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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Message-ID: <YiecBKGhVui1Gtb/@lunn.ch>
Date:   Tue, 8 Mar 2022 19:10:12 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Horatiu Vultur <horatiu.vultur@...rochip.com>
Cc:     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,
        richardcochran@...il.com, 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

> > So this is a function of the track length between the MAC and the PHY?
> 
> Nope.
> This latency represents the time it takes for the frame to travel from RJ45
> module to the timestamping unit inside the PHY. To be more precisely,
> the timestamping unit will do the timestamp when it detects the end of
> the start of the frame. So it represents the time from when the frame
> reaches the RJ45 to when the end of start of the frame reaches the
> timestamping unit inside the PHY.

I must be missing something here. How do you measure the latency
difference for a 1 meter cable vs a 100m cable? Does 100m cable plus
1cm of track from the RJ45 to the PHY make a difference compared to
100m cable plus 1.5cm of track? Isn't this error all just in the
noise?

   Andrew