netdev - Re: [PATCH v3 net-next 5/7] net: don't make false software transmit timestamps

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Message-ID: <20170517102707.GE4156@localhost>
Date:   Wed, 17 May 2017 12:27:07 +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 v3 net-next 5/7] net: don't make false software transmit
 timestamps

On Tue, May 16, 2017 at 06:34:38PM -0400, Willem de Bruijn wrote:
> On Tue, May 16, 2017 at 8:44 AM, Miroslav Lichvar <mlichvar@...hat.com> wrote:
> > If software timestamping is enabled by the SO_TIMESTAMP(NS) option
> > when a message without timestamp is already waiting in the queue, the
> > __sock_recv_timestamp() function will read the current time to make a
> > timestamp in order to always have something for the application.
> >
> > However, this applies also to outgoing packets looped back to the error
> > queue when hardware timestamping is enabled by the SO_TIMESTAMPING
> > option.
> 
> This is already the case for sockets that have both software receive
> timestamps and hardware tx timestamps enabled, independent from
> the new option SOF_TIMESTAMPING_OPT_TX_SWHW, right? If so,
> then this behavior must remain.

Even if we consider that it's not actually returning a valid TX
timestamp and it didn't behave as documented ("Only one field is
non-zero at any time")?

On the RX side this timestamp does make some sense as it could be
viewed as a very late timestamp, correctly ordered after the HW
timestamp, but on the TX side the order is reversed and returning a
timestamp later than the actual transmission might break a protocol.

If you don't see it as a bug fix, I think this weird interaction
between the SO_TIMESTAMPING(NS) and SO_TIMESTAMPING options needs to
be documented.

-- 
Miroslav Lichvar