[1.] One line summary of the problem: igc driver - Ethernet NIC speed not changing
[2.] Full description of the problem/report:
Trying to change my I225-V's connection speed to 1000 Mbps down from 2500 Mbps as a workaround to disconnection issues, but changes won't apply, regardless of using NetworkManager or ethtool.

NetworkManager displays the changed values, but they don't seem to actually apply.

Using ethtool to change the speed to 1000 Mbps (`ethtool -s enp6s0 speed 1000`) also fails.
The interface gets brought down then up with the same 2500 Mbps speed.

[3.] Keywords (i.e., modules, networking, kernel): driver, networking, igc, intel[4.] Kernel information
 [4.1.] Kernel version (from /proc/version): Linux version 6.4.6-arch1-1 (linux@archlinux) (gcc (GCC) 13.1.1 20230714, GNU ld (GNU Binutils) 2.40.0) #1 SMP PREEMPT_DYNAMIC Mon, 24 Jul 2023 20:19:38 +0000[4.2.] Kernel .config file: Cannot obtain
[5.] Most recent kernel version which did not have the bug: 6.2.9 or more recent?
[6.] Output of Oops.. message (if applicable) with symbolic information     resolved (see Documentation/admin-guide/bug-hunting.rst) N/A
[7.] A small shell script or example program which triggers the     problem (if possible)
# ethtool -s [INTERFACE] speed 1000
[8.] Environment
[8.1.] Software (add the output of the ver_linux script here) Cannot obtain
[8.2.] Processor information (from /proc/cpuinfo):
processor	: 0
vendor_id	: AuthenticAMD
cpu family	: 23
model		: 113
model name	: AMD Ryzen 5 3600 6-Core Processor
stepping	: 0
microcode	: 0x8701030
cpu MHz		: 2473.153
cache size	: 512 KB
physical id	: 0
siblings	: 12
core id		: 0
cpu cores	: 6
apicid		: 0
initial apicid	: 0
fpu		: yes
fpu_exception	: yes
cpuid level	: 16
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm constant_tsc rep_good nopl nonstop_tsc cpuid extd_apicid aperfmperf rapl pni pclmulqdq monitor ssse3 fma cx16 sse4_1 sse4_2 x2apic movbe popcnt aes xsave avx f16c rdrand lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs skinit wdt tce topoext perfctr_core perfctr_nb bpext perfctr_llc mwaitx cpb cat_l3 cdp_l3 hw_pstate ssbd mba ibpb stibp vmmcall fsgsbase bmi1 avx2 smep bmi2 cqm rdt_a rdseed adx smap clflushopt clwb sha_ni xsaveopt xsavec xgetbv1 cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local clzero irperf xsaveerptr rdpru wbnoinvd arat npt lbrv svm_lock nrip_save tsc_scale vmcb_clean flushbyasid decodeassists pausefilter pfthreshold avic v_vmsave_vmload vgif v_spec_ctrl rdpid overflow_recov succor smca sev sev_es
bugs		: sysret_ss_attrs spectre_v1 spectre_v2 spec_store_bypass retbleed smt_rsb
bogomips	: 7188.50
TLB size	: 3072 4K pages
clflush size	: 64
cache_alignment	: 64
address sizes	: 43 bits physical, 48 bits virtual
power management: ts ttp tm hwpstate cpb eff_freq_ro [13] [14]

