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

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Date:   Fri, 21 Jan 2022 09:20:45 -0800
From:   Tim Harvey <tharvey@...eworks.com>
To:     netdev <netdev@...r.kernel.org>
Subject: best way to disable ANEG and force ethernet link?

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