Re: Google Embedded Controller blob - how does that get to the EC?

On Sun, Dec 17, 2023 at 7:37 PM James Feeney <[email protected]> wrote:

> > Since my MrChromebox firmware does not use vboot, I have ported the
> EC-RW firmware update code from vboot to coreboot, and check if an EC-RW
> update is needed on each boot as part of EC init in ramstage. This code is
> in my tree on github for inspection.
>
> Well, there are over 100 files named "ramstage.c" in the github repository.
>

it's not in a file named ramstage; I was simply mentioning where it
occurred in the boot process, which is different than in Google-built images

https://github.com/MrChromebox/coreboot/commit/9f780d35a7bb029a27d96e44f7fe0797eecf5143


>
> Do I understand correctly that the MrChromebox firmware generated will
> have a custom ramstage, different from the stock coreboot ramstage, just to
> check if an EC-RW update is needed? Coreboot would not typically try to do
> that?


completely. My tree has over 100 commits in addition to the upstream tag on
which it is based. The addition of the EC firmware update is just one of
many.

coreboot does not update the EC firmware, because it's a function of vboot
and Google's payload (depthcharge).


>
> > It's not necessary to build your own EC-RW firmware or even update it
> unless there's a specific issue you're trying to resolve.
>
> But still, to play-around with different payloads, using mostly stock
> coreboot, the EC firmware could just be completely left out of the coreboot
> ROM image?
>

absolutely. If you're building from upstream coreboot, the RO EC firmware
will be used. if building from my tree, then the RW firmware will be used
(regardless if it is updated or not)


>
> And, if I still needed to update the EC firmware, I could use the Google
> flashrom fork, as in the Google example, just: flashrom -p internal:bus=lpc
> -w ec.bin ?
>

not necessarily. You'd need a version of flashrom which supports EC
firmware updates (it's actually being removed / may have been removed),
kernel permissions which allow it, an EC that's actually on the LPC bus (vs
eSPI). And you wouldn't want to write the entire EC firmware as in the
command above, you'd only want to update the RW portion. And you'd want to
have a mechanism to not boot the RW firmware automatically in case your
update failed, or in case it succeeded and the firmware was bad.

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