From: john stultz <[EMAIL PROTECTED]>
Date: Fri, 23 Feb 2007 16:53:27 -0800
> On Fri, 2007-02-23 at 16:34 -0800, David Miller wrote:
> > + .shift = 16,
> >
> > These shift selections all seem rather arbitrary.
> >
> > If it's not an arbitrary selection, it would be nice to have some
> > comments about how to go about choosing an appropriate shift.
> > I imagine the selections has to do with the possible range of
> > the frequencies the clocksource supports, and how much
> > accuracy you get for certain shift selections given that range.
>
> Correct. The higher the shift value, the more precise NTP multiplier
> adjustment we can make. However, too large w/ a high frequency
> clocksource and you'll risk overflowing 64bits on the mult.
>
> Although that's not super critical anymore since Roman implemented the
> high-res error accounting, the net effect should be the same over the
> long term(we just have to work harder w/ courser adjustments - resulting
> in very small clock frequency oscillations).
While we were discussing this I was debugging my clocksource sparc64
implementation, a shift of 32 didn't work whereas one of 16 (as
in your version of sparc64 support) did. :-)
There is also a very unfortunate inconsistency between how the shift
and multiplier are used in clock sources vs. clock events.
You can initialize your clocksource multiplier using:
mult = clocksource_hz2mult(ticks_per_hz, SHIFT);
but WOE BE TO HE who tries to use that for clockevents (like I
did initially) :-) You have to instead use something like:
mult = div_sc(ticks_per_hz, NSEC_PER_SEC, SHIFT);
I would have been done with the sparc64 dynticks support yesterday if
I didn't trip over all this stuff.
It also isn't clear from the comments that the first argument
to clocksource_hz2mult() and div_sc() is indeed "ticks_per_hz".
I had to gleen over the hpet and LAPIC drivers to kind of figure
this out. Another potential improvement for the comments :-)
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
