[TF-A] Proposal: TF-A to adopt hand-off blocks (HOBs) for information passing between boot stages

[Simon Glass]

Hi François,

On Thu, 15 Jul 2021 at 08:25, François Ozog <francois.ozog at linaro.org>
wrote:

>
>
> On Sat, 10 Jul 2021 at 01:07, Julius Werner <jwerner at chromium.org> wrote:
>
>> > - "bloblist" is a general term for concept of linked list and it's not
>> exactly U-boot implementation. The proposed solution will cause some degree
>> of changes in all the participating projects. For backward compatibility
>> issue, we have already though about it and proposed to have build configs
>> which will be enabled by platform integrators.
>>
>> Okay, let's gather the requirements and then decide how the existing
>> system needs to be expanded to meet them. Please keep me in the loop
>> on those discussions.
>>
>> > > U-Boot has an established mechanism which is used in particular on
>> all chrome
>> > > books in both x86 and Arm environments.  I have the impression that
>> > > U-Boot is the typical BL33 so I would import the mechanism into TFA,
>> > > not the other way round.
>>
>> There has been no Chromebook (x86 or Arm) shipping with U-Boot since
>> 2013 (before TF-A existed). In fact, Chromebooks are all running
>> coreboot today and using the bl_aux_params mechanism (it's 3 SoC
>> vendors, more than 3 SoCs, but the parameter code is shared at the
>> vendor level), and one of my concerns here is trying to limit
>> disruption to those existing setups. I don't have enough data to say
>> what the "typical" boot flow among all TF-A devices in existence is,
>> but at least for the millions of Chromebooks out there it is coreboot
>> + TF-A, not U-Boot.
>>
> My bad! I candidly inferred from Chromebook reference in U-Boot and
> Simon's VBE proposal that U-Boot was used.
>

This is a bit off-topic, but it is somewhat in the reverse. U-Boot's
verified boot (2014 I think) uses the crypto from Chromium OS (which I
think came from Android at some point). VBE is an attempt to provide a
standard boot flow for U-Boot devices and is again heavily influenced by
Chromium OS. U-Boot does ship on most Chromebooks (oddly enough, not the
ARM ones!) but just as an alternate bootloader and is not used in the
verified-boot flow. I won't get into the details of how the Chroimum OS
replacement came about, as that is way off topic!

To me, bl_aux_params is a pretty limited. I am looking here:
https://github.com/ARM-software/arm-trusted-firmware/blob/master/include/export/lib/bl_aux_params/bl_aux_params_exp.h

Is that the right place?

There is not a huge amount of code in TF-A that uses it (mediatek and
rockchip?) so I don't see a problem with updating it, given the benefits of
interoperability.

In fact, having taken a bit of a look, my view is that U-Boot's bloblist is
a much better starting point for this effort:

- fuller implementation with more features
- has comments / more documentation
- easily supports everything in one block instead of a linked list (easier
to allocate)
- avoids 64-bit tags/size which seem quite unnecessary
- has unit tests

plus some minor points I won't mention. So I think it is worth the pain to
switch TF-A over.

Perhaps we can gather the requirements that are needed for this? So far I
am aware of:

- devicetree bob tag
- vendor-private tag area
- ability to link to another bloblist (do we really need this?)

Regards,
Simon