netdev - Re: Extending socket timestamping API for NTP

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Message-ID: <20170323190733.GA30056@localhost.localdomain>
Date:   Thu, 23 Mar 2017 20:07:33 +0100
From:   Richard Cochran <richardcochran@...il.com>
To:     Miroslav Lichvar <mlichvar@...hat.com>
Cc:     netdev@...r.kernel.org, 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 Thu, Mar 23, 2017 at 05:21:45PM +0100, Miroslav Lichvar wrote:
> A better approach might be a control message that would provide the
> original interface index together with the length of the packet, so
> the application could transpose the HW timestamp and map the HW
> interface to the PHC.

This sounds better than trying to auto-magically transpose and correct
for link speed.

BTW, isn't there already a control message for "original interface
index"?
 
> The two values could be saved in the skb_shared_info structure. Now
> my question is if they could be useful also for other things than
> timestamping

such as?

> and if it should be a new socket option which would work
> on any socket independently from timestamping, or if it should rather
> be a new flag for the SO_TIMESTAMPING option. If the latter, would it
> make sense to put them in the skb_shared_hwtstamps structure and
> modify all drivers to set the values when a HW timestamp is captured
> instead of adding more code to __netif_receive_skb_core() or similar?

This information is solely for a highly specialized NTP application.
No normal program would ever need this, AFAICT.  So, if possible,
getting the original frame length should be done in a way that doesn't
affect users that don't need it.

Thanks,
Richard