(repeats 12x: 12 threads processor)[8.3.] Module information (from /proc/modules):
igc 188416 0 - Live 0x0000000000000000
[8.4.] Loaded driver and hardware information (/proc/ioports, /proc/iomem)
/proc/ioports
0000-0000 : PCI Bus 0000:00
  0000-0000 : dma1
  0000-0000 : pic1
  0000-0000 : timer0
  0000-0000 : timer1
  0000-0000 : keyboard
  0000-0000 : PNP0800:00
  0000-0000 : keyboard
  0000-0000 : rtc0
  0000-0000 : dma page reg
  0000-0000 : pic2
  0000-0000 : ACPI PM2_CNT_BLK
  0000-0000 : dma2
  0000-0000 : fpu
  0000-0000 : pnp 00:03
  0000-0000 : pnp 00:03
0000-0000 : PCI Bus 0000:00
0000-0000 : PCI Bus 0000:00
  0000-0000 : serial
  0000-0000 : pnp 00:04
  0000-0000 : pnp 00:04
  0000-0000 : pnp 00:04
  0000-0000 : pnp 00:04
    0000-0000 : ACPI PM1a_EVT_BLK
    0000-0000 : ACPI PM1a_CNT_BLK
    0000-0000 : ACPI PM_TMR
    0000-0000 : ACPI GPE0_BLK
  0000-0000 : pnp 00:04
  0000-0000 : pnp 00:04
  0000-0000 : pnp 00:04
    0000-0000 : piix4_smbus
  0000-0000 : pnp 00:04
    0000-0000 : piix4_smbus
  0000-0000 : pnp 00:04
  0000-0000 : pnp 00:04
  0000-0000 : pnp 00:04
  0000-0000 : pnp 00:04
  0000-0000 : pnp 00:04
  0000-0000 : pnp 00:04
  0000-0000 : pnp 00:04
0000-0000 : PCI conf1
0000-0000 : PCI Bus 0000:00
  0000-0000 : PCI Bus 0000:07
    0000-0000 : PCI Bus 0000:08
      0000-0000 : PCI Bus 0000:09
        0000-0000 : 0000:09:00.0

