netdev - Re: [PATCH net-next v6 4/7] net: dsa: hellcreek: Add support for hardware timestamping

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Message-ID: <87362pjev0.fsf@kurt>
Date:   Thu, 08 Oct 2020 10:34:11 +0200
From:   Kurt Kanzenbach <kurt@...utronix.de>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Richard Cochran <richardcochran@...il.com>,
        Kamil Alkhouri <kamil.alkhouri@...offenburg.de>,
        ilias.apalodimas@...aro.org
Subject: Re: [PATCH net-next v6 4/7] net: dsa: hellcreek: Add support for hardware timestamping

On Wed Oct 07 2020, Vladimir Oltean wrote:
> On Wed, Oct 07, 2020 at 12:39:49PM +0200, Kurt Kanzenbach wrote:
>> For instance the hellcreek switch has actually three ptp hardware
>> clocks and the time stamping can be configured to use either one of
>> them.
>
> The sja1105 also has a corrected and an uncorrected PTP clock that can
> take timestamps. Initially I had thought I'd be going to spend some time
> figuring out multi-PHC support, but now I don't see any practical reason
> to use the uncorrected PHC for anything.

Just out of curiosity: How do you implement 802.1AS then? My
understanding is that the free-running clock has to be used for the
calculation of the peer delays and such meaning there should be a way to
get access to both PHCs or having some form of cross timestamping
available.

The hellcreek switch can take cross snapshots of all three ptp clocks in
hardware for that purpose.

>> > So when you'll poll for TX timestamps, you'll receive a TX
>> > timestamp from the PHY and another one from the switch, and those will
>> > be in a race with one another, so you won't know which one is which.
>> 
>> OK. So what happens if the driver will accept to disable hardware
>> timestamping? Is there anything else that needs to be implemented? Are
>> there (good) examples?
>
> It needs to not call skb_complete_tx_timestamp() and friends.
>
> For PHY timestamping, it also needs to invoke the correct methods for RX
> and for TX, where the PHY timestamping hooks will get called. I don't
> think that DSA is compatible yet with PHY timestamping, but it is
> probably a trivial modification.

Hmm? If DSA doesn't support PHY timestamping how are other DSA drivers
dealing with it then? I'm getting really confused.

Furthermore, there is no hellcreek hardware available with timestamping
capable PHYs. How am I supposed to even test this?

For now, until there is hardware available, PHY timestamping is not
supported with the hellcreek switch.

Thanks,
Kurt

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