linux-kernel - Re: question about periodic clocks

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Date:	Sat, 10 Mar 2007 07:50:38 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	tglx@...utronix.de
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: question about periodic clocks

Thomas Gleixner wrote:
> Good point. I never thought about that and we set the period in the
> clock event device itself. You are right, the clockevents layer should
> hand over the period either with the set_mode call or seperately.
> Probably with the set_mode call, as it is needed exactly there and we
> don't want to have a "if (dev->mode == XXX)" check in set_next_event().
>
> I look into this.
>
>   

So, in the meantime, the period is 1/HZ?

I also have a question about clockevent cpumasks.  I was using the lapic
clockevent as a model, but as I understand it there's a lapic per CPU,
which explains why it registers a clockevent per cpu with that cpu alone
in the cpumask.

The Xen timer is a bit different; I guess more like hpet.  There's a
single (virtual-)machine-wide timer, which is "owned" by the last cpu
with programmed it; ie, that cpu is the one which gets the resulting
event interrupt.  Does this mean I should register a single clockevent
device with a cpumask of CPU_MASK_ALL?  Or should I constrain it to a
single cpu?

There's a comment in hpet.c saying

		 * Start hpet with the boot cpu mask and make it
		 * global after the IO_APIC has been initialized.

but I don't see any place where the hpet cpumask is updated.

Thanks,
    J

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