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

On Sat, Oct 10, 2020 at 12:21 AM Finn Thain [off-list ref] wrote:

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.

Yes, correct. I marked this patch as RFC instead of PATCH, as I'm
just trying to find out where it should be headed. I would hope the
other patches can just get merged.

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

I don't think so, I mainly did it to see if there is a problem with mixing
the two modes, and I couldn't find any. The behavior seems unchanged
before and after my patch, the main difference being a few extra kilobytes
in kernel .text for the generic clockevents code.

On Thu, 8 Oct 2020, Arnd Bergmann wrote:

quoted

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.

Good point, though I would be surprised if anything went wrong with
this patch on real hardware but not in emulation, as all the register-level
interactions with the timer are the same.

Adding oneshot mode is a completely different matter though, that
clearly needs to be tested on real hardware.

quoted

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.

Ah, good. This is partly what I had been hoping for, as my patch
can be used as a starting point for that if you want to give it a go.

     Arnd

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