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

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Date:   Mon,  4 Apr 2022 17:05:08 +0200
From:   Michael Walle <michael@...le.cc>
To:     richardcochran@...il.com
Cc:     andrew@...n.ch, davem@...emloft.net, grygorii.strashko@...com,
        kuba@...nel.org, kurt@...utronix.de, linux-kernel@...r.kernel.org,
        linux@...linux.org.uk, mlichvar@...hat.com, netdev@...r.kernel.org,
        qiangqing.zhang@....com, vladimir.oltean@....com,
        Michael Walle <michael@...le.cc>
Subject: Re: [PATCH RFC V1 net-next 3/4] net: Let the active time stamping layer be selectable.

Sorry for digging out this older thread, but it seems to be discussed
in [1].

> IMO, the default should be PHY because up until now the PHY layer was
> prefered.
> 
> Or would you say the MAC layer should take default priority?
> 
> (that may well break some existing systems)

Correct me if I'm wrong, but for systems with multiple interfaces,
in particular switches, you'd need external circuits to synchronize
the PHCs within in the PHYs. (And if you use a time aware scheduler
you'd need to synchronize the MAC, too). Whereas for switches there
is usually just one PHC in the MAC which just works.

On these systems, pushing the timestamping to the PHY would mean
that this external circuitry must exist and have to be in use/
supported. MAC timestamping will work in all cases without any
external dependencies.

I'm working on a board with the LAN9668 switch which has one LAN8814
PHY and two GPY215 PHYs and two internal PHYs. The LAN9668 driver
will forward all timestamping ioctls to the PHY if it supports
timestamping (unconditionally). As soon as the patches to add ptp
support to the LAN8814 will be accepted, I guess it will break the
PTP/TAS support because there is no synchronization between all the
PHCs on that board. Thus, IMHO MAC timestamping should be the default.

-michael

[1] https://lore.kernel.org/netdev/20220308145405.GD29063@hoboy.vegasvil.org/