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

Richard Cochran
2022-03-09T01:46:54Z

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