netdev - Re: [PATCH net-next v2 4/5] ptp: Support late timestamp determination

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Message-ID: <CANr-f5xriLzQ+3xtM+iV8ahu=J1mA7ixbc49f0i2jxkySthTdQ@mail.gmail.com>
Date:   Sun, 10 Apr 2022 16:42:19 +0200
From:   Gerhard Engleder <gerhard@...leder-embedded.com>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     Vinicius Costa Gomes <vinicius.gomes@...el.com>, yangbo.lu@....com,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, mlichvar@...hat.com,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2 4/5] ptp: Support late timestamp determination

> > > > @@ -887,18 +885,28 @@ void __sock_recv_timestamp(struct msghdr *msg, struct sock *sk,
> > > >       if (shhwtstamps &&
> > > >           (sk->sk_tsflags & SOF_TIMESTAMPING_RAW_HARDWARE) &&
> > > >           !skb_is_swtx_tstamp(skb, false_tstamp)) {
> > > > +             rcu_read_lock();
> > > > +             orig_dev = dev_get_by_napi_id(skb_napi_id(skb));
> > >
> > > __sock_recv_timestamp() is hot path.
> > >
> > > No need to call dev_get_by_napi_id() for the vast majority of cases
> > > using plain old MAC time stamping.
> >
> > Isn't dev_get_by_napi_id() called most of the time anyway in put_ts_pktinfo()?
>
> No.  Only when SOF_TIMESTAMPING_OPT_PKTINFO is requested.

You are right, my fault.

> > That's the reason for the removal of a separate flag, which signals the need to
> > timestamp determination based on address/cookie. I thought there is no need
> > for that flag, as netdev is already available later in the existing code.
> >
> > > Make this conditional on (sk->sk_tsflags & SOF_TIMESTAMPING_BIND_PHC).
> >
> > This flag tells netdev_get_tstamp() which timestamp is required. If it
> > is not set, then
> > netdev_get_tstamp() has to deliver the normal timestamp as always. But
> > this normal
> > timestamp is only available via address/cookie. So netdev_get_tstamp() must be
> > called.
>
> It should be this:
>
> - normal, non-vclock:   use hwtstamps->hwtstamp directly
> - vclock:               use slower path with lookup
>
> I don't see why you can't implement that.

I will try to implement it that way.

Thank you!

Gerhard