netdev - Re: [PATCH v3 1/4] Add correlated clocksource deriving system time from an auxiliary clocksource

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Message-ID: <20150904151021.GF18489@twins.programming.kicks-ass.net>
Date:	Fri, 4 Sep 2015 17:10:21 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	"Hall, Christopher S" <christopher.s.hall@...el.com>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"john.stultz@...aro.org" <john.stultz@...aro.org>,
	"richardcochran@...il.com" <richardcochran@...il.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>
Subject: Re: [PATCH v3 1/4] Add correlated clocksource deriving system time
 from an auxiliary clocksource

On Fri, Sep 04, 2015 at 03:02:19PM +0200, Thomas Gleixner wrote:
> > For example, supply the ART value as an argument and, in the case of
> > the realtime clock, keep a short history of clock changes.  It would
> 
> It's not only clock realtime which is affected by those.
> 
> > fail in cases where there are a lot of calls to adjtimex(),
> 
> That has nothing to do with lots of adjtimex calls. The kernel does a
> slow correction of the conversion values itself to avoid time jumping
> around.

I think what they're getting at is asking if there's a rate limit to
time adjustments, without that, saving the last n transition points will
still not cover any given length of history.

So what I think they're looking for; is given an upper bound on the DSP
delaying its data, come up with a fixed minimal amount of transitions
points we must store to cover the history.


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