Re: [RFC 13/13] m68k: mac: convert to generic clockevent

Finn Thain
2020-10-09T22:21:38Z

Hi Arnd,

Perhaps patch 13 does not belong in this series (?).

All m68k platforms will need conversion before the TODO can be removed
from Documentation/features/time/clockevents/arch-support.txt.

On m68k, HZ is fixed at 100. Without addressing that, would there be any
benefit from adopting GENERIC_CLOCKEVENTS as per this RFC patch?

On Thu, 8 Oct 2020, Arnd Bergmann wrote:

> Now that the infrastructure allows kernels to have both legacy timer
> ticks and clockevent drivers in the same image, start by moving one
> platform to generic clockevents.
>
> As qemu only supports the q800 platform among the classic m68k, use that
> as an example.
>

Correct VIA emulation is suprisingly difficult, so this kind of work
should be tested on real hardware.

I say that because when I did the clocksource conversion for m68k I ran
into a bug in QEMU (since fixed) and also because I once worked on some of
the bugs in the emulated VIA device used in MAME/MESS.

> I also tried adding oneshot mode, which was successful but broke the
> clocksource. It's probably not hard to make it work properly, but this
> is where I've stopped.
>

I'm not so sure that one timer is able to support both a clocksource
driver and a clockevent driver. In some cases we may have to drop the
clocksource driver (i.e. fall back on the jiffies clocksource).

Anyway, even on Macs with only one VIA chip we still have two timers. So I
think we should try to use Timer 1 as a freerunning clocksource and Timer
2 as a oneshot clock event. This may result in better accuracy and simpler
code. This may require some experimentation though.

> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> I have never tried implementing a clockevent or clocksource
> driver in the past, so this is really just an experiment and
> I expect I got something wrong.
>

Writing clockevent drivers is new to me too. I'm still trying to discover
what the implications might be if the only available clockevent device
offers oneshot mode or periodic mode but not both.