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

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Message-ID: <alpine.DEB.2.11.1510210941130.4000@nanos>
Date:	Wed, 21 Oct 2015 09:44:10 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	John Stultz <john.stultz@...aro.org>
cc:	Richard Cochran <richardcochran@...il.com>,
	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, 20 Oct 2015, John Stultz wrote:

> On Tue, Oct 20, 2015 at 12:11 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > On Tue, 20 Oct 2015, Richard Cochran wrote:
> >> 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.
> >
> > Darn, we don't want to have that kind of sampling in every driver
> > which has this kind of problem even if it looks like the simpler
> > choice for this particular use case. This is going to be something
> > which next generation chips will have on more than just the audio
> > interface and we realy want to have a generic solution for this.
> 
> I sort of agree with Richard that the timekeeper history approach
> doesn't seem like a generic solution here.
 
I'm not pushing that approach. I just want a generic facility of some
sort to solve that.

> And again, you seem to be speaking with a bigger picture in mind that
> at least I don't yet share (apologies for being thick headed here).
> Being able to have various hardware sharing a time base is quite
> useful, and methods for correlating timestamps together are useful.
> But I don't yet really understand why its important that we can
> translate a hardware timestamp from some time in the past to the
> correct system time in the past without error.

If your device can only provide timestamps from the past, then having
access to the history is important if you want to have precise
correlation.

Thanks,

	tglx
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