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 -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
