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

Richard Cochran
2022-03-04T13:55:52Z

On Fri, Mar 04, 2022 at 01:50:47PM +0100, Andrew Lunn wrote:
> Why does this need to be configured, rather than hard coded? Why would
> the latency for a given speed change? I would of thought though you
> would take the average length of a PTP packet and divide is by the
> link speed.

Latency is unrelated to frame length.

My understanding is that it is VERY tricky to measure PHY latency.
Studies have shown that some PHYs vary by link speed, and some vary
randomly, frame by frame.

So I can understand wanting to configure it. However, DTS is probably
the wrong place. The linuxptp user space stack has configuration
variables for this purpose:

egressLatency
Specifies the difference in nanoseconds between the actual
transmission time at the reference plane and the reported trans‐
mit time stamp. This value will be added to egress time stamps
obtained from the hardware. The default is 0.

ingressLatency
Specifies the difference in nanoseconds between the reported re‐
ceive time stamp and the actual reception time at reference
plane. This value will be subtracted from ingress time stamps
obtained from the hardware. The default is 0.

Thanks,
Richard