netdev - Re: Problems obtaining software TX timestamps

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Date:	Thu, 9 Sep 2010 19:32:28 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Ingo Kofler <ikofler@....uni-klu.ac.at>, netdev@...r.kernel.org
Subject: Re: Problems obtaining software TX timestamps

On Thu, Sep 09, 2010 at 04:02:38PM +0200, Eric Dumazet wrote:
> Should drivers call it at start_xmit() time, or at tx completion time ?

If I could tell you that, I could also describe the sound of one hand
clapping.

IMHO, it should be done as immediately after the buffer has been
handed off to the hardware. Not all MACs have a completion method, so
that would at least make the "rule" consistent.

In general, the goals of high volume and timestamp accuracy are in
conflict (unless you have HW timestamping). If users are relying on SW
timestamping, then they should be aware of the fact that high network
traffic will spoil their fun.

Having said that, I don't know enough about all the different kinds of
MACs out there to really answer the question definitively.

Richard
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