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

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Message-ID: <20220410134215.GA258320@hoboy.vegasvil.org>
Date:   Sun, 10 Apr 2022 06:42:15 -0700
From:   Richard Cochran <richardcochran@...il.com>
To:     Gerhard Engleder <gerhard@...leder-embedded.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

On Sun, Apr 10, 2022 at 02:54:36PM +0200, Gerhard Engleder wrote:
> > > @@ -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.

> 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.


Thanks,
Richard