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

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Message-ID: <alpine.DEB.2.11.1510201246250.3982@nanos>
Date:	Tue, 20 Oct 2015 12:48:03 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Richard Cochran <richardcochran@...il.com>
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, 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,

Thanks,

	tglx
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