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Date:	Fri, 28 Sep 2012 09:53:03 +0200
From:	Miroslav Lichvar <>
To:	Richard Cochran <>
Cc:, David Miller <>,
	Jacob Keller <>,
	John Stultz <>
Subject: Re: [PATCH RFC net-next 1/1] ptp: add an ioctl to compare PHC time
 with system time

On Thu, Sep 27, 2012 at 08:12:16PM +0200, Richard Cochran wrote:
> This patch adds an ioctl for PTP Hardware Clock (PHC) devices that allows
> user space to measure the time offset between the PHC and the system
> clock. Rather than hard coding any kind of estimation algorithm into the
> kernel, this patch takes the more flexible approach of just delivering
> an array of raw clock readings. In that way, the user space clock servo
> may be adapted to new and different hardware clocks.

Would it make sense to extend the ioctl to allow also comparing the
PHC with another PHC or perhaps even a different system clock than

I'm thinking if someone wanted to synchronize one PHC to another, it
should be better to work with phc1-phc2 offsets than combine phc1-sys
and sys-phc2 offsets.

Miroslav Lichvar
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