netdev - Re: Extending socket timestamping API for NTP

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Date:   Wed, 8 Feb 2017 11:26:23 +0100
From:   Miroslav Lichvar <mlichvar@...hat.com>
To:     sdncurious <sdncurious@...il.com>
Cc:     netdev@...r.kernel.org, Richard Cochran <richardcochran@...il.com>,
        Jiri Benc <jbenc@...hat.com>,
        "Keller, Jacob E" <jacob.e.keller@...el.com>,
        Denny Page <dennypage@...com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: Extending socket timestamping API for NTP

On Tue, Feb 07, 2017 at 12:37:15PM -0800, sdncurious wrote:
> On Tue, Feb 7, 2017 at 6:01 AM, Miroslav Lichvar <mlichvar@...hat.com> wrote:
> > 6) new SO_TIMESTAMPING option to get PHC index with HW timestamps
> >
> >    With bridges, bonding and other things it's difficult to determine
> >    which PHC timestamped the packet. It would be very useful if the
> >    PHC index was provided with each HW timestamp.
> >
> >    I'm not sure what would be the best place to put it. I guess the
> >    second timespec in scm_timestamping could be reused for this, but
> >    that sounds like a gross hack. Do we need to define a new struct?
> 
> What is the use case for this. even if the delay though the PHY's how
> would that be compensated ?

The idea was that applications like NTP servers and clients wouldn't
have to care about interfaces and how they map together with addresses
to PHCs over time. Currently, I use the interface index from
IP_PKTINFO to get the PHC, but that doesn't work with bridges and
other virtual interfaces. Another possibility would be an option to
modify the behavior of IP_PKTINFO to save the index of the real
interface. I'm not sure how would that compare in difficulty to
extending SCM_TIMESTAMPING with PHC index.

-- 
Miroslav Lichvar