Re: [PATCH v2 net] net: fec: fix hardware time stamping by external devices

[Vladimir Oltean <olteanv@gmail.com>]

On Tue, Jul 14, 2020 at 03:39:04PM +0300, Sergey Organov wrote:
> Vladimir Oltean <olteanv@gmail.com> writes:
> 
> > On Mon, Jul 13, 2020 at 01:32:09AM +0300, Sergey Organov wrote:
> 
> [...]
> 
> >> > From the perspective of the mainline kernel, that can never happen.
> >>
> >> Yet in happened to me, and in some way because of the UAPI deficiencies
> >> I've mentioned, as ethtool has entirely separate code path, that happens
> >> to be correct for a long time already.
> >>
> >
> > Yup, you are right:
> >
> 
> [...]
> 
> > Very bad design choice indeed...
> > Given the fact that the PHY timestamping needs massaging from MAC driver
> > for plenty of other reasons, now of all things, ethtool just decided
> > it's not going to consult the MAC driver about the PHC it intends to
> > expose to user space, and just say "here's the PHY, deal with it". This
> > is a structural bug, I would say.
> >
> >> > From your perspective as a developer, in your private work tree, where
> >> > _you_ added the necessary wiring for PHY timestamping, I fully
> >> > understand that this is exactly what happened _to_you_.
> >> > I am not saying that PHY timestamping doesn't need this issue fixed. It
> >> > does, and if it weren't for DSA, it would have simply been a "new
> >> > feature", and it would have been ok to have everything in the same
> >> > patch.
> >>
> >> Except that it's not a "new feature", but a bug-fix of an existing one,
> >> as I see it.
> >>
> >
> > See above. It's clear that the intention of the PHY timestamping support
> > is for MAC drivers to opt-in, otherwise some mechanism would have been
> > devised such that not every single one of them would need to check for
> > phy_has_hwtstamp() in .ndo_do_ioctl(). That simply doesn't scale. Also,
> > it seems that automatically calling phy_ts_info from
> > __ethtool_get_ts_info is not coherent with that intention.
> >
> > I need to think more about this. Anyway, if your aim is to "reduce
> > confusion" for others walking in your foot steps, I think this is much
> > worthier of your time: avoiding the inconsistent situation where the MAC
> > driver is obviously not ready for PHY timestamping, however not all
> > parts of the kernel are in agreement with that, and tell the user
> > something else.
> 
> You see, I have a problem on kernel 4.9.146. After I apply this patch,
> the problem goes away, at least for FEC/PHY combo that I care about, and
> chances are high that for DSA as well, according to your own expertise.
> Why should I care what is or is not ready for what to get a bug-fix
> patch into the kernel? Why should I guess some vague "intentions" or
> spend my time elsewhere?
> 
> Also please notice that if, as you suggest, I will propose only half of
> the patch that will fix DSA only, then I will create confusion for
> FEC/PHY users that will have no way to figure they need another part of
> the fix to get their setup to work.
> 
> Could we please finally agree that, as what I suggest is indeed a simple
> bug-fix, we could safely let it into the kernel?
> 
> Thanks,
> -- Sergey

I cannot contradict you, you have all the arguments on your side. The
person who added support for "ethtool -T" in commit c8f3a8c31069
("ethtool: Introduce a method for getting time stamping capabilities.")
made a fundamental mistake in that they exposed broken functionality to
the user, in case CONFIG_NETWORK_PHY_TIMESTAMPING is enabled and the MAC
driver doesn't fulfill the requirements, be they skb_tx_timestamp(),
phy_has_hwtstamp() and what not. So, therefore, any patch that is adding
PHY timestamping compatibility in a MAC driver can rightfully claim that
it is fixing a bug, a sloppy design. Fair enough.

The only reason why I mentioned about spending your time on useful
things is because in your previous series you seemed to be concerned
about that. In retrospect, I believe you agree with me that your
confusion would have been significantly lower if the output of "ethtool
-T" was in harmony with the actual source of hardware timestamps.
Now that we discussed it through and I did see your point, I just
suggested what I believe to be the fundamental issue here, don't shoot
the messenger. Of course you are free to spend your time however you
want to.

Acked-by: Vladimir Oltean <olteanv@gmail.com>

Thanks,
-Vladimir