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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Date:   Tue, 8 Mar 2022 23:14:04 +0100
From:   Horatiu Vultur <horatiu.vultur@...rochip.com>
To:     Andrew Lunn <andrew@...n.ch>
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

The 03/08/2022 19:10, Andrew Lunn wrote:
> 
> > > 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?

In the same way because the end result will be the same.
Lets presume that the cable introduce a 5ns latency per meter.
So, if we use a 1m cable and the mean path delay is 11, then
the latency is 11 - 5.
If we use a 100m cable then the mean path delay will be 506(if is not
506 then is something wrong) then the latency is 506 - 500.

> 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?

In that case I don't think you will see any difference.

> Isn't this error all just in the noise?

I am not sure I follow this question.

> 
>    Andrew

-- 
/Horatiu