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
This is much better than INT_MAX/2.
> > + } else {
> > + u64 usec = div_u64(nsec, 1000);
> > +
> > + if (usec > INT_MAX)
> > + usec = INT_MAX;
> > + return (int)usec;
> > + }
> > +}
>
>
>
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