netdev - Re: [PATCH net-next v2 0/2] timestamp: control SOF_TIMESTAMPING_RX

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Message-ID: <CAL+tcoCUhYH=udvB3rdVZm0gVypmAa5devPXryPwY+39mHscDA@mail.gmail.com>
Date: Fri, 30 Aug 2024 01:45:56 +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, willemb@...gle.com, 
	netdev@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v2 0/2] timestamp: control SOF_TIMESTAMPING_RX_SOFTWARE
 feature per socket

On Fri, Aug 30, 2024 at 12:23 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> Jason Xing wrote:
> > On Thu, Aug 29, 2024 at 10:14 PM Willem de Bruijn
> > <willemdebruijn.kernel@...il.com> wrote:
> > >
> > > Jason Xing wrote:
> > > > From: Jason Xing <kernelxing@...cent.com>
> > > >
> > > > Prior to this series, when one socket is set SOF_TIMESTAMPING_RX_SOFTWARE
> > > > which measn the whole system turns on this button, other sockets that only
> > > > have SOF_TIMESTAMPING_SOFTWARE will be affected and then print the rx
> > > > timestamp information even without SOF_TIMESTAMPING_RX_SOFTWARE flag.
> > > > In such a case, the rxtimestamp.c selftest surely fails, please see
> > > > testcase 6.
> > > >
> > > > In a normal case, if we only set SOF_TIMESTAMPING_SOFTWARE flag, we
> > > > can't get the rx timestamp because there is no path leading to turn on
> > > > netstamp_needed_key button in net_enable_timestamp(). That is to say, if
> > > > the user only sets SOF_TIMESTAMPING_SOFTWARE, we don't expect we are
> > > > able to fetch the timestamp from the skb.
> > >
> > > I already happened to stumble upon a counterexample.
> > >
> > > The below code requests software timestamps, but does not set the
> > > generate flag. I suspect because they assume a PTP daemon (sfptpd)
> > > running that has already enabled that.
> >
> > To be honest, I took a quick search through the whole onload program
> > and then suspected the use of timestamp looks really weird.
> >
> > 1. I searched the SOF_TIMESTAMPING_RX_SOFTWARE flag and found there is
> > no other related place that actually uses it.
> > 2. please also see the tx_timestamping.c file[1]. The author similarly
> > only turns on SOF_TIMESTAMPING_SOFTWARE report flag without turning on
> > any useful generation flag we are familiar with, like
> > SOF_TIMESTAMPING_TX_SOFTWARE, SOF_TIMESTAMPING_TX_SCHED,
> > SOF_TIMESTAMPING_TX_ACK.
> >
> > [1]: https://github.com/Xilinx-CNS/onload/blob/master/src/tests/onload/hwtimestamping/tx_timestamping.c#L247
> >
> > >
> > > https://github.com/Xilinx-CNS/onload/blob/master/src/tests/onload/hwtimestamping/rx_timestamping.c
> > >
> > > I suspect that there will be more of such examples in practice. In
> > > which case we should scuttle this. Please do a search online for
> > > SOF_TIMESTAMPING_SOFTWARE to scan for this pattern.
> >
> > I feel that only the buggy program or some program particularly takes
> > advantage of the global netstamp_needed_key...
>
> My point is that I just happen to stumble on one open source example
> of this behavior.
>
> That is a strong indication that other applications may make the same
> implicit assumption. Both open source, and the probably many more non
> public users.
>
> Rule #1 is to not break users.

Yes, I know it.

>
> Given that we even have proof that we would break users, we cannot
> make this change, sorry.

Okay. Your concern indeed makes sense. Sigh, I just finished the v3
patch series :S

>
> A safer alternative is to define a new timestamp option flag that
> opt-in enables this filter-if-SOF_TIMESTAMPING_RX_SOFTWARE is not
> set behavior.

At the first glance, It sounds like it's a little bit similar to
SOF_TIMESTAMPING_OPT_ID_TCP that is used to replace
SOF_TIMESTAMPING_OPT_ID in the bytestream case for robustness
consideration.

Are you suggesting that if we can use the new report flag combined
with SOF_TIMESTAMPING_SOFTWARE, the application will not get a rx
timestamp report, right? The new flag goes the opposite way compared
with SOF_TIMESTAMPING_RX_SOFTWARE, indicating we don't expect a rx sw
report.

If that is so, what would you recommend to name the new flag which is
a report flag (not a generation flag)? How about calling
"SOF_TIMESTAMPING_RX_SOFTWARE_CTRL". I tried, but my English
vocabulary doesn't help, sorry :(

Thanks,
Jason