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

[Oleksij Rempel <o.rempel@pengutronix.de>]

On Fri, Oct 24, 2025 at 10:46:14AM +0200, Maxime Chevallier wrote:
> 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.

Ack, I agree. I would be better to have information and configuration
for all loopbacks in one place.
Will it be possible to reflect the chain of components and level of
related loopbacks? I guess, at least each driver would be able to know
own loopback levels/order.

Please add me in CC if you decide to jump in to this rabbit hole :D

Best Regards,
Oleksij
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