netdev - Re: best way to disable ANEG and force ethernet link?

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Date:   Sat, 22 Jan 2022 10:33:55 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Tim Harvey <tharvey@...eworks.com>, netdev <netdev@...r.kernel.org>
Subject: Re: best way to disable ANEG and force ethernet link?

On 21.01.2022 18:20, Tim Harvey wrote:
> Greetings,
> 
> I'm troubleshooting a network issue and am looking for the best way to
> force link speed/duplex without using auto-negotiation.
> 
> What is the best way to do this in Linux? Currently from a userspace
> perspective I have only been able to using 'ifconfig' to bring the
> link up, then 'mii-tool -F 100baseTx-FD <dev>' to force it. The
> ethtool methods do not work on my MAC/PHY (not clear what ethtool
> support is needed for that on a driver level). For testing purposes I
> would like to avoid the link coming up at 1000mbps in the first place
> and force it to 100mbps.
> 
> Before hacking the heck out of the phy layer in the kernel I was
> hoping for some advice for the best place to do this.
> 
> In case it matters I have two boards that I would like to do this on:
> an IMX8MM with FEC MAC and a CN803X with an RGMII (thunderx) vnic MAC.
> Both have a GPY111 (Intel Xway) PHY.
> 
> Best regards,
> 
> Tim

ifconfig is a legacy tool, better use ip. ethtool should work for you.
If not, please provide the details: Which command are you using, any error?
The Intel Xway PHY driver is loaded?
For just disabling Gbit mode you don't have to switch to forced mode.
You can just disable advertising Gbit mode:
ethtool -s <if> advertise 1000baseT/Full off