RTL8125D breaks at 2.5Gbps

Hi Heiner, ChunHao,

Last autumn, my RTL8125D stopped working at 2.5 Gbps due to excessive
(2/3) packet loss.  This happened after an Ubuntu kernel update, but
booting the old kernel didn't help. As I couldn't investigate the issue
at that time, I moved the machine to a 1 Gbps switch.  I regularly tried newer
Ubuntu kernels, checked cables, but nothing fixed the issue...
Last weekend, I decided to have a deeper look, and restored all relevant
boot logs from backup.  Suddenly my eye caught:

    Unable to load firmware rtl_nic/rtl8125d-1.fw

in the logs of working kernels.

After moving /lib/firmware/rtl_nic/rtl8125d-1.fw.zst out of the way,
and power-cycling the machine, 2.5 Gbps started working again (280
MIB/s)! Presumably the firmware in ROM does support 2.5 Gbps, while
rtl8125d-1.fw is limited to 1 Gbps?

My motherboard is ASUS System Product Name/TUF GAMING B850M-PLUS WIFI,
BIOS 1057 06/02/2025.  I am using Ubuntu's 7.0.0-29-generic kernel,
whose r8169 driver is almost the same as in upstream v7.0.

According to the r8169 output, I have RTL8125D XID 688, while lspci
says RTL8125 2.5GbE Controller (rev 0c).

However, according to the r8169 driver sources:
  - RTL8125D ([1] says rev.a?) has XID 688 and uses
    rtl_nic/rtl8125d-1.fw,
  - RTL8125D rev.b (matches [2]) has XID 689 and uses
    rtl_nic/rtl8125d-2.fw.

Does XID 688 rev.c needs different firmware?
Would it be worthwhile/safe to replace
/lib/firmware/rtl_nic/rtl8125d-1.fw.zstd by rtl8125d-2.fw.zstd?
Is there any other info you need?

Thanks in advance!

[1] https://gitlab.com/kernel-firmware/linux-firmware/-/merge_requests/332
[2] https://gitlab.com/kernel-firmware/linux-firmware/-/merge_requests/387

Gr{oetje,eeting}s,

                        Geert