Subject: Re: [PATCH 2/2] net: sfp: support 25G long-range modules (extended compliance code 0x3)

From: "Russell King (Oracle)" <[email-masked]>
Date: Mon, 19 Jan 2026 10:48:41 +0000

On Mon, Jan 19, 2026 at 07:30:20AM +0000, Josua Mayer wrote:
> On 18/01/2026 18:01, Andrew Lunn wrote:
> > On Sun, Jan 18, 2026 at 04:07:38PM +0200, Josua Mayer wrote:
> >> The extended compliance code value SFF8024_ECC_100GBASE_ER4_25GBASE_ER
> >> (0x3) means either 4-lane 100G or single lane 25G.
> > Is there a way to tell them apart?
> The physical connectors are different, so we can know from the
> device-tree compatible string.
> 
> For now sfp driver does not support qsfp.

And likely will never do.

I did look at QSFP support due to the LX2160A SR board, and I did
scratch some code together, but I didn't get far with it:

(a) LX2160A is just not flexible enough to consider the possibilities
    properly to implement support (no run-time reconfiguration of the
    interface mode.)

(b) QSFPs can be used as a single interface, or as multiple interfaces.
    There is no way that the SFP and phylink layers can cope with that
    as they are currently structured.

(c) QSFP EEPROMs have a completely different structure to SFP EEPROMs.

(d) I couldn't see any way that the QSFP EEPROM distinguished between
    e.g. a cable that had QSFP at one end and 4x SFP at the other vs
    a cable that had QSFP at each end, thus making it impossible to
    know whether 100G as 4 25G lanes would be possible.

(e) I'm aware that there's devlink which I believe can deal with some
    of this "single network interface of 4 lanes" vs "four network
    interfaces of 1 lane" configuration, but I've never used it, and
    when I looked at it, it wasn't clear how. I have no hardware that
    makes use of devlink to play with to find out.

Basically, QSFP support is something I have little knowledge of, there
is precious little on the 'net about its use, I have no hardware
experience with, and I don't see how it fits into Linux networking.
So I decided it would be a fools errand to attempt to implement
anything.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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