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

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Message-ID: <aQDQgBMp0R/RM0r4@test-OptiPlex-Tower-Plus-7010>
Date: Tue, 28 Oct 2025 19:47:36 +0530
From: Hariprasad Kelam <hkelam@...vell.com>
To: Oleksij Rempel <o.rempel@...gutronix.de>
CC: Maxime Chevallier <maxime.chevallier@...tlin.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 2025-10-24 at 14:41:09, Oleksij Rempel (o.rempel@...gutronix.de) wrote:
> 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
> 
    ACK, will define new ethtool ops for loopback as a first step.
 Will try to add generic loopback modes to the list.

Thanks,
Hariprasad k