Messages in this thread | | | Date | Mon, 21 Jan 2013 16:51:58 -0800 | From | John Stultz <> | Subject | Re: One of these things (CONFIG_HZ) is not like the others.. |
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On 01/21/2013 02:54 PM, Matt Sealey wrote: > On Mon, Jan 21, 2013 at 4:36 PM, John Stultz <john.stultz@linaro.org> wrote: >> On 01/21/2013 01:14 PM, Matt Sealey wrote: >>> My question really has to be is CONFIG_SCHED_HRTICK useful, what >>> exactly is it going to do on ARM here since nobody can ever have >>> enabled it? Is it going to keel over and explode if nobody registers a >>> non-jiffies sched_clock (since the jiffies clock is technically >>> reporting itself as a ridiculously high resolution clocksource..)? >> ??? Not following this at all. jiffies is the *MOST* coarse resolution >> clocksource there is (at least that I'm aware of.. I recall someone wanting >> to do a 60Hz clocksource, but I don't think that ever happened). > Is that based on it's clocksource rating (probably worse than a real > hrtimer) or it's reported resolution? Because on i.MX51 if I force it > to use the jiffies clock the debug on the kernel log is telling me it > has a higher resolution (it TELLS me that it ticks "as fast" as the > CPU frequency and wraps less than my real timer). So the clocksource rating is supposed to be defined by the clocksource driver writer, and just provides a way for the clocksource core to select the best clocksource given a set of clocksources. It is not defined as any sort of calculated mapping to any property of the clocksource itself (although some driver writers might compute a ratings value in that way, but I feel the static ranking is much simpler). The comment above struct clocksource in clocksource.h tries to explain this.
As far as jiffies rating, from jiffies.c: .rating = 1, /* lowest valid rating*/
So I'm not sure what you mean by "the debug on the kernel log is telling me it has a higher resolution".
> I know where the 60Hz clocksource might come from, the old Amiga > platforms have one based on the PSU frequency (50Hz in Europe, 60Hz > US/Japan). Even a 60Hz clocksource is useful though (on the Amiga, at > least, it is precisely the vsync clock for synchronizing your display > output on TV-out, which makes it completely useful for the framebuffer > driver), but.. you just won't expect to assign it as sched_clock or > your delay timer. And if anyone does I'd expect they'd know full well > it'd not run so well.
Yes, in the case I was remembering, the 60HZ was driven by the electrical line.
thanks -john
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