/proc/iomem
00000000-00000000 : Reserved
00000000-00000000 : System RAM
00000000-00000000 : Reserved
  00000000-00000000 : PCI Bus 0000:00
  00000000-00000000 : System ROM
00000000-00000000 : System RAM
00000000-00000000 : Reserved
00000000-00000000 : System RAM
00000000-00000000 : ACPI Non-volatile Storage
00000000-00000000 : System RAM
00000000-00000000 : Reserved
00000000-00000000 : System RAM
00000000-00000000 : System RAM
00000000-00000000 : System RAM
00000000-00000000 : System RAM
00000000-00000000 : System RAM
00000000-00000000 : Reserved
00000000-00000000 : System RAM
00000000-00000000 : Reserved
00000000-00000000 : System RAM
00000000-00000000 : Reserved
00000000-00000000 : System RAM
00000000-00000000 : Reserved
  00000000-00000000 : MSFT0101:00
    00000000-00000000 : MSFT0101:00
  00000000-00000000 : MSFT0101:00
    00000000-00000000 : MSFT0101:00
00000000-00000000 : ACPI Tables
00000000-00000000 : ACPI Non-volatile Storage
00000000-00000000 : Reserved
00000000-00000000 : System RAM
00000000-00000000 : Reserved
00000000-00000000 : PCI Bus 0000:00
  00000000-00000000 : PCI MMCONFIG 0000 [bus 00-7f]
    00000000-00000000 : pnp 00:00
  00000000-00000000 : PCI Bus 0000:0b
    00000000-00000000 : 0000:0b:00.3
      00000000-00000000 : xhci-hcd
    00000000-00000000 : 0000:0b:00.1
      00000000-00000000 : ccp
    00000000-00000000 : 0000:0b:00.4
      00000000-00000000 : ICH HD audio
    00000000-00000000 : 0000:0b:00.1
      00000000-00000000 : ccp
  00000000-00000000 : PCI Bus 0000:07
    00000000-00000000 : PCI Bus 0000:08
      00000000-00000000 : PCI Bus 0000:09
        00000000-00000000 : 0000:09:00.0
        00000000-00000000 : 0000:09:00.0
        00000000-00000000 : 0000:09:00.1
          00000000-00000000 : ICH HD audio
    00000000-00000000 : 0000:07:00.0
  00000000-00000000 : PCI Bus 0000:02
    00000000-00000000 : PCI Bus 0000:03
      00000000-00000000 : PCI Bus 0000:06
        00000000-00000000 : 0000:06:00.0
          00000000-00000000 : igc
        00000000-00000000 : 0000:06:00.0
          00000000-00000000 : igc
    00000000-00000000 : 0000:02:00.1
    00000000-00000000 : 0000:02:00.1
      00000000-00000000 : ahci
    00000000-00000000 : 0000:02:00.0
      00000000-00000000 : xhci-hcd
  00000000-00000000 : PCI Bus 0000:01
    00000000-00000000 : 0000:01:00.0
      00000000-00000000 : nvme
00000000-00000000 : pnp 00:01
  00000000-00000000 : MSFT0101:00
00000000-00000000 : amd_iommu
00000000-00000000 : Reserved
00000000-00000000 : IOAPIC 0
00000000-00000000 : IOAPIC 1
00000000-00000000 : Reserved
  00000000-00000000 : pnp 00:04
00000000-00000000 : Reserved
  00000000-00000000 : AMDIF030:00
    00000000-00000000 : AMDIF030:00 AMDIF030:00
00000000-00000000 : Reserved
  00000000-00000000 : HPET 0
    00000000-00000000 : PNP0103:00
00000000-00000000 : Reserved
00000000-00000000 : Reserved
  00000000-00000000 : AMDI0030:00
    00000000-00000000 : AMDI0030:00 AMDI0030:00
00000000-00000000 : pnp 00:04
00000000-00000000 : Reserved
  00000000-00000000 : AMDI0010:03
    00000000-00000000 : AMDI0010:03 AMDI0010:03
00000000-00000000 : Reserved
00000000-00000000 : Local APIC
  00000000-00000000 : pnp 00:04
00000000-00000000 : pnp 00:04
00000000-00000000 : System RAM
  00000000-00000000 : Kernel code
  00000000-00000000 : Kernel rodata
  00000000-00000000 : Kernel data
  00000000-00000000 : Kernel bss
00000000-00000000 : Reserved
00000000-00000000 : PCI Bus 0000:00
  00000000-00000000 : PCI Bus 0000:07
    00000000-00000000 : PCI Bus 0000:08
      00000000-00000000 : PCI Bus 0000:09
        00000000-00000000 : 0000:09:00.0
        00000000-00000000 : 0000:09:00.0
00000000-00000000 : 0000:09:00.0[8.5.] PCI information ('lspci -vvv' as root)
06:00.0 Ethernet controller: Intel Corporation Ethernet Controller I225-V (rev 02)
Subsystem: ASUSTeK Computer Inc. Ethernet Controller I225-V
Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx+
Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
Latency: 0, Cache Line Size: 64 bytes
Interrupt: pin A routed to IRQ 36
IOMMU group: 15
