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

Horatiu Vultur
2022-03-08T22:11:16Z

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