netdev - RE: phy: fixed link 1000 or 100 set with autoneg off

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Message-ID: <1fc7704bfd664746bf93c65591efa071@AcuMS.aculab.com>
Date: Mon, 23 Oct 2023 14:57:43 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Heiner Kallweit' <hkallweit1@...il.com>, "Jia, Fang"
	<fang.jia@...driver.com>, Andrew Lunn <andrew@...n.ch>, Florian Fainelli
	<f.fainelli@...il.com>, "David S. Miller" <davem@...emloft.net>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: phy: fixed link 1000 or 100 set with autoneg off

From: Heiner Kallweit
> Sent: 23 October 2023 08:52
> 
> On 23.10.2023 09:01, Jia, Fang wrote:
> > Hi Experts,
> >
> > We use NXP LS1046 board and face an issue about the eth interface speed.
> >
> > 1) Scenario
> >
> > we set fixed link 1000Mb/s in device tree.
> >
> > However, after we set the auto-neg off, then the eth1's speed changed to 10M and Duplex changed to Half.
> > The value of /sys/class/net/eth1/speed is 10 and /sys/class/net/eth1/duplex is half
> >
> Why do you set aneg to off? Leave aneg on, that's the only supported
> mode in swphy. 1000Mbps requires aneg anyway per standard.

Limiting the advertised modes has always worked better than
trying to set a fixed mode.
Although connect to a 10M hub (that sends out single link test pulses)
and you'll end up 10M/HDX regardless of the ANAR register. 

Not least of the problems is that you need to get the far end
to use exactly the same mode.

Otherwise it is very easy to get a mismatch.
In the 'old days' a HDX/FDX mismatch was easy to get and very confusing.

	David

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