netdev - Re: Extending socket timestamping API for NTP

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Date:   Wed, 8 Feb 2017 15:34:52 -0800
From:   sdncurious <sdncurious@...il.com>
To:     Miroslav Lichvar <mlichvar@...hat.com>
Cc:     netdev <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 Wed, Feb 8, 2017 at 2:26 AM, Miroslav Lichvar <mlichvar@...hat.com> wrote:
> 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.

Why not just return the digest that is in the message ?
Though I am not sure if the least 32 bits will result in too many collisions.

RMS

>
> --
> Miroslav Lichvar