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SubjectRe: [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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