Region 0: Memory at fcc00000 (32-bit, non-prefetchable) [size=1M]
Region 3: Memory at fcd00000 (32-bit, non-prefetchable) [size=16K]
Capabilities: [40] Power Management version 3
Flags: PMEClk- DSI+ D1- D2- AuxCurrent=0mA PME(D0+,D1-,D2-,D3hot+,D3cold+)
Status: D0 NoSoftRst+ PME-Enable- DSel=0 DScale=1 PME-
Capabilities: [50] MSI: Enable- Count=1/1 Maskable+ 64bit+
Address: 0000000000000000  Data: 0000
Masking: 00000000  Pending: 00000000
Capabilities: [70] MSI-X: Enable+ Count=5 Masked-
Vector table: BAR=3 offset=00000000
PBA: BAR=3 offset=00002000
Capabilities: [a0] Express (v2) Endpoint, MSI 00
DevCap:	MaxPayload 512 bytes, PhantFunc 0, Latency L0s <512ns, L1 <64us
ExtTag- AttnBtn- AttnInd- PwrInd- RBE+ FLReset+ SlotPowerLimit 0W
DevCtl:	CorrErr+ NonFatalErr+ FatalErr+ UnsupReq+
RlxdOrd+ ExtTag- PhantFunc- AuxPwr- NoSnoop+ FLReset-
MaxPayload 512 bytes, MaxReadReq 512 bytes
DevSta:	CorrErr- NonFatalErr- FatalErr- UnsupReq- AuxPwr+ TransPend-
LnkCap:	Port #9, Speed 5GT/s, Width x1, ASPM L1, Exit Latency L1 <4us
ClockPM- Surprise- LLActRep- BwNot- ASPMOptComp+
LnkCtl:	ASPM Disabled; RCB 64 bytes, Disabled- CommClk+
ExtSynch- ClockPM- AutWidDis- BWInt- AutBWInt-
LnkSta:	Speed 5GT/s, Width x1
TrErr- Train- SlotClk+ DLActive- BWMgmt- ABWMgmt-
DevCap2: Completion Timeout: Range ABCD, TimeoutDis+ NROPrPrP- LTR+
10BitTagComp- 10BitTagReq- OBFF Not Supported, ExtFmt- EETLPPrefix-
EmergencyPowerReduction Not Supported, EmergencyPowerReductionInit-
FRS- TPHComp- ExtTPHComp-
AtomicOpsCap: 32bit- 64bit- 128bitCAS-
DevCtl2: Completion Timeout: 50us to 50ms, TimeoutDis- LTR- 10BitTagReq- OBFF Disabled,
AtomicOpsCtl: ReqEn-
LnkCtl2: Target Link Speed: 5GT/s, EnterCompliance- SpeedDis-
Transmit Margin: Normal Operating Range, EnterModifiedCompliance- ComplianceSOS-
Compliance Preset/De-emphasis: -6dB de-emphasis, 0dB preshoot
LnkSta2: Current De-emphasis Level: -6dB, EqualizationComplete- EqualizationPhase1-
EqualizationPhase2- EqualizationPhase3- LinkEqualizationRequest-
Retimer- 2Retimers- CrosslinkRes: unsupported
Capabilities: [100 v2] Advanced Error Reporting
UESta:	DLP- SDES- TLP- FCP- CmpltTO- CmpltAbrt- UnxCmplt- RxOF- MalfTLP- ECRC- UnsupReq- ACSViol-
UEMsk:	DLP- SDES- TLP- FCP- CmpltTO- CmpltAbrt- UnxCmplt- RxOF- MalfTLP- ECRC- UnsupReq- ACSViol-
UESvrt:	DLP+ SDES+ TLP- FCP+ CmpltTO- CmpltAbrt- UnxCmplt- RxOF+ MalfTLP+ ECRC- UnsupReq- ACSViol-
CESta:	RxErr- BadTLP- BadDLLP- Rollover- Timeout- AdvNonFatalErr-
CEMsk:	RxErr- BadTLP- BadDLLP- Rollover- Timeout- AdvNonFatalErr+
AERCap:	First Error Pointer: 00, ECRCGenCap+ ECRCGenEn- ECRCChkCap+ ECRCChkEn-
MultHdrRecCap- MultHdrRecEn- TLPPfxPres- HdrLogCap-
HeaderLog: 00000000 00000000 00000000 00000000
Capabilities: [140 v1] Device Serial Number 24-4b-fe-ff-ff-5a-40-86
Capabilities: [1c0 v1] Latency Tolerance Reporting
Max snoop latency: 0ns
Max no snoop latency: 0ns
Capabilities: [1f0 v1] Precision Time Measurement
PTMCap: Requester:+ Responder:- Root:-
PTMClockGranularity: 4ns
PTMControl: Enabled:+ RootSelected:-
PTMEffectiveGranularity: Unknown
Capabilities: [1e0 v1] L1 PM Substates
L1SubCap: PCI-PM_L1.2- PCI-PM_L1.1+ ASPM_L1.2- ASPM_L1.1+ L1_PM_Substates+
L1SubCtl1: PCI-PM_L1.2- PCI-PM_L1.1- ASPM_L1.2- ASPM_L1.1-
L1SubCtl2:
Kernel driver in use: igc
Kernel modules: igc[8.6.] SCSI information (from /proc/scsi/scsi) Empty
[8.7.] Other information that might be relevant to the problem       (please look in /proc and include all information that you       think to be relevant):
Relevant logs after running `ethtool -s enp6s0 speed 1000`:
juil. 25 21:14:56 kkiotarch NetworkManager[459]: <info>  [1690312496.0963] device (enp6s0): carrier: link connected
juil. 25 21:14:56 kkiotarch kernel: igc 0000:06:00.0 enp6s0: NIC Link is Up 2500 Mbps Full Duplex, Flow Control: RX/TX

[X.] Other notes, patches, fixes, workarounds:
As a temporary solution, I have forced port speed to be set at 1000 Mbps via my router.

Apologies if this should have submitted to my distribution's bug report first (Arch Linux); on my current kernel version, there should be no patches applied here compared to upstream.

Thank you,
KKIOT