linux-kernel - Re: [PATCH net-next 2/3] dt-bindings: net: micrel: Configure latency values and timestamping check for LAN8814 phy

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Date:   Tue, 8 Mar 2022 17:46:47 -0800
From:   Richard Cochran <richardcochran@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Horatiu Vultur <horatiu.vultur@...rochip.com>,
        Divya.Koppera@...rochip.com, netdev@...r.kernel.org,
        hkallweit1@...il.com, linux@...linux.org.uk, davem@...emloft.net,
        kuba@...nel.org, robh+dt@...nel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, UNGLinuxDriver@...rochip.com,
        Madhuri.Sripada@...rochip.com, Manohar.Puri@...rochip.com
Subject: Re: [PATCH net-next 2/3] dt-bindings: net: micrel: Configure latency
 values and timestamping check for LAN8814 phy

On Wed, Mar 09, 2022 at 12:36:54AM +0100, Andrew Lunn wrote:

> I'm assuming the ptp protocol does not try to measure the cable delay,
> since if it did, there would be no need to know the RJ45-PHY delay, it
> would be part of that.

The PTP does indeed measure the cable delay.  With a well tuned
system, you can tell the copper cable length directly from the
measured delay.

The problem with uncorrected PHY time stamps is that they affect the
boundary point between the node and the network.  A static error there
will create a path asymmetry that can neither be measured nor
corrected by the PTP, and a variable error degrades the time signal.

Thanks,
Richard