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

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Message-ID: <20151020115113.GA23428@netboy>
Date:	Tue, 20 Oct 2015 13:51:13 +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 12:48:03PM +0200, Thomas Gleixner wrote:
> On Tue, 20 Oct 2015, Richard Cochran wrote:
> 
> > On Mon, Oct 19, 2015 at 05:36:56PM -0700, John Stultz wrote:
> > > If we're only tracking 4ms of history, how does this solution
> > > measurably improve the error over using the timestamps to generate
> > > MONOTONIC_RAW clock deltas (which doesn't require keeping any history)
> > > and using getnstime_raw_and_real to take an anchor point to calculate
> > > the delta from?  Why is adding complexity necessary?
> > 
> > This idea is variant of what I suggested in another reply in this
> > thread.  To my understanding, there is no need at all to keep a
> > history arbitrarily 4 ms long.  Instead, the DSP driver (or whoever
> > else may need such a thing) can simply sample the system time at the
> > rate needed for that particular application.
> 
> That's complete nonsense. The whole point is to have a proper
> correlation from ART/audio timestamps to system time. Sampling system
> time does not help in any way,

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.

Thanks,
Richard

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