netdev - Re: [PATCH net-next v1 0/9] Ethernet Cable test support

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Message-ID: <4cae330197a5bdd1559dcea3482f0732@walle.cc>
Date:   Thu, 30 Apr 2020 22:01:55 +0200
From:   Michael Walle <michael@...le.cc>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Florian Fainelli <f.fainelli@...il.com>, cphealy@...il.com,
        davem@...emloft.net, hkallweit1@...il.com, mkubecek@...e.cz,
        netdev@...r.kernel.org
Subject: Re: [PATCH net-next v1 0/9] Ethernet Cable test support

Am 2020-04-30 21:41, schrieb Andrew Lunn:
>> ECD. The registers looks exactly like the one from the Marvell PHYs,
>> which makes me wonder if both have the same building block or if one
>> imitated the registers of the other. There are subtle differences
>> like one bit in the broadcom PHY is "break link" and is self-clearing,
>> while the bit on the Marvell PHY is described as "perform diagnostics
>> on link break".
> 
> Should we be sharing code between the two drivers?

If they are indeed the same sure, but it doesn't look like that the
marvell procedure works for the broadcom PHY.

>> What do you mean by calibrate it?
> 
> Some of the Marvell documentation talks about calibrating for losses
> on the PCB. Run a diagnostics with no cable plugged in, and get the
> cable length to the 'fault'. This gives you the distance to the RJ45
> socket. You should then subtract that from all subsequent results.
> But since this is board design specific, i decided to ignore it. I
> suppose it could be stuffed into a DT property, but i got the feeling
> it is not worth it, given the measurement granularity of 80cm.

Oh so, the time delta counter of the Marvell PHY also runs at 125MHz? ;)

        /* Accoring to the datasheet the distance to the fault is
         * DELTA_TIME * 0.824 meters.
         *
         * The author suspect the correct formula is:
         *  distance in meters = (c * VF) / (2 * 125MHz)
         * where c is the speed of light, VF is the velocity factor of
         * the twisted pair cable, 125MHz the counter frequency and
         * the factor 2 because the hardware will measure the round
         * trip time.
         * With a VF of 0.69 we get the factor 0.824 mentioned in the
         * datasheet.
         */


-michael