Messages in this thread | | | Date | Fri, 20 Sep 2013 23:30:20 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH] clockevents: Sanitize ticks to nsec conversion |
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On Fri, 20 Sep 2013, Uwe Kleine-König wrote: > On Fri, Sep 20, 2013 at 11:56:27AM +0200, Thomas Gleixner wrote: > > On Thu, 19 Sep 2013, Uwe Kleine-König wrote: > > > > + * from nsec to device ticks will be correct. > > > > + * > > > > + * For mult > (1 << shift), i.e. device frequency is > 1GHz we > > > > + * need to be careful. Adding mult - 1 will result in a value > > > > + * which when converted back to device ticks will be larger > > > s/will/can/ > > > > No, it will always be larger. > Hmm, consider a 1.25 GHz clock with shift = 2 and mult = 5. Then > ns2clc(clc2ns(1000)) = 1000. So it's not always larger! > In the fast-clock-case we have: > With x << shift = n * mult - k for k in [0 .. mult-1] and an integer n: > > ns2clc(clc2ns(x)) > = ns2clc(((x << shift) + mult - 1) / mult) > = ((((x << shift) + mult - 1) / mult) * mult) >> shift > = n * mult >> shift > = ((x << shift) + k) >> shift > = x + (k >> shift) > > So ns2clc(clc2ns(x)) = x for all x > 0 that have > > k = mult - ((x << shift) - 1) % mult - 1 < 1 << shift > > So my correction still stands.
Fair enough.
> > 1) We cannot add if we'd overflow > > > > 2) For mult <= 1 << shift it's always correct > > > > 3) for mult > 1 << shift we only apply it to the min value not the max > > Yeah, I didn't say your code is wrong *here*. I just think that my > easier (and so probably faster) code is good enough.
Granted. I was stuck in the correctness discussion. So yeah, it does not matter if we steal 30 usec of maximum idle sleep time from a 32kHz clock. OTOH it does not matter much in the setup slow path to take another conditional. :)
> Best regards and thanks for the nice discussion,
Ditto! You saved me from actually sitting down and using the pencil to do the proper math.
Thanks,
tglx | |