Re: [PATCH net-next 02/11] ethtool: Add loopback netlink UAPI definitions

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

On Wed, Mar 11, 2026 at 07:50:52PM -0700, Jakub Kicinski wrote:
> On Wed, 11 Mar 2026 20:26:39 +0100 Andrew Lunn wrote:
> > > For that we have what we need with phy_link_topology, as each PHY has
> > > its index, we should be good to go in that regard hopefully :)  
> > 
> > So depth would be local to a component? We could have two PHY
> > components, each with a different index, and depth = 0?
> > 
> > I _think_ Jakub's depth was more at a global level? But then it would
> > need to be passed down as we do the enumeration.
> 
> Oh, sorry, I responded without reading the whole discussion :)
> No, I imagined the depth would be within a single component, 
> so under control of a single driver (instance). The ordering
> between components should be defined by PHY topology etc so
> it's outside of the loopback config.

As for me, it is problematic to help the user to understand the datapath
depth on a switch. For example:

CPU -- xMII --- MAC1 [loop] --- fabric --- MAC2 [loop] --- xMII -- PHY
                                    \----- MACx [loop] ---

... each port has two xMII loop configurations: towards the xMII or towards
the fabric. From a driver perspective, a loop towards the xMII is
"remote." However, from a system perspective, a "remote" loop on MAC1 is
a local loop at depth=0, whereas a "local" loop on MAC2 is a local loop
at depth=1.

Other example would be where we have a chain of components which are
attached on the system in a unexpected direction, where the MDI
interface is pointing towards the main CPU, so the remote loopbacks
became to local loop.

One more issue is the test data generator location. The data generator
is not always the CPU. We have HW generators located in components like
PHYs or we may use external source (remote loopback).

This is why i would prefer to have description of topology by not
hard coding the perspective of view.

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