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

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Message-ID: <20200714144409.ymnj6fhlnztsg6ir@skbuf>
Date:   Tue, 14 Jul 2020 17:44:09 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Sergey Organov <sorganov@...il.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Fugang Duan <fugang.duan@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH v2 net] net: fec: fix hardware time stamping by external
 devices

On Tue, Jul 14, 2020 at 05:23:24PM +0300, Vladimir Oltean wrote:
> On Tue, Jul 14, 2020 at 03:39:04PM +0300, Sergey Organov wrote:
> > Vladimir Oltean <olteanv@...il.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@...il.com>
> 
> Thanks,
> -Vladimir

Of course, it would be good if you sent a new version with the sha1sum
of the Fixes: tag having the right length, otherwise people will
complain.

Thanks,
-Vladimir