linux-kernel - Re: [patch 3/3] xtensa: ccount clocksource

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Date:	Tue, 3 Mar 2009 20:54:01 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Daniel Walker <dwalker@...o99.com>
Cc:	Johannes Weiner <jw@...ix.com>, Chris Zankel <chris@...kel.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 3/3] xtensa: ccount clocksource

On Tue, Mar 03, 2009 at 09:55:45AM -0800, Daniel Walker wrote:
> On Tue, 2009-03-03 at 16:30 +0100, Johannes Weiner wrote:
> > @ -29,6 +30,19 @@ unsigned long ccount_per_jiffy;              /* per 
> >  unsigned long nsec_per_ccount;         /* nsec per ccount increment
> > */
> >  #endif
> >  
> > +static cycle_t ccount_read(void)
> > +{
> > +       return (cycle_t)get_ccount();
> > +}
> > +
> > +static struct clocksource ccount_clocksource = {
> > +       .name = "ccount",
> > +       .rating = 200,
> > +       .read = ccount_read,
> > +       .mask = CLOCKSOURCE_MASK(32),
> > +       .mult = NSEC_PER_CCOUNT,
> > +};
> 
> You don't want to use the shift field?

Thanks for pointing it out.

To make sure I understood this:

If shift is 0, then a walltime adjustment would be done in 1/2^0
steps, meaning an adjustment of counting one nanosecond more or less
per ccount.

To give this a finer granularity and smooth out adjustments, the shift
should be a trade-off between too much adjustment and no adjustment
progress in a sane amount of time (and, of course, to stay within
bounds of the used type).

Does that make sense?

I found a patch of yours that introduced clocksource_hz2shift() but it
seems it hasn't been merged (yet).  Is it yet to get integrated?

Thanks in advance,

	Hannes
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