linux-kernel - RTL8127AF doesn't get a link over SFP+ DAC

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Message-ID: <CAN9vWDK=36NUdTtZhPMu7Yh15kGv+gkE35A93dU0qg01z5VkbA@mail.gmail.com>
Date: Thu, 13 Nov 2025 18:30:51 +0100
From: Michael Zimmermann <sigmaepsilon92@...il.com>
To: Heiner Kallweit <hkallweit1@...il.com>, nic_swsd@...ltek.com, 
	Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: RTL8127AF doesn't get a link over SFP+ DAC

Hi,

I have a RT8127AF card from DIEWU:
https://24wireless.info/diewu-txa403-and-txa405 .
The card is detected just fine:
[125201.683763] r8169 0000:08:00.0 eth1: RTL8127A, xx:xx:xx:xx:xx:xx,
XID 6c9, IRQ 143
[125201.683770] r8169 0000:08:00.0 eth1: jumbo features [frames: 16362
bytes, tx checksumming: ko]
[125201.688543] r8169 0000:08:00.0 enp8s0: renamed from eth1
[125201.715519] Realtek Internal NBASE-T PHY r8169-0-800:00: attached
PHY driver (mii_bus:phy_addr=r8169-0-800:00, irq=MAC)
[125202.277034] r8169 0000:08:00.0 enp8s0: Link is Down

This is what ethtool shows:
Settings for enp8s0:
        Supported ports: [ TP    MII ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
                                10000baseT/Full
                                2500baseT/Full
                                5000baseT/Full
        Supported pause frame use: Symmetric Receive-only
        Supports auto-negotiation: Yes
        Supported FEC modes: Not reported
        Advertised link modes:  10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
                                10000baseT/Full
                                2500baseT/Full
                                5000baseT/Full
        Advertised pause frame use: Symmetric Receive-only
        Advertised auto-negotiation: Yes
        Advertised FEC modes: Not reported
        Speed: Unknown!
        Duplex: Unknown! (255)
        Auto-negotiation: on
        master-slave cfg: preferred slave
        master-slave status: unknown
        Port: Twisted Pair
        PHYAD: 0
        Transceiver: internal
        MDI-X: Unknown
        Supports Wake-on: pumbg
        Wake-on: d
        Link detected: no

and `ip a`:
10: enp8s0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc
fq_codel state DOWN group default qlen 1000
    link/ether xx:xx:xx:xx:xx:xx brd ff:ff:ff:ff:ff:ff
    altname enxXXXXXXXXXXXX

And that's it, the link never comes up. The 10G Mikrotik switch on the
other side sees that the module is inserted on its side, but doesn't
show any change when I plug in the RTL8127AF.

It works in Windows 11 and it also works with the r8127 Linux driver
downloaded from Realteks website:
https://www.realtek.com/Download/List?cate_id=584 :

[129318.976134] r8127: This product is covered by one or more of the
following patents: US6,570,884, US6,115,776, and US6,327,625.
[129318.976175] r8127  Copyright (C) 2025 Realtek NIC software team
<nicfae@...ltek.com>
                 This program comes with ABSOLUTELY NO WARRANTY; for
details, please see <http://www.gnu.org/licenses/>.
                 This is free software, and you are welcome to
redistribute it under certain conditions; see
<http://www.gnu.org/licenses/>.
[129318.988293] r8127 0000:08:00.0 enp8s0: renamed from eth1
[129318.997092] enp8s0: 0xffffd49ec9140000, xx:xx:xx:xx:xx:xx, IRQ 137
[129319.421629] r8127: enp8s0: link up

ethtool with realteks driver shows something quite interesting:
Settings for enp8s0:
        Supported ports: [ TP ]
        Supported link modes:   1000baseT/Full
                                10000baseT/Full
        Supported pause frame use: No
        Supports auto-negotiation: No
        Supported FEC modes: Not reported
        Advertised link modes:  1000baseT/Full
                                10000baseT/Full
        Advertised pause frame use: No
        Advertised auto-negotiation: No
        Advertised FEC modes: Not reported
        Speed: 10000Mb/s
        Duplex: Full
        Auto-negotiation: off
        Port: Twisted Pair
        PHYAD: 0
        Transceiver: internal
        MDI-X: on
        Supports Wake-on: pumbg
        Wake-on: g
        Current message level: 0x00000033 (51)
                               drv probe ifdown ifup
        Link detected: yes

auto-negotiation is off, even though it's enabled on my Mikrotik
switch. "ethtool -s enp8s0 autoneg on" (or off) on the realtek driver
succeeds but doesn't change what ethtools status shows. "ethtool -s
enp8s0 autoneg off" on the mainline driver does fail with:
netlink error: link settings update failed
netlink error: Invalid argument

So while I have no idea why things are not working, my best theory is
that auto-negotiation isn't supported (properly) and the mainline
driver doesn't support disabling it.

Thanks
Michael