Re: [PATCH v4 1/4] Produce system time from correlated clocksource

Richard Cochran
2015-10-20T14:55:30Z

On Tue, Oct 20, 2015 at 01:51:13PM +0200, Richard Cochran wrote:
> You can, in fact, achieve "proper" correlation by sampling. As John
> said, the question is whether the method in the patch set "measurably
> improves the error" over using another, simpler method.

Here is a short example to put some numbers on the expected error.
Let the driver sample at an interval of 1 ms. If the system time's
frequency hasn't changed between two samples, A and B, then the driver
may interpolate without introducing any error.

If the frequency is changed between the sample times, then the
interpolated value will have some error. Because 1 ms is smallest HZ
value, the frequency can change at most once during the sample. If
the frequency changes near point A or B, then the error is minimal.
The worst case occurs when the frequency is changed half way between A
and B.

Suppose the frequency is changed by 10 PPM, at point C, half way
between A and B. This change results in a 5 nanosecond time
difference at B (10 PPM over C -> B). The driver will interpolate
using line A-B with slope increased by 5 PPM, and the worst case
error, found at point C, is then 2.5 nanoseconds.

Thanks,
Richard