Ethtool is missing C2C link modes

Ethtool is missing C2C link modes

[D H, Siddaraju]

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.

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.

	10G_SFI_C2C		SFF-8418
	25G_AUI_C2C		IEEE 802.3 Annex 109A
	CAUI4 C2C		IEEE 802.3 Annex 83D
	LAUI2-C2C		IEEE 802.3 Annex 135B
	50GAUI-1 C2C		IEEE 802.3 Annex 135F
	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

- Thank you,
Siddaraju D H

Re: Ethtool is missing C2C link modes

[Andrew Lunn]

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

Re: Ethtool is missing C2C link modes

[David Laight]

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
> 

Re: Ethtool is missing C2C link modes

[Maxime Chevallier]

Hi,

On 7/11/26 08:31, David Laight wrote:
> 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.

Note that there's ongoing work[1] to better support nics with multiple connectors,
which also includes reporting what the 'media-side' of a MAC / PHY can do :

[1]: https://lore.kernel.org/netdev/20260701110427.143945-1-maxime.chevallier@bootlin.com/#t

Taking the example of a combo-port (SFP + RJ45), it would look like that :

 ethtool --show-ports eth1

Port for eth1:
	Port id: 1
	Supported link modes:  10baseT/Half 10baseT/Full
	                       100baseT/Half 100baseT/Full
	                       1000baseT/Full
	                       10000baseT/Full
	                       2500baseT/Full
	                       5000baseT/Full
	Port type: mdi

Port for eth1:
	Port id: 2
	Supported MII interfaces : 10gbase-r
	Port type: sfp

or even

# ethtool --show-ports eth3

Port for eth3:
	Port id: 1
	Supported MII interfaces : sgmii, 1000base-x, 2500base-x
	Port type: sfp


Now, with this infrastucture also comes the ability to list the media-side
interfaces that are not MDI but rather MII.

The main goal is combo-port support, but also media-converters, e.g. things
like :

MAC ------ PHY ------- <something>
     rgmii      sgmii

(for now, the ongoing series focus on supporting this through SFP, but extending
that to other link types is something I'd like to achieve)

As Andrew says, when you use "ethtool ethX", the list you get is the media-side
modes that you can use at the connector, so it's an aggregated list of the MDI
that are usable based on the MAC, PCS, PHY you're using (MAC and PCS would limit
speed/encoding, PHY defines the actual MDI modes)

For the C2C modes, it's not clear to me if these are MDI modes, i.e:

  ETHTOOL_LINK_MODE_25000baseC2C

Or a phy_interface_t, i.e:

  PHY_INTERFACE_MODE_25GAUI

We already have XAUI and RXAUI as phy_interface_t as of today, so it looks like
we don't want an ethool linkmode for that but rather a phy_interface_t

Maxime