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

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Message-ID: <CAF=yD-JGUw6pryNV3YevFMxC5=1A62qujAfFO7LjQi2LtsSg4g@mail.gmail.com>
Date:   Thu, 18 May 2017 15:38:30 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Miroslav Lichvar <mlichvar@...hat.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 10:07 AM, Miroslav Lichvar <mlichvar@...hat.com> wrote:
> The scm_timestamping struct may return multiple non-zero fields, e.g.
> when both software and hardware RX timestamping is enabled, or when the
> SO_TIMESTAMP(NS) option is combined with SCM_TIMESTAMPING and a false
> software timestamp is generated in the recvmsg() call in order to always
> return a SCM_TIMESTAMP(NS) message.
>
> CC: Richard Cochran <richardcochran@...il.com>
> CC: Willem de Bruijn <willemb@...gle.com>
> Signed-off-by: Miroslav Lichvar <mlichvar@...hat.com>

Thanks for adding this!

> +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.

> 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.