linux-kernel - Re: Query regarding Phy loopback support

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Message-ID: <defa4c07-0f8f-43cc-ba8d-0450998a8598@lunn.ch>
Date: Sun, 21 Sep 2025 20:42:55 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Hariprasad Kelam <hkelam@...vell.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, kuba@...nel.org,
	davem@...emloft.net, sgoutham@...vell.com, gakula@...vell.com,
	sbhatta@...vell.com, naveenm@...vell.com, edumazet@...gle.com,
	pabeni@...hat.com, andrew+netdev@...n.ch, bbhushan2@...vell.com
Subject: Re: Query regarding Phy loopback support

On Sun, Sep 21, 2025 at 11:14:55PM +0530, Hariprasad Kelam wrote:
> On 2025-09-16 at 22:13:20, Andrew Lunn (andrew@...n.ch) wrote:
> > On Tue, Sep 16, 2025 at 04:48:56PM +0530, Hariprasad Kelam wrote:
> > > We're looking for a standard way to configure PHY loopback on a network 
> > > interface using common Linux tools like ethtool, ip, or devlink.
> > > 
> > > Currently, ethtool -k eth0 loopback on enables a generic loopback, but it 
> > > doesn't specify if it's an internal, external, or PHY loopback. 
> > > Need suggestions to implement this feature in a standard way.
> > 
> > What actually do you mean by PHY loopback?
> 
> The Octeon silicon series supports both MAC (RPM) and PHY (GSERM) loopback 
> modes for testing.
> 
> We are seeking a solution to support the following loopback types:
> 
> MAC Level
> 
> Far-end loopback: Ingress data is routed back to egress data (MAC-to-MAC).
> 
> Near-end external loopback: Egress traffic is routed back to ingress traffic at the PCS layer.
> 
> PHY Level
> 
> Near-end digital loopback
> 
> Near-end analog loopback
> 
> Far-end digital loopback
> 
> Far-end analog loopback
> 
> We need suggestions on how to enable and manage these specific modes.

Whatever you put in place, it needs to be generic to support other
modes. So you need some sort of enum which can be extended. When
describing the different modes, please try to reference 802.3, so it
is clear what each actually means. And if it is a vendor mode, please
describe it well, so other vendors know what it is, and can match
their vendor names to it.

Frames received on the Media loopback vs host transmitted frames
should be another property.

Are you wanting to use this with ethtool --test? That operation is
still using IOCTL. So you will want to add netlink support, both in
ethtool(1) and net/ethtool/netlink.c, so you can add the extra
optional parameters to indicate where loopback should be
performed. And them plumb this through the MAC ethtool to phylink and
phylib, and maybe the PCS layer, if you have a linux PCS involved.

	Andrew