netdev - Re: [DSA] fallback PTP to master port when switch does not support it

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Message-ID: <20220407094439.ubf66iei3wgimx7d@skbuf>
Date:   Thu, 7 Apr 2022 12:44:39 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Matej Zachar <zachar.matej@...il.com>, netdev@...r.kernel.org
Subject: Re: [DSA] fallback PTP to master port when switch does not support it

On Tue, Apr 05, 2022 at 12:48:51PM -0700, Jakub Kicinski wrote:
> On Tue, 5 Apr 2022 00:04:30 +0200 Andrew Lunn wrote:
> > What i don't like about your proposed fallback is that it gives the
> > impression the slave ports actually support PTP, when they do not.
>
> +1, running PTP on the master means there is a non-PTP-aware switch
> in the path, which should not be taken lightly.

+2, the change could probably be technically done, and there are aspects
worth discussing, but the goal presented here is questionable and it's
best to not fool ourselves into thinking that the variable queuing delays
of the switch are taken into account when reporting the timestamps,
which they aren't.

I think that by the time you realize that you need PTP hardware
timestamping on switch ports but you have a PTP-unaware switch
integrated *into* your system, you need to go back to the drawing board.