linux-kernel - Re: [PATCH net-next v3 2/4] net: mdio: add PTP offset compensation to mdiobus_write

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Message-ID: <20190821080709.GO891@localhost>
Date:   Wed, 21 Aug 2019 10:07:09 +0200
From:   Miroslav Lichvar <mlichvar@...hat.com>
To:     Hubert Feurstein <h.feurstein@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>, netdev <netdev@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Richard Cochran <richardcochran@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net-next v3 2/4] net: mdio: add PTP offset compensation
 to mdiobus_write_sts

On Tue, Aug 20, 2019 at 06:56:56PM +0200, Hubert Feurstein wrote:
> Am Di., 20. Aug. 2019 um 17:40 Uhr schrieb Miroslav Lichvar
> > I think a large jitter is ok in this case. We just need to timestamp
> > something that we know for sure happened after the PHC timestamp. It
> > should have no impact on the offset and its stability, just the
> > reported delay. A test with phc2sys should be able to confirm that.
> > phc2sys selects the measurement with the shortest delay, which has
> > least uncertainty. I'd say that applies to both interrupt and polling.
> >
> > If it is difficult to specify the minimum interrupt delay, I'd still
> > prefer an overly pessimistic interval assuming a zero delay.
> >
> Currently I do not see the benefit from this. The original intention was to
> compensate for the remaining offset as good as possible.

That's ok, but IMHO the change should not break the assumptions of
existing application and users.

> The current code
> of phc2sys uses the delay only for the filtering of the measurement record
> with the shortest delay and for reporting and statistics. Why not simple shift
> the timestamps with the offset to the point where we expect the PHC timestamp
> to be captured, and we have a very good result compared to where we came
> from.

Because those reports/statistics are important in calculation of
maximum error. If someone had a requirement for a clock to be accurate
to 1.5 microseconds and the ioctl returned a delay indicating a
sufficient accuracy when in reality it could be worse, that would be a
problem.

BTW, phc2sys is not the only user of the ioctl.

-- 
Miroslav Lichvar