netdev - Re: [PATCH v5 net-next 5/7] net: fix documentation of struct scm

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Message-ID: <20170519101118.GD21003@localhost>
Date:   Fri, 19 May 2017 12:11:18 +0200
From:   Miroslav Lichvar <mlichvar@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Richard Cochran <richardcochran@...il.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH v5 net-next 5/7] net: fix documentation of struct
 scm_timestamping

On Thu, May 18, 2017 at 03:38:30PM -0400, Willem de Bruijn wrote:
> On Thu, May 18, 2017 at 10:07 AM, Miroslav Lichvar <mlichvar@...hat.com> wrote:
> > +Note that if the SO_TIMESTAMP or SO_TIMESTAMPNS option is enabled
> > +together with SO_TIMESTAMPING using SOF_TIMESTAMPING_SOFTWARE, a false
> > +software timestamp will be generated in the recvmsg() call and passed
> > +in ts[0] when a real software timestamp is missing.
> 
> With receive software timestamping this is expected behavior? I would make
> explicit that this happens even on tx timestamps.

How about adding ", e.g. when receive timestamping is enabled
between receiving the message and the recvmsg() call, or it is a
message with a hardware transmit timestamp." ?

> > For this reason it
> > +is not recommended to combine SO_TIMESTAMP(NS) with SO_TIMESTAMPING.
> 
> And I'd remove this. The extra timestamp is harmless, and we may be missing
> other reasons why someone would want to enable both on the same socket.

Ok. I'm just concerned people will inadvertently use the timestamp as
a real timestamp and then wonder why SW TX timestamping is so bad. I
have fallen into this trap.

-- 
Miroslav Lichvar