Re: Ethtool is missing C2C link modes

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 :-)

With the existing ethtool link modes, we were not able to fit these C2C
interfaces on any others (we fitted **SGMII interfaces to baseT link modes)
and we see this as a gap. If you acknowledge this, we plan to send an
RFC patch to define below listed C2C link modes to ethtool.

	25G_AUI_C2C		IEEE 802.3 Annex 109A

So 109 is about 25GBASE-R. We have the following link modes for that:

       ETHTOOL_LINK_MODE_25000baseCR_Full_BIT  = 31,
       ETHTOOL_LINK_MODE_25000baseKR_Full_BIT  = 32,
       ETHTOOL_LINK_MODE_25000baseSR_Full_BIT  = 33,

Why break the pattern? Why not add:

       ETHTOOL_LINK_MODE_25000baseC2C_Full_BIT

Is C2C that different to CR, KR, DR?

	200GAUI-4 C2C		IEEE 802.3 Annex 120D
	100GAUI-1 C2C		IEEE 802.3ck Annex 120F
	200GAUI-2 C2C		IEEE 802.3ck Clause 162
	400GAUI-4 C2C		IEEE 802.3ck Clause 163

If you look at the existing pattern for link modes which need to
specify the number of lanes:

        ETHTOOL_LINK_MODE_800000baseCR8_Full_BIT
        ETHTOOL_LINK_MODE_400000baseDR4_Full_BIT
	ETHTOOL_LINK_MODE_200000baseKR2_Full_BIT

why put the number in the middle?

Since you are breaking the existing pattern, it would be good to
include a justification why you picked your pattern.

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.

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.

	Andrew