netdev - PHYs with advanced cable diagnostic support

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Date:   Tue, 26 May 2020 09:06:00 +0200
From:   Oleksij Rempel <o.rempel@...gutronix.de>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Michal Kubecek <mkubecek@...e.cz>, netdev@...r.kernel.org,
        mkl@...gutronix.de, kernel@...gutronix.de,
        David Jander <david@...tonic.nl>
Subject: PHYs with advanced cable diagnostic support

Hi Andrew,

I was looking for PHYs with more advanced cable testing support and
information how it should be done properly.

So far I found this links:

A general information with some interesting examples:
https://www.ti.com/lit/an/slyt755/slyt755.pdf

A PHY with the impulse amplitude and times storage, for multiple
impulses:
http://www.ti.com/lit/ds/symlink/dp83867cr.pdf
--------------------------------------------------------------------------------
TDR measurement is allowed in the DP83867 in the following scenarios:
* While Link partner is disconnected - cable is unplugged at the other side
* Link partner is connected but remains quiet (for example,in power-down mode)
* TDR could be automatically activated when the link fails or is dropped by
setting bit 7 of register 0x0009(CFG1). The results of the TDR run after the
link fails are saved in the TDR registers.
--------------------------------------------------------------------------------

It has following configuration bits:
- TDR AUTO RUN: Automatic TDR on Link Down (can be done is software for
  all PHYs)
- TDR_AVG_NUM: number Of TDR Cycles (is it good to know, how many TDR
  cycle we do?)
- TDR_CYCLE_TIME: Set the time for each TDR cycle (is it needed to
  sample only nearest damage? Probably this well need an UAPI for this)

Making an automatic cable test on the link drop seems to be an
interesting idea. May be we should do it for all PHY with cable test
support?

Regards,
Oleksij

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