netdev - Re: Ethtool: advance phy debug support

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Message-ID: <174e07b4-68cc-4fbc-8350-a429f3f4e40f@lunn.ch>
Date: Tue, 18 Nov 2025 18:29:38 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Alexander Duyck <alexander.duyck@...il.com>
Cc: Lee Trager <lee@...ger.us>,
	Maxime Chevallier <maxime.chevallier@...tlin.com>,
	Susheela Doddagoudar <susheelavin@...il.com>,
	netdev@...r.kernel.org, mkubecek@...e.cz,
	Hariprasad Kelam <hkelam@...vell.com>,
	Alexander Duyck <alexanderduyck@...com>
Subject: Re: Ethtool: advance phy debug support

> Right. With PRBS you already should know what the link partner is
> configured for. It is usually a manual process on both sides. That is
> why I mentioned the cable/module EEPROM. The cable will indicate the
> number of lanes present and the recommended/maximum frequency and
> modulation it is supposed to be used at.

But i also expect that is standard ksetting_set behaviour. If user
space asks you do a specific link mode, you are going to check if it
is supported and return -EINVAL if it is not. So in the general case,
all this already exists. I call

ethtool -s eth42 autoneg off 20000baseCR2

and it should check if it is supported, and then configure the MAC,
PCS and anything else in the path for that link mode. Or return
-EINVAL if it is not supported.

[I forget the exact ethtool syntax, its not something i use very
often, to set a specific link mode]

> With that you can essentially
> determine what the correct setup would be to test it as this is mostly
> just a long duration cable test. The only imitation is if there is
> something in between such as a PMA/PMD that is configured for 2 lanes
> instead of 4.

And the CR2 indicates you want to use 2 lanes. And you would then run
RPBS on both of them. We could consider additional properties on the
netlink API to run PRBS on a subset. But would you not actually want
them all active to see if there is cross talk noise to deal with?

> Yes. Again most of these settings appear to be per-lane in both the IP
> we have and the IEEE specification. For example it occurs to me that a
> device could be running a 25G or 50G link over a single QSFP cable,
> and still have testing enabled for the unused 2 or 3 lanes on the
> cable potentially assuming the PMA/PMD is a 4 lane link and is only
> using one or two lanes for the link.

So that would be 25000baseCR and 50000baseCR2, for the 25G and 50G,
using 1 lane and 2 lanes, leaving the other 3/2 lanes unused?

	Andrew