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

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Date:   Thu, 27 Apr 2017 18:17:00 +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>,
        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

On Wed, Apr 26, 2017 at 08:00:02PM -0400, Willem de Bruijn wrote:
> > +       if (!hwtstamps && !(sk->sk_tsflags & SOF_TIMESTAMPING_OPT_TX_SWHW) &&
> > +           skb_shinfo(orig_skb)->tx_flags & SKBTX_IN_PROGRESS)
> > +               return;
> > +
> 
> This check should only happen for software transmit timestamps, so simpler to
> revise the check in sw_tx_timestamp above to
> 
>   if (skb_shinfo(skb)->tx_flags & SKBTX_SW_TSTAMP &&
> -        !(skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS))
> +      (!(skb_shinfo(orig_skb)->tx_flags & SKBTX_IN_PROGRESS)) ||
> +      (skb->sk && skb->sk->sk_tsflags & SOF_TIMESTAMPING_OPT_TX_SWHW)

Good point. This will avoid unnecessary calls of skb_tstamp_tx() in
the common case when SOF_TIMESTAMPING_OPT_TX_SWHW will not be enabled.

> > @@ -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?

-- 
Miroslav Lichvar