Re: [PATCH v2 4/4] kernel/time/clockevents: compensate for monotonic clock's dynamic frequency

[Thomas Gleixner]

On Mon, 11 Jul 2016, Nicolai Stange wrote:
> > +	raw = ((u64)interval >> 32) * raw_mult; /* Upper half of interval */
> > +	if (raw >> 32)
> > +		return KTIME_MAX;
> > +	raw <<= 32;
> > +	tmp = ((u64)interval & U32_MAX) * raw_mult; /* Lower half of interval */
> > +	if (U64_MAX - raw < tmp)
> > +		return KTIME_MAX;
> > +	raw += tmp;
> > +
> > +	/* Finally, do raw /= mono_mult with proper rounding. */
> > +	if (U64_MAX - raw < mono_mult / 2)
> > +		return KTIME_MAX;
> > +	raw += mono_mult / 2;
> > +	do_div(raw, mono_mult);
> > +
> > +	return (s64)raw;

That's a complete insanity. No way that we are going to do all this math in
the CE programming path.

If you want to address the issue, then you need to find a way to adjust the
mult/shift factors of the clock event device occasionally.

Thanks,

	tglx