On Wed, 29 Jun 2016, Nicolas Pitre wrote: > On Wed, 29 Jun 2016, Daniel Lezcano wrote: > > > On 06/29/2016 09:06 AM, Shreyas B. Prabhu wrote: > > > diff --git a/drivers/cpuidle/cpuidle.h b/drivers/cpuidle/cpuidle.h > > > index f87f399..c8ea5ad 100644 > > > --- a/drivers/cpuidle/cpuidle.h > > > +++ b/drivers/cpuidle/cpuidle.h > > > @@ -68,4 +68,27 @@ static inline void > > > cpuidle_coupled_unregister_device(struct cpuidle_device *dev) > > > } > > > #endif > > > > > > +/* > > > + * Used for calculating last_residency in usec. Optimized for case > > > + * where last_residency in nsecs is < INT_MAX/2 by using faster > > > + * approximation. Approximated value has less than 1% error. > > > + */ > > > +static inline int convert_nsec_to_usec(u64 nsec) > > > +{ > > > + if (likely(nsec < INT_MAX / 2)) { > > > > UINT_MAX ? > > Actually this can be better than that. > > > > + int usec = (int)nsec; > > First, you'll want an unsigned type. Given the provided argument is u64, > we can assume there won't be any negative values here. > > Then it would be wise to use a type with an explicit width, like U32. > > > > + usec += usec >> 5; > > > + usec = usec >> 10; > > > + return usec; > > And now you want to maximize the available range. So not to overflow the > first addition, we must respect: > > usec + (usec >> 5) <= 0xffffffff > usec + usec/32 <= 0xffffffff > usec <= (0xffffffff * 32) / 33 > > Therefore: > > nsec <= 0xf83e0f82
And to be sure, you should use 0xf83e0f82UL to avoid any potential sign extension. Nicolas _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev