netdev - Re: [PATCH net-next v2 2/5] ptp: Request cycles for TX timestamp

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Message-ID: <CANr-f5zrwe6Dea9B3OshtN39mia-U2q0Kw7x6fHYqTaORs0mFw@mail.gmail.com>
Date:   Sun, 10 Apr 2022 14:40:10 +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 2/5] ptp: Request cycles for TX timestamp

> > The free running cycle counter of physical clocks called cycles shall be
> > used for hardware timestamps to enable synchronisation.
> >
> > Introduce new flag SKBTX_HW_TSTAMP_USE_CYCLES, which signals driver to
> > provide a TX timestamp based on cycles if cycles are supported.
> >
> > Signed-off-by: Gerhard Engleder <gerhard@...leder-embedded.com>
> > ---
> >  include/linux/skbuff.h |  3 +++
> >  net/core/skbuff.c      |  2 ++
> >  net/socket.c           | 11 ++++++++++-
> >  3 files changed, 15 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > index 3a30cae8b0a5..aeb3ed4d6cf8 100644
> > --- a/include/linux/skbuff.h
> > +++ b/include/linux/skbuff.h
> > @@ -578,6 +578,9 @@ enum {
> >       /* device driver is going to provide hardware time stamp */
> >       SKBTX_IN_PROGRESS = 1 << 2,
> >
> > +     /* generate hardware time stamp based on cycles if supported */
> > +     SKBTX_HW_TSTAMP_USE_CYCLES = 1 << 3,
> > +
> >       /* generate wifi status information (where possible) */
> >       SKBTX_WIFI_STATUS = 1 << 4,
> >
> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > index 10bde7c6db44..c0f8f1341c3f 100644
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
> > @@ -4847,6 +4847,8 @@ void __skb_tstamp_tx(struct sk_buff *orig_skb,
> >               skb_shinfo(skb)->tx_flags |= skb_shinfo(orig_skb)->tx_flags &
> >                                            SKBTX_ANY_TSTAMP;
> >               skb_shinfo(skb)->tskey = skb_shinfo(orig_skb)->tskey;
> > +     } else {
> > +             skb_shinfo(skb)->tx_flags &= ~SKBTX_HW_TSTAMP_USE_CYCLES;
>
> Why is this needed?

It prevents that SKBTX_HW_TSTAMP_USE_CYCLES is set due to the call of
skb_clone(),
when the timestamp is delivered back to the socket. It lowers the flag
usage, but it is not
absolutely needed. I could remove that code.

Thank you!

Gerhard