Issue with DisplayLink Ethernet speeds

[FingerlessGloves]

Hi,

I've been debugging a issue this week where the DisplayLink USB network in my docking station wouldn't do much more than 400mbit/s download on a 1gbit/s capable network. On a windows device it worked no problem, but if you were running Linux the speed would drop right down.

After a lot of trial and error, I've found the cause of the problem, at least in the case of my Toshiba docking station, both single and dual screen model of docks are affected.

The udev rules that sets `cdc_ncm/rx_max` and `cdc_ncm/tx_max` to the correct max values, doesn't fire on my docking station, because the value of ENV{PRODUCT} is blank and does not start with "17e9/*". I then added a different version of the cdc_ncm rules to detect the DisplayLink driver differently, so it doesn't apply the rule to non DisplayLink docks. to use VENDOR ID instead of PRODUCT

ACTION=="add|move|bind", SUBSYSTEM=="net", DRIVERS=="cdc_ncm", ENV{ID_USB_VENDOR_ID}=="17e9", \
    ATTR{cdc_ncm/rx_max}=="?*", ATTR{cdc_ncm/dwNtbInMaxSize}=="?*", ATTR{cdc_ncm/rx_max}="$attr{cdc_ncm/dwNtbInMaxSize}"
ACTION=="add|move|bind", SUBSYSTEM=="net", DRIVERS=="cdc_ncm", ENV{ID_USB_VENDOR_ID}=="17e9", \
    ATTR{cdc_ncm/tx_max}=="?*", ATTR{cdc_ncm/dwNtbOutMaxSize}=="?*", ATTR{cdc_ncm/tx_max}="$attr{cdc_ncm/dwNtbOutMaxSize}"

I add this outside of the `ENV{PRODUCT}!="17e9/*", GOTO="not_dl"` and `LABEL="not_dl"` lines of the rules file created by displaylink-driver-6.3.0-48.run, so effectively at the end of the current udev rule file.

Hopefully this issue can be fixed in the installer you provide, no doubt other users are having this issue with the current way the rules are, which is failing to apply the corrected values

If it helps this is the udevadm info for my dock's ethernet

udevadm info --query=property --path=/sys/class/net/eth0

DEVPATH=/devices/pci0000:00/0000:00:08.1/0000:74:00.4/usb4/4-1/4-1.1/4-1.1:1.5/net/eth0
INTERFACE=eth0
IFINDEX=12
SUBSYSTEM=net
USEC_INITIALIZED=3640388761
ID_NET_DRIVER=cdc_ncm
ID_BUS=usb
ID_MODEL=dynadock_U3.0
ID_MODEL_ENC=dynadock\x20U3.0
ID_MODEL_ID=4305
ID_SERIAL=DisplayLink_dynadock_U3.0_10084571
ID_SERIAL_SHORT=10084571
ID_VENDOR=DisplayLink
ID_VENDOR_ENC=DisplayLink
ID_VENDOR_ID=17e9
ID_REVISION=3015
ID_TYPE=generic
ID_USB_MODEL=dynadock_U3.0
ID_USB_MODEL_ENC=dynadock\x20U3.0
ID_USB_MODEL_ID=4305
ID_USB_SERIAL=DisplayLink_dynadock_U3.0_10084571
ID_USB_SERIAL_SHORT=10084571
ID_USB_VENDOR=DisplayLink
ID_USB_VENDOR_ENC=DisplayLink
ID_USB_VENDOR_ID=17e9
ID_USB_REVISION=3015
ID_USB_TYPE=generic
ID_USB_INTERFACES=:ff0003:fe0101:010120:010220:020d00:0a0001:
ID_USB_INTERFACE_NUM=05
ID_USB_DRIVER=cdc_ncm
ID_USB_CLASS_FROM_DATABASE=Miscellaneous Device
ID_USB_PROTOCOL_FROM_DATABASE=Interface Association
ID_VENDOR_FROM_DATABASE=DisplayLink
ID_NET_NAMING_SCHEME=v258
ID_NET_NAME_MAC=enx0050b65b7c2b
ID_OUI_FROM_DATABASE=GOOD WAY IND. CO., LTD.
ID_NET_NAME_PATH=enp116s0f4u1u1i5
ID_MM_CANDIDATE=1
ID_PATH_WITH_USB_REVISION=pci-0000:74:00.4-usbv3-0:1.1:1.5
ID_PATH=pci-0000:74:00.4-usb-0:1.1:1.5
ID_PATH_TAG=pci-0000_74_00_4-usb-0_1_1_1_5
ID_NET_LINK_FILE=/usr/lib/systemd/network/99-default.link
ID_NET_NAME=eth0
SYSTEMD_ALIAS=/sys/subsystem/net/devices/eth0
TAGS=:systemd:
CURRENT_TAGS=:systemd:

Any more questions, I'll be happy to answer.

[Szymon]

Thanks for raising this and sharing the analysis results! I confirm your points are valid and we will use it to improve next version of the driver package.

Many thanks,
Szymon