Re: [PATCH 1/2 v3] SGI RTC: add clocksource driver

[john stultz <johnstul@us.ibm.com>]

On Thu, 2008-11-20 at 15:08 -0800, Andrew Morton wrote:
> On Wed, 19 Nov 2008 15:23:50 -0600
> Dimitri Sivanich <sivanich@sgi.com> wrote:
> 
> > This patch provides a driver for SGI RTC clocks and timers.
> > 
> > This provides a high resolution clock and timer source using the SGI
> > system-wide synchronized RTC clock/timer hardware.
[snip]
> > +static struct clocksource clocksource_uv = {
> > +	.name		= RTC_NAME,
> > +	.rating		= 400,
> > +	.read		= uv_read_rtc,
> > +	.mask		= (cycle_t)UVH_RTC_REAL_TIME_CLOCK_MASK,
> > +	.shift		= 0,
> > +	.flags		= CLOCK_SOURCE_IS_CONTINUOUS,
> > +};

Hey Dimitri,
	One issue with this clocksource is the shift value. The clocksource
frequency is calculated from the mult/shift pair (f = mult/2^shift).
When NTP steers the clock in the generic timekeeping code, it does so by
tweaking the mult value up or down. However if the shift value is too
low, that ends up being quite a large change to the frequency.

So in order to get fine grained frequency adjustments I'd recommend
using a larger shift value (8 at least). Ideally you want the largest
shift value possible, but some care is needed that it isn't too big,
because larger shift values mean larger mult values, and you don't want
to overflow on the multiplication.

A rule of thumb I use is to find a shift value so the resulting mult
value won't overflow 1 second worth of cycles.

thanks
-john