[coreboot] RFC: Changing CBMEM console to run as a persistent ring-buffer

[Julius Werner]

>
> I'm very concerned with compatibility. You can't guarantee that coreboot
> and payload match. And in case of mismatch you get a memory corruption that
> is very hard to trace. Can we please change signature of cbmem entry?


I would rather allow older implementations to continue working as best as
possible. Changing the signature would mean that an old payload (or 'cbmem'
command line tool, etc.) couldn't interoperate at all with the console. The
changes I made are as backwards-compatible as possible: in many cases (e.g.
before the console rolled over once) the old payload will continue working
just fine. If the console did roll over, the old payload can no longer
append lines and may print existing contents out of order. It will never
access invalid memory, though. (You may have been worried about bit 31 I'm
setting in the "cursor" field, but all existing implementations were
already required to check (cursor < size) before trusting the cursor to be
accessible since the old console would continue counting characters even
after the buffer filled up.)


> You mentioned having trouble building GRUB. Can you detail those?
> What do you mean by not having hardware supported by grub-coreboot?
> Grub-coreboot should work on all coreboot-supported boards.
>

I am able to build it, I just had to figure out how and install some
dependencies. But I tried booting it on an HP Chromebook 14 2013 (falco)
and it doesn't seem to recognize my keyboard and will reboot a few seconds
after displaying the GRUB console. (Most of the devices I have are ARM
unfortunately, which I'm assuming it doesn't support since all the coreboot
code is in i386 directories? And even if it did, it probably wouldn't have
a keyboard driver for them either. I assume there's no way to make it run
over the UART instead?)
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