Re: Ethtool is missing C2C link modes

[David Laight <david.laight.linux@gmail.com>]

On Sat, 11 Jul 2026 00:54:00 +0200
Andrew Lunn <andrew@lunn.ch> wrote:

> On Fri, Jul 10, 2026 at 09:45:43PM +0000, D H, Siddaraju wrote:
> > Hello Linux Ethernet team, Maxime, Andrew & Michal,
> > 
> > The IEEE AUI chip-to-chip (C2C) is the accepted standard for connecting
> > chips that handle subfunctions within the OSI physical layer. Just to
> > pick, the C2C is widely used when connecting Ethernet SoCs with retimers
> > and PCS SerDes terminated external-phys to offload PHY sublayer functions.  
> 
> It cannot be that widely used if Linux does not support it yet :-)

It also seems like something that is fixed for a physical board.
So while a common MAC driver would need to be told how to configure
its output, the user wouldn't be changing the value so it would
be more of a DT parameter than an ethtool one.

...
> Also, an architecture question...
> 
> It sounds like you use this between the MAC and the PCS. The PCS can
> then be connected to a PHY, and the PHY then has a line side. (I'm
> being a bit loose with the terms here, i should probably be saying
> PMA, PMD etc.)
> 
> Should ethtool be saying:
> 
> Settings for eth0:
> 	Supported ports: [ TP	 MII ]
> 	Supported link modes:   25000baseC2C
> 
> or should it be reporting:
> 
> Settings for eth0:
> 	Supported ports: [ TP	 MII ]
> 	Supported link modes:   25000baseSR
> 
> I _think_ ethtool reports the media, not some intermediary format.

You'd want to use ethtool to set the final link parameters of the
external phy?
So I think you's still want to be able to select (say) 100MHDX
for a TP link.

Remember the history.
The parameter was originally used to select between the the AUI, COAX and TP
connectors on a 10M ethernet card.
Then the internal TP gained extra speeds.
We then get MII for external 10M and 100M PHY, later RGMII for external Ge PHY.
But you rarely get boards (not MAC chips) that have a choice of interfaces
any more.

> Is ETHTOOL_LINK_MODE_25000baseC2C_Full_BIT actually needed? I suppose
> one use case would be when you directly connect two MACs together, PMA
> to PMA. So a 25G NIC directly connected to a switch port, with no
> 'media' in the middle. Then ethtool probably should report
> 25000baseC2C.

That might be similar to using RGMII crossover to connect to MAC together
(Which does get used on-board).
I'm not sure how we selected that, but overloading the media is 'sort of'
ok because there is no media in that case.

	David

> 
> 	Andrew
>