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

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Date:	Fri, 27 Aug 2010 15:50:40 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Richard Cochran <richardcochran@...il.com>
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.

> 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

ptp *clocks*

> 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.

For a new syscall you could equally make it

	(clockid_t id, void *args)

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

If you only have one clock that you are calling 'the PTP clock' - but is
that a good assumption ?

I agree with your fundamental arguments as I understand them

- That it's another clock or clocks possibly not synchronized with the
  system clock

- That there should be a sensible API for doing slews and steps on other
  clocks but the systen clock.

I'm concerned about the assumption that there is a single magic PTP
clock, and calling it a PTP clock for two reasons

- There can be more than one

- PTP is just a protocol, in five years time it might be TICTOC or
  something newer and more wonderous, in some environments it'll be a
  synchronous distributed clock generation not PTP etc. Wiring PTP or
  IEE1588v2 into the clock name doesn't make sense.

I'd be happier with a model which says we have some arbitary number of
synchronization sources which may or may not have a connection to system
time, and may be using all sorts of synchronization methods. Clock in
fact seems almost a misnomer.

Alan


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