[RFC/RFT 1/3] iop: clocksource support

[Ralf Baechle]

On Tue, Sep 01, 2009 at 11:48:57PM +0200, Linus Walleij wrote:

> >  > Calculate this using the algorithm in arch/mips/kernel/time.c
> >  > they have a dynamically changing clocksource...
> >
> > The algorithm always computes the largest shift/mult pair that
> > solves the equation:
> >
> >        // 0 <= shift && shift <= 32
> >        u64 mult = ((u64)1E9 << shift) / hz;
> >        (mult >> 32) == 0
> >
> > Can I assume that this is to minimize precision loss?
> 
> So I think, I asked the question of how to calculate div but noone
> answered IIRC, then I found the MIPS code and it contained this
> clue. Ralf Baechle from the MIPS camp wrote this code so lets
> ask him.

The code you're talking about is in arch/mips/kernel/time.c:

[...]
void __init clocksource_set_clock(struct clocksource *cs, unsigned int clock)
{
	u64 temp;
	u32 shift;

	/* Find a shift value */
	for (shift = 32; shift > 0; shift--) {
		temp = (u64) NSEC_PER_SEC << shift;
		do_div(temp, clock);
		if ((temp >> 32) == 0)
			break;
	}
	cs->shift = shift;
	cs->mult = (u32) temp;
}

void __cpuinit clockevent_set_clock(struct clock_event_device *cd,
	unsigned int clock)
{
	u64 temp;
	u32 shift;

	/* Find a shift value */
	for (shift = 32; shift > 0; shift--) {
		temp = (u64) clock << shift;
		do_div(temp, NSEC_PER_SEC);
		if ((temp >> 32) == 0)
			break;
	}
	cd->shift = shift;
	cd->mult = (u32) temp;
}
[...]

The algorithm tries to minimize the loss of precision.  Like two years ago
I had already agreed with Thomas Gleixner to move this function into
generic code but somehow that never happened.

  Ralf