Messages in this thread | | | Date | Fri, 20 Sep 2013 22:41:25 +0200 | From | Uwe Kleine-König <> | Subject | Re: [PATCH] clockevents: Sanitize ticks to nsec conversion |
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Hi Thomas,
On Fri, Sep 20, 2013 at 11:56:27AM +0200, Thomas Gleixner wrote: > On Thu, 19 Sep 2013, Uwe Kleine-König wrote: > > > + * For mult <= (1 << shift) we can safely add mult - 1 to > > > + * prevent integer rounding loss. So the backwards conversion > > It doesn't prevent inexactness to add mult - 1. It (only) asserts that > > the ns2delta(delta2ns(latch)) >= latch instead of ... <= latch when not > > doing it. > > For mult <= 1 << shift the conversion is always ending up with the > same latch value. Ah right, I missed that we're in the slow-clock-case.
> > > + * 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. > > > + * than latch by (mult / (1 << shift)) - 1. For the min_delta > > s/by/by up to/ From the calculation above you can also see that this term is wrong. k is smaller than mult (and there are values that realize k = mult - 1). So the converted back value can be larger than latch by up to (mult - 1) >> shift. This is zero for the slow-clock-case.
In the 1.25 GHz example above that means that the difference is up to 1, not 0 as your term would imply. 1004 is an example where the conversion to nano seconds and back to ticks results in a difference of 1.
> > > + * calculation we still want to apply this in order to stay > > > + * above the minimum device ticks limit. For the upper limit > > > + * we would end up with a latch value larger than the upper > > > + * limit of the device, so we omit the add to stay below the > > > + * device upper boundary. > > > + * > > > + * Also omit the add if it would overflow the u64 boundary. > > > + */ > > > + if ((~0ULL - clc > rnd) && > > > + (!ismax || evt->mult <= (1U << evt->shift))) > > > + clc += rnd; > > I would expect that > > > > if (!ismax) > > if (~0ULL - clc > rnd) > > clc += rnd; > > else > > clc = ~0ULL; > > > > is enough (and a tad more exact in the presence of an overflow). I have > > to think about that though. > > Errm. > > 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. > > > clockevents_calc_mult_shift(dev, freq, sec); > > > - dev->min_delta_ns = clockevent_delta2ns(dev->min_delta_ticks, dev); > > > - dev->max_delta_ns = clockevent_delta2ns(dev->max_delta_ticks, dev); > > > + dev->min_delta_ns = cev_delta2ns(dev->min_delta_ticks, dev, false); > > > + dev->max_delta_ns = cev_delta2ns(dev->max_delta_ticks, dev, true); > > Another improvement that came to my mind just now. For min_delta_ns you > > want to assert that it results in a value >= min_delta_ticks when > > converted back. For max_delta_ns you want ... value <= max_delta_ticks. > > What about the values in between? They for sure should land in > > [min_delta_ticks ... max_delta_ticks] when converted back and ideally > > should be most exact. The latter part would mean to add (rnd / 2) > > instead of rnd. I don't know yet how that would behave at the borders of > > the [min_delta_ns ... max_delta_ns] interval, but I think you still need > > to special-case that. > > Again: > > 1) For mult <= 1 << shift the backwards conversion is always the same as > the input value. > > 2) For mult > 1 << shift the backwards conversion of the min value is > always > than the input value. And the backwards conversion of the > max value is always < than the input value. > > The values between that are completely uninteresting as the > program_events code always converts from nsec to device ticks. > > We clamp the delta between min_ns and max_ns. So due to the above any > > min_ns <= delta <= max_ns > > will after conversion fulfil > > min_tick <= delta_tick <= max_tick > > So what are you going to improve? Either the math works or it does not. Right, my idea is nice, but useless.
So I suggest you resend your patch with the compile fix and the corrected comment and I will think about my suggestion to simplify the if condition independently as it's only a small runtime improvent and so not important enough to stop the correctness issue your patch is fixing.
Best regards and thanks for the nice discussion, Uwe
-- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | http://www.pengutronix.de/ | -- 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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