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Date:	Fri, 28 Sep 2012 10:26:38 +0200
From:	Richard Cochran <>
To:	Miroslav Lichvar <>
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 Fri, Sep 28, 2012 at 09:53:03AM +0200, Miroslav Lichvar wrote:
> 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.

Yes, I did think of that, too. There are some reserved fields in the
ioctl. It would be possible to use one field as a clockid_t for the
second phc device. For the static CLOCK_XYZ clock ids we could have a
switch/case. Then we could read any two clocks in the same way as in
this patch.

I am guessing it would be possible to synchronize two PHC devices to
within a few microseconds this way. Probably that is not good enough
to implement a boundary clock, for example, so I have my doubts about
the utility of this. But in any case, it is possible, and I think that
feature can wait for now.

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