Messages in this thread | | | Date | Fri, 23 Feb 2007 21:52:44 -0800 (PST) | Subject | Re: sparc generic time / clockevents | From | David Miller <> |
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From: john stultz <johnstul@us.ibm.com> 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 majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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