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

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Message-ID: <4C7A6164.60208@riesch.at>
Date:	Sun, 29 Aug 2010 15:32:20 +0200
From:	Christian Riesch <christian@...sch.at>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Richard Cochran <richardcochran@...il.com>,
	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.

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.

Not necessarily. AFAIK, the time distributed by IEEE1588v2 can either be 
based on the "PTP epoch" (timePropertiesDS.ptpTimescale=TRUE) and thus 
represent TAI or be based on an implementation specific arbitrary epoch 
(timePropertiesDS.ptpTimescale=FALSE) and represent time on some 
arbitrary time scale.

Christian


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