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

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Message-ID: <e9aa0470-2bd2-4825-8333-ad9dbc7f40a0@bootlin.com>
Date: Fri, 24 Oct 2025 10:46:14 +0200
From: Maxime Chevallier <maxime.chevallier@...tlin.com>
To: Hariprasad Kelam <hkelam@...vell.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>,
 Jonathan Corbet <corbet@....net>, Sunil Goutham <sgoutham@...vell.com>,
 Geetha sowjanya <gakula@...vell.com>, Subbaraya Sundeep
 <sbhatta@...vell.com>, Bharat Bhushan <bbhushan2@...vell.com>,
 Andrew Lunn <andrew+netdev@...n.ch>,
 Kory Maincent <kory.maincent@...tlin.com>, Gal Pressman <gal@...dia.com>,
 Jianbo Liu <jianbol@...dia.com>, Edward Cree <ecree.xilinx@...il.com>,
 Breno Leitao <leitao@...ian.org>, Oleksij Rempel <o.rempel@...gutronix.de>,
 Russell King <linux@...linux.org.uk>
Subject: Re: [RFC net-next] net: loopback: Extend netdev features with new
 loopback modes

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