Re: [PATCH 2/2] firmware: coreboot: Export active CBFS partition

patrick.rudolph
2019-10-10T09:47:00Z

On Wed, 2019-10-09 at 14:19 -0700, Julius Werner wrote:
> > Somehow we've gotten /sys/firmware/log to be the coreboot log, and
> > quite
> > frankly that blows my mind that this path was accepted upstream.
> > Userspace has to know it's running on coreboot firmware to know
> > that
> > /sys/firmware/log is actually the coreboot log.
>
> Not really sure I understand your concern here? That's the generic
> node for the log from the mainboard firmware, whatever it is. It was
> originally added for non-coreboot firmware and that use is still
> supported. If some other non-coreboot firmware wants to join in, it's
> welcome to do so -- the interface is separated out enough to make it
> easy to add more backends.
>
> I do agree that if we want to add other, more coreboot-specific
> nodes,
> they should be explicitly namespaced.
>

I'll add a new namespace called 'coreboot'.

> > But I also wonder why this is being exposed by the kernel at all?
>

It's difficult for userspace tools to find out how to access the flash
and then to find the FMAP, which resides somewhere in it on 4KiB boundary.
The "boot media params" usually give the offset to the FMAP from beginning
of the flash, but tell nothing about how to access it.

> I share Stephen's concern that I'm not sure this belongs in the
> kernel
> at all. There are existing ways for userspace to access this
> information like the cbmem utility does... if you want it accessible
> from fwupd, it could chain-call into cbmem or we could factor that
> functionality out into a library. If you want to get away from using
> /dev/mem for this, we could maybe add a driver that exports CBMEM or
> coreboot table areas via sysfs... but then I think that should be a
> generic driver which makes them all accessible in one go, rather than
> having to add yet another driver whenever someone needs to parse
> another coreboot table blob for some reason. We could design an
> interface like /sys/firmware/coreboot/table/<tag> where every entry
> in
> the table gets exported as a binary file.

I don't even consider using binaries that operate on /dev/mem.
In my opinion CBMEM is something coreboot internal, the OS or userspace
shouldn't even known about.

> I think a specific sysfs driver only makes sense for things that are
> human readable and that you'd actually expect a human to want to go
> read directly, like the log. Maybe exporting FMAP entries one by one
> like Stephen suggests could be such a case, but I doubt that there's
> a
> common enough need for that since there are plenty of existing ways
> to
> show FMAP entries from userspace (and if there was a generic
> interface
> like /sys/firmware/coreboot/table/37 to access it, we could just add
> a
> new flag to the dump_fmap utility to read it from there).

I'll expose the coreboot tables using a sysfs driver, which then can be
used by coreboot tools instead of accessing /dev/mem. As it holds the
FMAP and "boot media params" that's all I need for now.

The downside is that the userspace tools need to be keep in sync with
the binary interface the coreboot tables are providing.