netdev - Re: [PATCH net-next] dp83640: Adjust ptp event timestamps

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Date:	Thu, 10 Jul 2014 09:13:51 +0000
From:	Sørensen, Stefan 
	<Stefan.Sorensen@...ctralink.com>
To:	"richardcochran@...il.com" <richardcochran@...il.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] dp83640: Adjust ptp event timestamps

On Wed, 2014-07-09 at 19:41 +0200, Richard Cochran wrote:
> On Wed, Jul 09, 2014 at 12:22:23PM +0200, Stefan Sørensen wrote:
> > From the dp86340 Software Development Guide:
> >   Event timestamp values should be adjusted by 3*reference clock period +
> >   11 ns = 35 ns to compensate for input path and synchronization delays.
> > 
> > So subtract 35ns from event timestamps.
> 
> I have avoided adding this kind of thing into drivers because I think
> people will want to make the correction in the delayAsymmetry field of
> their user space stack. After all, this is exactly what that value is
> supposed to cover.

This is not adjusting the packet timestamps but the gpio event
timestamps - the ptp protocol is not involved here. Without this
adjustment, looping back a pps signal from one gpio to another will
result in a ~35ns offset.

> Some parts have fixed delays (or at least they claim to) and some have
> variable delays that can depend on link speed or other factors. So my
> gut feeling tells me to be consistent and leave all such corrections
> out, because in that way the end user will not have to research
> whether a particular driver is "pre-correcting" or not.

Then the end user will have to figure out what delay corrections need to
be made and configure the applications(s) to adjust for this. But I
don't see any good solution to this either. 

Stefan