[PATCH 0/2] ARM: EXYNOS: Allow to use architected timer

Arnd Bergmann arnd at arndb.de
Tue Oct 6 03:03:09 PDT 2015

On Tuesday 06 October 2015 12:44:54 Pavel Fedin wrote:
>  Hello!
> > My feeling is that is shouldn't be keyed off the presence of the device in DT
> > though. Maybe we can find a way that allows you to put the device into DT
> > but not have it used by default unless the user explicitly enables it, e.g. though
> > a kernel command line option like "force_broken_archtimer"?
>  For this purpose we have status = "disabled" in the device tree. We can add CP15 timer node and
> disable it. If the user needs it, the bootloader could just change "disabled" to "ok", and it's
> done.
>  The only problem here would be that CP15 timer on Exynos is actually tied to MCT. We need to enable
> MCT, otherwise clocks don't tick. That's why i decided to make CP15 timer a subnode.
>  My current patch lacks "status" property check, i could add it if you agree with this approach.

It's not ideal, because the hardware is always there and always slightly
broken, so it's not really a property of the platform. I'd be happier with
something that is easier for users to change if they know what they are
doing and also makes it clear that it is slightly risky.

> > Or maybe we could find a way to keep using MCT in the host but use the
> > arch timer in the guest only?
>  That's an interesting question, but, perhaps, it would require more changes to the code, and KVM
> maintainers don't like these things, calling them "broken non-compliant hardware". So, i guess, we
> could move from simple things to more complex ones.

It's probably worth trying to see how much complexity you need for that
approach, maybe it's not as hard in the end.

> > For all I can tell, nobody every /saw/ the problem, we just know that it
> > hasn't passed verification (or something like that) and you shouldn't
> > use it in production.
>  Yes, and i agreed that this should be an option, not the default. 
>  P.S. I cc'ed to linux-samsung-soc, but i'm not subscribed. Will it reach there?

I think it should, at least I didn't get an email back for my reply,
and I'm not subscribed either.


More information about the linux-arm-kernel mailing list