Known issues on Linux

Problems we know about on Linux, how to tell if your computer is affected, and what you can do.

Connection stays at about 9 Gb/s on a 20 Gbps cable

On some Linux laptops, copies top out at about 9 Gb/s on a USB4 20 Gbps cable. The cable carries two 10 Gb/s lanes, and only one is used. The other computer may still report the link as 20 Gbps.

Why it happens

Either Linux or the computer’s firmware manages the Thunderbolt/USB4 controller, and the firmware decides which at startup. Linux can join the two lanes of a computer-to-computer link only when it manages the controller. When the firmware manages it, the link stays on one lane. Neither DirectConnect nor the other computer can change this.

We found this on a Lenovo ThinkPad P1 with an Intel Tiger Lake processor, connected to a Windows 11 laptop. The cable measured 9.1 Gb/s.

Check whether you’re affected

Connect the cable, then run these in a terminal:

sudo dmesg | grep "USB4 _OSC"
cat /sys/bus/thunderbolt/devices/*/rx_lanes

If the first command prints nothing, the firmware manages the controller. If the second prints 1, the connection is using one lane.

What you can do

Use a 40 Gbps cable. A Thunderbolt 4 or USB4 40 Gbps cable runs each lane at 20 Gb/s, so one lane is enough. On one 20 Gb/s lane, a Linux laptop reaches 16.3–17.8 Gb/s with a Mac and 17.3 Gb/s to a Windows PC. See which USB-C cable you need.
Update the firmware. Install the latest BIOS or UEFI update for your computer, and look for a Thunderbolt or USB4 setting that lets the operating system manage the ports. Afterwards, the first command above prints lines starting with USB4 _OSC.