linux-stable/Documentation/time_interpolators.txt

Time Interpolators
------------------

Time interpolators are a base of time calculation between timer ticks and
allow an accurate determination of time down to the accuracy of the time
source in nanoseconds.

The architecture specific code typically provides gettimeofday and
settimeofday under Linux. The time interpolator provides both if an arch
defines CONFIG_TIME_INTERPOLATION. The arch still must set up timer tick
operations and call the necessary functions to advance the clock.

With the time interpolator a standardized interface exists for time
interpolation between ticks. The provided logic is highly scalable
and has been tested in SMP situations of up to 512 CPUs.

If CONFIG_TIME_INTERPOLATION is defined then the architecture specific code
(or the device drivers - like HPET) may register time interpolators.
These are typically defined in the following way:

static struct time_interpolator my_interpolator {
	.frequency = MY_FREQUENCY,
	.source = TIME_SOURCE_MMIO32,
	.shift = 8,		/* scaling for higher accuracy */
	.drift = -1,		/* Unknown drift */
	.jitter = 0		/* time source is stable */
};

void time_init(void)
{
	....
	/* Initialization of the timer *.
	my_interpolator.address = &my_timer;
	register_time_interpolator(&my_interpolator);
	....
}

For more details see include/linux/timex.h and kernel/timer.c.

Christoph Lameter <christoph@lameter.com>, October 31, 2004