netdev - Re: [Intel-wired-lan] [RFC v2 net-next 01/10] net: Add a new socket option for a future transmit time.

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Message-ID: <20180213095652.GA11459@localhost>
Date:   Tue, 13 Feb 2018 10:56:52 +0100
From:   Miroslav Lichvar <mlichvar@...hat.com>
To:     Jesus Sanchez-Palencia <jesus.sanchez-palencia@...el.com>
Cc:     netdev@...r.kernel.org, john.stultz@...aro.org,
        Richard Cochran <rcochran@...utronix.de>, jiri@...nulli.us,
        ivan.briano@...el.com, richardcochran@...il.com, henrik@...tad.us,
        jhs@...atatu.com, levi.pearson@...man.com,
        intel-wired-lan@...ts.osuosl.org, xiyou.wangcong@...il.com,
        tglx@...utronix.de, anna-maria@...utronix.de
Subject: Re: [Intel-wired-lan] [RFC v2 net-next 01/10] net: Add a new socket
 option for a future transmit time.

On Mon, Feb 12, 2018 at 02:39:06PM -0800, Jesus Sanchez-Palencia wrote:
> On 01/18/2018 12:42 AM, Miroslav Lichvar wrote:
> > Please keep in mind that the PHCs and the system clock don't have to
> > be synchronized to each other. If I understand the rest of the series
> > correctly, there is an assumption that the PHCs are keeping time in
> > TAI and CLOCK_TAI can be used as a fallback.
> 
> Just to double-check, imagine that I've configured the qdisc for
> SW best-effort and with clockid CLOCK_REALTIME. When it receives a
> packet with the clockid of a /dev/ptpX, the qdisc should just drop that
> packet, right?

Yes, I think it should drop it. The kernel does not know the offset
between the two clocks (they don't even have to be synchronized), so
it cannot convert a PHC-based TX time to the system time.

-- 
Miroslav Lichvar