[U-Boot] SPL load ARM Trusted Firmware BL31?

[Dan Handley]

Hi all

> -----Original Message-----
> From: Masahiro Yamada [mailto:yamada.masahiro at socionext.com]
> Sent: 05 October 2016 04:19
> Hi.
>
> Recently I implemented ARM Trusted Firmware BL31 for my SoCs.
>
> But, I am wondering how the boot-flow should be.
>
I'm no U-Boot expert but as ARM Trusted Firmware (TF) maintainer, I feel I should say something here.

In general we expect partners to take the bits of TF that work best for them. So all the bootflows discussed in this thread are valid as far as I'm concerned:

Proprietary Boot ROM -> Proprietary firmware -> TF BL31 -> U-Boot -> Linux

Proprietary Boot ROM -> U-Boot SPL -> TF BL31 -> Full U-Boot -> Linux

TF BL1 -> TF BL2 -> TF BL31 -> U-Boot or UEFI (BL33) -> Linux

We've gone to some effort to make TF as interoperable as possible with other boot firmware. If there's more we can do here, please let us know.

As Michal said in an earlier mail, which bootflow you choose depends on what brings you the most value. Others have stated the advantages of U-Boot SPL over TF BL2, which I can't disagree with. But there may be other factors to consider, for example:

* Interoperability with other boot firmware (e.g. UEFI) or other OS.
* Integration with proprietary secure software (which permissively licensed software allows).
* Alignment with ARM's Trusted Board Boot Requirements (TBBR) document.

Each vendor needs to make their own decisions here.

Regarding TF BL31, we'd prefer this to remain the EL3 Runtime Software on AArch64 platforms, because:

* It is the deployment vehicle for ARM support code, which is constantly evolving (e.g. ARM architectural support, ARM CPU support, errata workarounds, plus other fundamental IP support).
* It has upstream integration with various Trusted OS.
* It helps standardization by providing reference code for PSCI and potentially other future industry standards.

There may be other runtime resident firmware present, but we expect these to execute at a lower EL.

One area that may be worth exploring is whether more can be done to help integration of the early TF secure world initialization code into other boot firmware (e.g. proprietary Boot ROMs or U-Boot SPL). This could be useful on platforms where TF BL1 is not used or TF BL31 is not the first software to execute on the CPU after reset.

I hope that helps (and is not too controversial!).

Regards

Dan.

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