linux-kernel - Re: [PATCH 1/5] ptp: Added a brand new class driver for ptp clocks.

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Message-ID: <20100827140205.GA3293@riccoc20.at.omicron.at>
Date:	Fri, 27 Aug 2010 16:02:05 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	john stultz <johnstul@...ibm.com>,
	Christian Riesch <christian.riesch@...cron.at>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, devicetree-discuss@...ts.ozlabs.org,
	linux-arm-kernel@...ts.infradead.org,
	Krzysztof Halasa <khc@...waw.pl>,
	Rodolfo Giometti <giometti@...ux.it>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 1/5] ptp: Added a brand new class driver for ptp clocks.

On Fri, Aug 27, 2010 at 01:41:54PM +0100, Alan Cox wrote:
> > The master node in a PTP network probably takes its time from a
> > precise external time source, like GPS. The GPS provides a 1 PPS
> > directly to the PTP clock hardware, which latches the PTP hardware
> > clock time on the PPS edge. This provides one sample as input to a
> > clock servo (in the PTPd) that, in turn, regulates the PTP clock
> > hardware.
> 
> A PTP clock is TAI, Unix time is UTC.

But TAI and UTC progress at the same rate, and UTC differs from TAI by
a constant offset. In fact, the needed conversion is provided by the
protocol, so it is not hard to take a 1 PPS from GPS and set the PTP
clock to TAI.

> > This is the core issue and source of misunderstanding, in my view. The
> > fact of the matter is, the current generation of computers has
> > multiple clocks, and these are usually unsynchronized. I think we
> > should not try too hard to cover up or work around this. It is a fact
> > of life.
> 
> In this case I don't think you can. Their divergence is rather difficult
> to handle unless you have a GPS to hand.
> 
> But all this talk of "PTP this" and "PTP that" is not helpful. Any
> interface for additional time sources should be generic with PTP being
> one use case.

To tell the truth, my original motivation for the patch set was to
support PTP clocks and applications. I don't think that is such a bad
idea. After all, the adjtimex interface was added just to support NTP.

At the same time, I can understand the desire to have a generic
hardware clock adjustment API. Let me see if I can understand and
summarize what people are asking for:

	  clock_adjtime(clockid_t id, struct timex *t);

and struct timex gets some new fields at the end.

Using the call, NTPd can call clock_adjtime(CLOCK_REALTIME) and PTPd
can call clock_realtime(CLOCK_PTP) and everyone is happy, no?

Thanks,
Richard
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