Credit: https://github.com/Anime4000/RTL960x/discussions/250#discussion-6288339

• OpenWrt router with:
  • SFP port(s) (any speed) with I2C bus
  • Packages ethtool-full i2c-tools

1. SSH into OpenWrt
2. Run one of the following commands to determine which I2C bus your SFP is on
   • If your router has a single SFP:
     1. BUS=$(for bus in /sys/bus/i2c/devices/i2c-*; do b=${bus##*-}; i2cdetect -y -r $b 2>/dev/null | grep -q " 50 " && i2cdetect -y -r $b 2>/dev/null | grep -q " 51 " && echo $b; done)
     2. echo $BUS and check there is a value before proceeding (you can also run the multiple SFP script below to verify the SFP vendor and S/N)
   • If your router has multiple SFPs:

     1. for bus in /sys/bus/i2c/devices/i2c-*; do
            b=${bus##*-}
            if i2cdetect -y -r $b 2>/dev/null | grep -q " 50 " && i2cdetect -y -r $b 2>/dev/null | grep -q " 51 "; then
                vendor=""
                for reg in $(seq 20 35); do
                    vendor="$vendor$(printf "\\x$(i2cget -y $b 0x50 $reg | sed 's/0x//')")"
                done
                serial=""
                for reg in $(seq 68 83); do
                    serial="$serial$(printf "\\x$(i2cget -y $b 0x50 $reg | sed 's/0x//')")"
                done
                echo ""
                echo "Bus: $b"
                echo "├─▶ Vendor: $vendor"
                echo "└─▶ S/N: $serial"
            fi
        done

     2. Determine the correct bus number, e.g. 5
     3. BUS=5 (replace 5 with the respective bus number)
3. Run the following commands to set the SFP EEPROM bits in OpenWrt according to the EEPROM Bits table below
   1. i2cset -y $BUS 0x50 0x06 0x00
   2. i2cset -y $BUS 0x50 0x0a 0x05
   3. i2cset -y $BUS 0x50 0x0c 0x1f
   4. i2cset -y $BUS 0x50 0x3f $(i2cdump -y -r 0-62 $BUS 0x50 b | awk 'NR>1 {for(i=2;i<=NF;i++) if($i~/^[0-9a-f]{2}$/) {cmd="printf \"%d\" 0x"$i; cmd|getline d; close(cmd); sum+=d}} END {printf "0x%02x", sum % 256}')
4. Physically re-seat the SFP
5. Verify the new speeds are detected:
   ethtool -m [ethX]
   

           Supported link modes:   2500baseX/Full
                                   1000baseX/Full

⚠️ Warning: These settings are untested. Proceed with caution.

Notes:

• Due to Linux kernel limitations, only a single speed can be advertised via EEPROM for XG-PON/XGS-PON. Multi-speed combinations (e.g. 2.5/5Gbps, 5/10Gbps, 2.5/5/10Gbps) are not achievable through EEPROM editing alone
• You do not have to update the 0x5F checksum (CC_EXT) unless you edit addresses 0x40–0x5E, which are out of scope of this guide; but in case you need it, this command will update CC_EXT:
  i2cset -y $BUS 0x50 0x5f $(i2cdump -y -r 64-94 $BUS 0x50 b | awk 'NR>1 {for(i=2;i<=NF;i++) if($i~/^[0-9a-f]{2}$/) {cmd="printf \"%d\" 0x"$i; cmd|getline d; close(cmd); sum+=d}} END {printf "0x%02x", sum % 256}')

Notes:

• Set 0x06 to 0x00 (disable 1000BASE-LX) for all configurations above except 1000BASE-X, then set 0x0A and 0x0C according to the desired speed