netdev - Re: Extending socket timestamping API for NTP

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Date:   Thu, 09 Feb 2017 12:25:43 -0800
From:   Denny Page <dennypage@...com>
To:     Miroslav Lichvar <mlichvar@...hat.com>
Cc:     netdev@...r.kernel.org, Richard Cochran <richardcochran@...il.com>,
        Jiri Benc <jbenc@...hat.com>,
        "Keller, Jacob E" <jacob.e.keller@...el.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: Extending socket timestamping API for NTP


> On Feb 08, 2017, at 16:45, Denny Page <dennypage@...com> wrote:
> 
> [Resend as plain text]
> 
> 
>> On Feb 07, 2017, at 06:01, Miroslav Lichvar <mlichvar@...hat.com> wrote:
>> 
>> 5) new SO_TIMESTAMPING options to get transposed RX timestamps
>> 
>>  PTP uses preamble RX timestamps, but NTP works with trailer RX
>>  timestamps. This means NTP implementations currently need to
>>  transpose HW RX timestamps. The calculation requires the link speed
>>  and the length of the packet at layer 2. It seems this can be
>>  reliably done only using raw sockets. It would be very nice if the
>>  kernel could tranpose the timestamps automatically.
>> 
>>  The existing SOF_TIMESTAMPING_RX_HARDWARE flag could be aliased to
>>  SOF_TIMESTAMPING_RX_HARDWARE_PREAMBLE and the new flag could be
>>  SOF_TIMESTAMPING_RX_HARDWARE_TRAILER.
>> 
>>  PTP has a similar problem with SW RX timestamps, which are closer
>>  to the trailer timestamps rather than preamble timestamps. A new
>>  SOF_TIMESTAMPING_RX_SOFTWARE_PREAMBLE flag could be added for PTP
>>  implementations to get transposed timestamps in order to improve
>>  accuracy.
>> 
>> 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?
> 
> 
> Miroslav, if #5 were implemented, would #6 still needed?
> 
> Denny

Miroslav, please ignore this. Of course you still need the index in order to get the PHC offset. My bad.

Denny