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

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Date:	Tue, 20 Oct 2015 16:55:23 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	John Stultz <john.stultz@...aro.org>,
	Christopher Hall <christopher.s.hall@...el.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"x86@...nel.org" <x86@...nel.org>,
	intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
	lkml <linux-kernel@...r.kernel.org>, kevin.b.stanton@...el.com
Subject: Re: [PATCH v4 1/4] Produce system time from correlated clocksource

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
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