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

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Message-ID: <20220304135540.GD16032@hoboy.vegasvil.org>
Date:   Fri, 4 Mar 2022 05:55:40 -0800
From:   Richard Cochran <richardcochran@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Divya Koppera <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 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