netdev - Re: [PATCH net-next v5 1/2] net-timestamp: introduce SOF_TIMESTAMPING_OPT_RX

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Message-ID: <CAL+tcoBHP7pZdWB+b1tj12=8VGSeMwvEezkQAvB3qOf81Uqk5Q@mail.gmail.com>
Date: Sat, 7 Sep 2024 09:45:00 +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 v5 1/2] net-timestamp: introduce
 SOF_TIMESTAMPING_OPT_RX_FILTER flag

On Sat, Sep 7, 2024 at 9:23 AM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> On Sat, Sep 7, 2024 at 7:24 AM 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."
> > >
> > > Similarly, this new flag could also be used for hardware case where we
> > > can set it with SOF_TIMESTAMPING_RAW_HARDWARE, then we won't receive
> > > hardware receive 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>
> > > Signed-off-by: Jason Xing <kernelxing@...cent.com>
> >
> > High level: where is the harm in receiving unsolicited timestamps?
> > A process can easily ignore them. I do wonder if the only use case is
> > an overly strict testcase. Was reminded of this as I tried to write
> > a more concise paragraph for the documentation.
>
> You raised a good question.
>
> I think It's more of a design consideration instead of a bugfix
> actually. So it is not solving a bug which makes the apps wrong but
> gives users a hint that we can explicitly and accurately do what we
> want and we expect.

One more thing: if I recall correctly, the initial reason I proposed
is that the rx report flag is not controlled under per socket but
maybe affected by others. It's against the expectation.