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

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Date:   Wed, 17 May 2017 09:52:12 -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 v3 net-next 5/7] net: don't make false software transmit timestamps

On Wed, May 17, 2017 at 6:27 AM, Miroslav Lichvar <mlichvar@...hat.com> wrote:
> 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")?

Even if it is not a very useful timestamp. Applications may read it nonetheless.
I think that the documentation is newer than the feature, and wrong in
this case.

> 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,

Actually, on Rx it is equally problematic. It can easily generate out of
order timestamps. When enabling timestamping, the first packets will
have a timestamp generated at recvmsg while the following have one
generated at __netif_receive_skb_core.

> 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.

I agree. I don't think that it's all that useful, but it is
established behavior.