netdev - Re: [PATCH net-next v4 1/4] net-timestamp: filter out report when setting SOF_TIMESTAMPING

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Message-ID: <CAL+tcoD=hF3OE0e2e2O8BLVyFtQe_XOttJY_8NKnyX+uEh0EQw@mail.gmail.com>
Date: Thu, 5 Sep 2024 21:45:16 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	pabeni@...hat.com, dsahern@...nel.org, shuah@...nel.org, willemb@...gle.com, 
	linux-kselftest@...r.kernel.org, netdev@...r.kernel.org, 
	Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v4 1/4] net-timestamp: filter out report when
 setting SOF_TIMESTAMPING_SOFTWARE

On Thu, Sep 5, 2024 at 9:37 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> Jason Xing wrote:
> > From: Jason Xing <kernelxing@...cent.com>
> >
> > introduce a new flag SOF_TIMESTAMPING_OPT_RX_FILTER in the receive
> > path. User can set it with SOF_TIMESTAMPING_SOFTWARE to filter
> > out rx software timestamp report, especially after a process turns on
> > netstamp_needed_key which can time stamp every incoming skb.
> >
> > Previously, we found out if an application starts first which turns on
> > netstamp_needed_key, then another one only passing SOF_TIMESTAMPING_SOFTWARE
> > could also get rx timestamp. Now we handle this case by introducing this
> > new flag without breaking users.
> >
> > Quoting Willem to explain why we need the flag:
> > "why a process would want to request software timestamp reporting, but
> > not receive software timestamp generation. The only use I see is when
> > the application does request
> > SOF_TIMESTAMPING_SOFTWARE | SOF_TIMESTAMPING_TX_SOFTWARE."
> >
> > In this way, we have two kinds of combination:
> > 1. setting SOF_TIMESTAMPING_SOFTWARE|SOF_TIMESTAMPING_RX_SOFTWARE, it
> > will surely allow users to get the rx software timestamp report.
> > 2. setting SOF_TIMESTAMPING_SOFTWARE|SOF_TIMESTAMPING_OPT_RX_FILTER
> > while the skb is timestamped, it will stop reporting the rx software
> > timestamp.
> >
> > Another thing about errqueue in this patch I have a few words to say:
> > In this case, we need to handle the egress path carefully, or else
> > reporting the tx timestamp will fail. Egress path and ingress path will
> > finally call sock_recv_timestamp(). We have to distinguish them.
> > Errqueue is a good indicator to reflect the flow direction.
> >
> > Suggested-by: Willem de Bruijn <willemb@...gle.com>
> > Reviewed-by: Willem de Bruijn <willemb@...gle.com>
>
> nit: Reviewed-by tags are only sticky if no changes are made.

I got it. I will remove it.

>
> > diff --git a/Documentation/networking/timestamping.rst b/Documentation/networking/timestamping.rst
> > index 5e93cd71f99f..37ead02be3b1 100644
> > --- a/Documentation/networking/timestamping.rst
> > +++ b/Documentation/networking/timestamping.rst
> > @@ -266,6 +266,18 @@ SOF_TIMESTAMPING_OPT_TX_SWHW:
> >    two separate messages will be looped to the socket's error queue,
> >    each containing just one timestamp.
> >
> > +SOF_TIMESTAMPING_OPT_RX_FILTER:
> > +  Used in the receive software timestamp. Enabling the flag along with
> > +  SOF_TIMESTAMPING_SOFTWARE will not report the rx timestamp to the
> > +  userspace so that it can filter out the case where one process starts
> > +  first which turns on netstamp_needed_key through setting generation
> > +  flags like SOF_TIMESTAMPING_RX_SOFTWARE, then another one only passing
> > +  SOF_TIMESTAMPING_SOFTWARE report flag could also get the rx timestamp.
>
> This raises the question: why would a process request
> report flag SOF_TIMESTAMPING_SOFTWARE without generate flag
> SOF_TIMESTAMPING_RX_SOFTWARE? The only sensible use case I see is when
> it sets SOF_TIMSETAMPING_TX_SOFTWARE. Probably good to mention that.
>
> May also be good to mention that existing applications sometimes set
> SOF_TIMESTAMPING_SOFTWARE only, because they implicitly came to depend
> on another (usually daemon) process to enable rx timestamps systemwide.

Much better. Thanks. I will add them too.

>
> > +
> > +  SOF_TIMESTAMPING_OPT_RX_FILTER prevents the application from being
> > +  influenced by others and let the application choose whether to report
> > +  the timestamp in the receive path or not.
> > +
>
> I'd drop this paragraph. It's more of a value statement.
>

I see. Will drop it.

Thanks,
Jason