Re: [RFC net-next] net: loopback: Extend netdev features with new loopback modes

[Maxime Chevallier]

Hi,

+Russell +Oleksij

On 24/10/2025 06:48, Hariprasad Kelam wrote:
> This patch enhances loopback support by exposing new loopback modes
> (e.g., MAC, SERDES) to userspace. These new modes are added extension
> to the existing netdev features.
> 
> This allows users to select the loopback at specific layer.
> 
> Below are new modes added:
> 
> MAC near end loopback
> 
> MAC far end loopback
> 
> SERDES loopback
> 
> Depending on the feedback will submit ethtool changes.

Good to see you're willing to tackle this work. However as Eric says,
I don't think the netdev_features is the right place for this :
 - These 3 loopback modes here may not be enough for some plaforms
 - This eludes all PHY-side and PCS-side loopback modes that we could
   also use.

If we want to expose these loopback modes to userspace, we may actually
need a dedicated ethtool netlink command for loopback configuration and
control. This could then hit netdev ethtool ops or phy_device ethtool
ops depending on the selected loopback point.

If you don't want to deal with the whole complexity of PHY loopback, you
can for now only hook into a newly introduced netdev ethtool ops dedicated
to loopback on the ethnl side, but keep the door open for PHY-side
loopback later on.

Maxime