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

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Message-ID: <aPtCrc6EPpn_hcYp@pengutronix.de>
Date: Fri, 24 Oct 2025 11:11:09 +0200
From: Oleksij Rempel <o.rempel@...gutronix.de>
To: Maxime Chevallier <maxime.chevallier@...tlin.com>
Cc: Hariprasad Kelam <hkelam@...vell.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"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>,
	Russell King <linux@...linux.org.uk>
Subject: Re: [RFC net-next] net: loopback: Extend netdev features with new
 loopback modes

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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