netdev - Re: [PATCH v1 net-next 5/6] net: allow simultaneous SW and HW transmit timestamping

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Message-ID: <CAF=yD-+HK-dCG_XjqBKfkSF1bjJavTr7EFgeFNH2yRc2CXgOxA@mail.gmail.com>
Date:   Thu, 27 Apr 2017 12:21:00 -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>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        "Keller, Jacob E" <jacob.e.keller@...el.com>,
        Denny Page <dennypage@...com>, Jiri Benc <jbenc@...hat.com>
Subject: Re: [PATCH v1 net-next 5/6] net: allow simultaneous SW and HW
 transmit timestamping

>> > @@ -720,6 +720,7 @@ void __sock_recv_timestamp(struct msghdr *msg, struct sock *sk,
>> >                 empty = 0;
>> >         if (shhwtstamps &&
>> >             (sk->sk_tsflags & SOF_TIMESTAMPING_RAW_HARDWARE) &&
>> > +           (empty || !skb_is_err_queue(skb)) &&
>> >             ktime_to_timespec_cond(shhwtstamps->hwtstamp, tss.ts + 2)) {
>>
>> I find skb->tstamp == 0 easier to understand than the condition on empty.
>>
>> Indeed, this is so non-obvious that I would suggest another helper function
>> skb_is_hwtx_tstamp with a concise comment about the race condition
>> between tx software and hardware timestamps (as in the last sentence of
>> the commit message).
>
> Should it include also the skb_is_err_queue() check? If it returned
> true for both TX and RX HW timestamps, maybe it could be called
> skb_has_hw_tstamp?

For the purpose of documenting why this complex condition exists,
I would call the skb_is_err_queue in that helper function and make
it tx + hw specific.