netdev - LinkMD Feature on KSZ8873MLL

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Message-ID: <20190204163135.snvxvpdwvt7nlod6@pengutronix.de>
Date:   Mon, 4 Feb 2019 17:31:35 +0100
From:   Michael Grzeschik <mgr@...gutronix.de>
To:     nicolas.ferre@...rochip.com
Cc:     netdev@...r.kernel.org, kernel@...gutronix.de
Subject: LinkMD Feature on KSZ8873MLL

Hi Nicolas

We are currently debugging an issue with the KSZ8873 with the
LinkMD Feature to measure Cable-Failures. We implemented all
steps described in corresponding section of the Datasheet:

3.1.12 LINKMD ®  CABLE DIAGNOSTICS

It is also already clear that this feature is only available to the second of
the ports due to an errata on the status register of the first port.

What we see is, that in some of the measurements the detection returns
"11 = cable diagnostic test failed (invalid test)".
In the Datasheet of the KSZ8001 this failure is a little bit
more detailed desribed:

<quote>
The ‘11’ case, Invalid test, occurs when it is not possible for the KSZ8001 to
shut down the link partner.  In this case, the test is not run, since it
would not be possible for the KSZ8001 to determine if the detected
signal is a reflection of the signal generated or a signal from
another source.
</quote>

The datasheet also mentions that it needs to disable HP Auto MDIX (switch to
Micrel MDIX) which was left out in the KSZ8873 Datasheet. We fiddled with all
mentioned bits but did not get any stable measurement with the KSZ8873.

The odd thing about it, is the fact that it works for some repeatable times
without changing anything in the whole system and from now to then it returns
the invalid test failure. We also connected two KSZ8873 switches to disable
MDIX on both sides before the test.

Could you help us or poke somebody who could describe the issue we see?

Thanks,
Michael

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