Messages in this thread |  | | Date | Fri, 20 Sep 2013 11:56:27 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH] clockevents: Sanitize ticks to nsec conversion |
| |
On Thu, 19 Sep 2013, Uwe Kleine-König wrote: > > + u64 rnd = (u64) evt->mult - 1; > > > > if (unlikely(!evt->mult)) { > > evt->mult = 1; > > WARN_ON(1); > > } > I suggest to move the assignment to rnd below this if block as it > changes mult.
True.
> > + /* > + * Upper bound sanity check. If the backwards conversion is > + * not equal latch, we know that the above shift overflowed. > + */ > + if (clc >> evt->shift) != (u64)latch) You didn't compile test, did you? Also the cast on the rhs isn't needed.
I did. I just missed to refresh the patch before sending it :)
> > + * 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.
> > + * 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.
> > + * than latch by (mult / (1 << shift)) - 1. For the min_delta > s/by/by up to/ > > > + * 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 > > 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.
Thanks,
tglx |  |