On Thu, 19 Feb 2026 at 14:01, Björn Töpel [off-list ref] wrote:

Hi!

Background
==========

This series adds initial ethtool support for CMIS loopback.

For the brave, there's userland support here [1], and use, e.g., as below:

  | [root@machine ~/ethtool]# ./ethtool --show-module eth0
  | Module parameters for eth0:
  | loopback-capabilities: media-side-input, host-side-input
  | loopback-enabled: none
  | [root@machine ~/ethtool]# ./ethtool --set-module eth0
loopback-enabled host-side-input
  | netlink error: Netdevice is up, so setting loopback is not permitted
  | netlink error: Device or resource busy
  | [root@machine ~/ethtool]# ip link set dev eth0 down
  | [root@machine ~/ethtool]# ./ethtool --set-module eth0
loopback-enabled host-side-input
  | [root@machine ~/ethtool]# ./ethtool --show-module eth0
  | Module parameters for eth0:
  | loopback-capabilities: media-side-input, host-side-input
  | loopback-enabled: host-side-input
  | [root@machine ~/ethtool]# ip link set dev eth0 up


[1] https://github.com/fb-bjorn/ethtool/tree/module-loopback