCs: The BootROM is using AArch32, but the
> SPL runs in AArch64.
> Returning back to AArch32 was not working as expected, since the RMR
> reset into 32-bit mode always starts execution in the BootROM, but not
> in the FEL routine.
>
> After some debug and research and with help via IRC, the CPU hotplug
> mechanism emerged as a solution: If a certain R_CPUCFG register contains
> some magic, the BootROM will immediately branch to an address stored in
> some other register. This works well for our purposes.
>
> Enable the FEL feature by providing early AArch32 code to first save the
> FEL state, *before* initially entering AArch64.
> If we eventually determine that we should return to FEL, we reset back
> into AArch32, and use the CPU hotplug mechanism to run some small
> AArch32 code snippet that restores the initially saved FEL state.
>
> That allows the normal AArch64 SPL build to be loaded via the sunxi-fel
> tool, with it returning into FEL mode, so that other payloads can be
> transferred via FEL as well.
>
> Tested on A64, H5 and H6.
>
> Signed-off-by: Andre Przywara <andre.przywara at arm.com>
> ---

Acked-by: Jagan Teki <jagan at amarulasolutions.com>