linux-kernel - Re: [RFC PATCH v2 0/2] AT8031 PHY timestamping support

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Message-ID: <20200228181507.GA4744@localhost>
Date:   Fri, 28 Feb 2020 10:15:07 -0800
From:   Richard Cochran <richardcochran@...il.com>
To:     Michael Walle <michael@...le.cc>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [RFC PATCH v2 0/2] AT8031 PHY timestamping support

On Fri, Feb 28, 2020 at 07:02:24PM +0100, Michael Walle wrote:
>  (1) The PHY doesn't support atomic reading of the (timestamp,
>      messageType, sequenceId) tuple. The workaround is to read the
>      timestamp again and check if it has changed. Actually, you'd have
>      to read the complete tuple again.

This HW is broken by design :(

> But if you're using a P2P clock with peer delay requests this whole
> thing falls apart because of caveat (3). You'll often see messages like
>   received SYNC without timestamp
> or
>  received PDELAY_RESP without timestamp
> in linuxptp. Sometimes it working for some time and then it starts to
> loosing packets. I suspect this depends on how the PDELAY messages are
> interleaved with the SYNC message. If there is not enough time to until
> the next event message is received either of these two messages won't
> have a timestamp.

And even the case where a Sync and a DelayResp arrive at nearly the
same time will fail.

> The PHY also supports appending the timestamp to the actual ethernet frame,
> but this seems to only work when the PHY is connected via RGMII. I've never
> get it to work with a SGMII connection.

This is the way to go.  I would try to get the vendor's help in making
this work.

Thanks,
Richard