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

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Message-ID: <alpine.DEB.2.11.1510132113470.25029@nanos>
Date:	Tue, 13 Oct 2015 21:15:51 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Richard Cochran <richardcochran@...il.com>
cc:	"Christopher S. Hall" <christopher.s.hall@...el.com>,
	jeffrey.t.kirsher@...el.com, hpa@...or.com, mingo@...hat.com,
	john.stultz@...aro.org, peterz@...radead.org, x86@...nel.org,
	intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kevin.b.stanton@...el.com
Subject: Re: [PATCH v4 1/4] Produce system time from correlated clocksource

On Tue, 13 Oct 2015, Richard Cochran wrote:

> On Tue, Oct 13, 2015 at 09:51:02AM +0200, Thomas Gleixner wrote:
> > 
> > You are restricting the problem space to this particular use
> > case. There are other use cases where PTP is not available or not the
> > relevant reference, but you still want to correlate time domains to
> > ART.
> 
> They may well be other use cases, but they have not been identified
> here.  The PTP to media clock problem has a very simple solution.  You
> do not need a history of system time stamps to solve it.

Well, these use cases are not in the focus of Christopher, but I have
a few in my head. Think industrial fieldbusses.
 
> Even if you wanted to correlate the system time's UTC with the media
> clock, still you don't need any shadow history for that.  Just feed
> (ART, UTC) pairs into the DSP at a regular rate, and let the DSP do
> the math.  This does not need to be part of the central time keeping
> code at all.

That's not working. The firmware is not going to change, no matter
what.

Thanks,

	tglx
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