linux-kernel - Re: [PATCH RFC V1 net-next 3/4] net: Let the active time stamping layer be selectable.

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Date:   Tue, 05 Apr 2022 13:19:58 +0200
From:   Kurt Kanzenbach <kurt@...utronix.de>
To:     Michael Walle <michael@...le.cc>
Cc:     Andrew Lunn <andrew@...n.ch>, richardcochran@...il.com,
        davem@...emloft.net, grygorii.strashko@...com, kuba@...nel.org,
        linux-kernel@...r.kernel.org, linux@...linux.org.uk,
        mlichvar@...hat.com, netdev@...r.kernel.org,
        qiangqing.zhang@....com, vladimir.oltean@....com
Subject: Re: [PATCH RFC V1 net-next 3/4] net: Let the active time stamping
 layer be selectable.

On Tue Apr 05 2022, Michael Walle wrote:
> Am 2022-04-05 11:01, schrieb Kurt Kanzenbach:
>> On Mon Apr 04 2022, Michael Walle wrote:
>>> That would make sense. I guess what bothers me with the current
>>> mechanism is that a feature addition to the PHY in the *future* (the
>>> timestamping support) might break a board - or at least changes the
>>> behavior by suddenly using PHY timestamping.
>> 
>> Currently PHY timestamping is hidden behind a configuration option
>> (NETWORK_PHY_TIMESTAMPING). By disabling this option the default
>> behavior should stay at MAC timestamping even if additional features
>> are added on top of the PHY drivers at later stages. Or not?
>
> That is correct. But a Kconfig option has several drawbacks:
> (1) Doesn't work with boards where I might want PHY timestamping
>      on *some* ports, thus I need to enable it and then stumple
>      across the same problem.
> (2) Doesn't work with generic distro support, which is what is
>      ARM pushing right now with their SystemReady stuff (among other
>      things also for embeddem system). Despite that, I have two boards
>      which are already ready for booting debian out of the box for
>      example. While I might convince Debian to enable that option
>      (as I see it, that option is there to disable the additional
>      overhead) it certainly won't be on a per board basis.
>      Actually for keeping the MAC timestamping as is, you'd need to
>      convince a distribution to never enable the PHY timestamping
>      kconfig option.
>
> So yes, I agree it will work when you have control over your
> kconfig options, after all (1) might be more academic. But I'm
> really concerned about (2).

Yes, the limitations described above are exactly one of the reasons to
make the timestamping layer configurable at run time as done by these
patches.

Thanks,
Kurt

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