Re: dynamic-hz

[Tony Lindgren]

* linux-os <linux-os@xxxxxxxxxxxxxxxxxx> [041214 15:04]:
> On Tue, 14 Dec 2004, Pavel Machek wrote:
> 
> >Hi!
> >
> >>>>The patch in question is at:
> >>>>
> >>>>http://linux-omap.bkbits.net:8080/main/user=tmlind/patch@xxxxxxxxxxx?nav=!-|index.html|stats|!+|index.html|ChangeSet@-12w|cset@xxxxxxxxxxx
> >>>
> >>>Wow, that's basically 8 lines of code plus driver for new
> >>>hardware... Is it really that simple?
> >>
> >>Yeah, the key things are reprogramming the timer in the idle loop
> >>based on next_timer_interrupt(), and calling timer_interrupt from
> >>other interrupts as well :)
> >>
> >>Should we try a similar patch for x86/amd64? I'm not sure which timers
> >>to use though? One should be programmable length for the interrupt,
> >>and the other continuous for the timekeeping.
> >
> >Yes, it would certainly be interesting. 5% power savings, and no
> >singing capacitors, while keeping HZ=1000. Sounds good to me.
> >
> >There are about 1000 timers available in PC, each having its own
> >quirks. CMOS clock should be able to generate 1024Hz periodic timer
> >(we currently do not use) and TSC we currently use for periodic timer
> >should be usable in single-shot mode.
> >								Pavel
> >--
> 
> If you use that RTC timer, it needs to be something that can be
> turned OFF. Many embedded applications use that because its the
> only timer that the OS doesn't muck with. It also has very low
> noise which makes in a good timing source for IIR filters for
> high precision, low data-rate data acquisition (like 24 bits).

OK, thanks for the information. That could be the continuous timer
then, and TSC the periodic timer.

> Since it generates an edge, its interrupt can't be shared.
> I certainly hope that you don't use it. One can read the
> time without disturbing the interrupt rate. One just
> needs to use the existing rtc_lock and not spin with
> the lock being held.

Yeah, the timer update would be just a read from the RTC timer.

> Currently the kernel RTC software allocates the RTC interrupt
> even though it doesn't use it. This makes it necessary to
> compile the RTC as a module and then remove it when another
> driver needs to use the RTC interrupt source.

The interrupt could be used for timer wrap only.

Tony
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