netdev - 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 17:58:41 -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 Tue, Mar 08, 2022 at 05:46:47PM -0800, Richard Cochran wrote:
> 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.

And FWIW, the imperfections in the cable *also* introduce path
asymmetry and thus uncorrected offsets.  The twisted pairs are never
exactly the same length in both directions, for example.

However the PHY delays can be in the microseconds, but cable delay
deltas in the nanoseconds, so correcting PHY delay is much more
important.

Thanks,
Richard