netdev - Re: [PATCH] net: lan743x_ptp: convert to ktime_get_clocktai

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Message-ID: <CAK8P3a3guebtDLNhbQwNmA2nq=raKiP8ztGRpb03r0XD8h+ZNA@mail.gmail.com>
Date:   Wed, 15 Aug 2018 22:44:34 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Bryan.Whitehead@...rochip.com
Cc:     UNGLinuxDriver@...rochiptechnology.mail.onmicrosoft.com,
        David Miller <davem@...emloft.net>,
        YueHaibing <yuehaibing@...wei.com>,
        Networking <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH] net: lan743x_ptp: convert to ktime_get_clocktai_ts64

On Wed, Aug 15, 2018 at 10:41 PM <Bryan.Whitehead@...rochip.com> wrote:
>
> > > > Question: this is the only ptp driver that sets the hardware time to
> > > > the current system time in TAI. Why does it do that?
> > >
> > > This is done when the driver starts up after reset. Otherwise the clock is off
> > by 48 years.
> > > It seemed to me that the system time was the most appropriate clock to
> > sync to.
> > > If my reasoning is incorrect, please enlighten me.
> >
> > I've never worked with PTP, but my understanding from looking at the other
> > drivers is that the time normally gets set either from another host through
> > the PTP protocol, or using clock_settime() from user space with the current
> > time.
>
> Those methods will still work. But if it's not set by those methods, I thought the
> clock should at least be set once on driver startup to align with the system clock.
> After that, other methods are free to reset it again.

(adding Richard Cochran to Cc for more insight here)

I would argue that it's more important that the driver behaves like all other
PTP implementations. If we want the behavior to be that the initial PTP time is
set to the ktime_get_clocktai_ts64() value, then this should be done by the
PTP core rather than the device driver. If there is a good reason that the
other drivers don't do it like this, then I would assume the same reason applies
to lan743x.

        Arnd