netdev - Re: [PATCH net-next 5/5] ptp: start virtual clocks at current system time.

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Date:   Mon, 31 Jan 2022 08:32:40 -0800
From:   Richard Cochran <richardcochran@...il.com>
To:     Miroslav Lichvar <mlichvar@...hat.com>
Cc:     netdev@...r.kernel.org, Yangbo Lu <yangbo.lu@....com>
Subject: Re: [PATCH net-next 5/5] ptp: start virtual clocks at current system
 time.

On Mon, Jan 31, 2022 at 11:21:08AM +0100, Miroslav Lichvar wrote:
> I tried to find the discussion around this decision, but failed. Do
> you have a link?

I'll dig it up for you.

> To me, it seems very strange to start the PHC at 0. It makes the
> initial clock correction unnecessarily larger by ~7 orders of
> magnitude. The system clock is initialized from the RTC, which can
> have an error comparable to the TAI-UTC offset, especially if the
> machine was turned off for a longer period of time, so why not
> initialize the PHC from the system time? The error is much smaller
> than billions of seconds.

When the clock reads Jan 1, 1970, then that is clearly wrong, and so a
user might suspect that it is uninititalized.

When the clock is off by 37 seconds, the user will likely post a vague
complaint to linuxptp-users asking why linuxptp doesn't handle leap
seconds.

I prefer the clarity of the first case.

Thanks,
Richard