linux-kernel - Re: [RFC PATCH net-next 7/9] net: dsa: microchip: ksz9477: add hardware time stamping support

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Date:   Mon, 2 Nov 2020 14:28:12 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Christian Eggers <ceggers@...i.de>
Cc:     Richard Cochran <richardcochran@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Helmut Grohne <helmut.grohne@...enta.de>,
        Paul Barker <pbarker@...sulko.com>,
        Codrin Ciubotariu <codrin.ciubotariu@...rochip.com>,
        George McCollister <george.mccollister@...il.com>,
        Marek Vasut <marex@...x.de>,
        Tristram Ha <Tristram.Ha@...rochip.com>,
        "David S . Miller" <davem@...emloft.net>,
        Woojung Huh <woojung.huh@...rochip.com>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>,
        netdev@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH net-next 7/9] net: dsa: microchip: ksz9477: add
 hardware time stamping support

On Mon, Nov 02, 2020 at 11:35:00AM +0100, Christian Eggers wrote:
> Maybe my mail from October, 22 was ambiguous. I meant that despite of the
> presence of filtering, a BCMA algorithm should be about to work (as no
> Announce messages are filtered out).
> 
> Additionally I said, that switching between "master" and "slave" mode could
> not be done automatically by the driver, as the driver could at most detect
> the presence of Sync messages (indication for master mode), but would do hard
> to detect a transition to slave mode.
> 
> I see a chance that user space (ptp4l) could configure the appropriate
> "hardware filter setup" for master/slave mode.

The concept that you want from user space is hard to define.
You want ptp4l to make the driver aware of the port state, which is
something that happens at runtime and is not part of the "hardware
filter setup" as you say. Then, even if ptp4l notifies the driver of
port state, E2E BC setups would still be broken because that's how the
hardware works and no user space assistance can help with that. Also,
what abstraction do you plan using for programming the PTP port state
into the kernel.

Maybe you should optimize for what you plan to use. If you need to use
an E2E profile, maybe it would be worth the effort to find an
appropriate abstraction for this port state thing. If you only use it
for testing, then maybe it would be a good idea to keep the sysfs in
your tree.

> > Why am I mentioning this? Because the setting that's causing trouble for
> > us is 'port state of the host port OC', which in the context of what I
> > said above is nonsense. There _is_ no host port OC. There are 2 switch
> > ports which can act as individual OCs, or as a BC, or as a TC.
> But the switch has only one clock at all. I assume that the switch cannot be a
> boundary clock, only TC seems possible.

Why would a P2P BC not work? It does not require more than one clock.

> As said above, having "filter setups" for E2E/P2P and for MASTER/SLAVE would
> probably fit well for this kind of hardware.

For E2E/P2P is one thing, for master/slave is a completely different
thing.