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

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Date:   Tue, 14 Jul 2020 17:35:36 +0300
From:   Sergey Organov <sorganov@...il.com>
To:     Vladimir Oltean <olteanv@...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

Vladimir Oltean <olteanv@...il.com> writes:

> 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.

OK, thanks!

>
> 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.

I do care about these things indeed, it's only that right now what I
care most is to get the fixes into the kernel.

Then we can think without hurry about how all this could be improved.

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

Thanks for reviewing, and again for helpful and beneficial discussion!

-- Sergey