Subject: Re: Extending socket timestamping API for NTP

From: Richard Cochran <[email-masked]>
Date: Thu, 23 Mar 2017 20:07:33 +0100

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