OpenBSD and Thunderbolt issue on ThinkPad T480s

  2025-03-25      253 words, 2 minutes

• bios
• openbsd
• thinkpad
• thunderbolt

As I was testing OpenBSD on my refurbished ThinkPad T480s, I noticed that the fan kept blowing even though the laptop was IDLE. Given that I am a vigilant reader of misc@ , I already knew a few ThinkPads had been reported to be sluggish because of some BIOS settings regarding Thunderbolt.

Whether I was running OpenBSD 7.6 or 7.7-BETA, I could notice two invariant behaviours:

• the OS would boot and the fan wouldn’t stop blowing;
• the OS would boot and the fan would behave normally.
  Until I put the laptop into sleep mode and resume it.
  Then, the fan wouldn’t stop blowing.

In both cases, top(1) wouldn’t reveal anything special except the load average would be too high for an IDLE machine. But using htop(1) would reveal a single core being used at 100%. Not always the same core, but still with a 100% usage.

So I decided to drop into the BIOS and change a few parameters.

• In the Config section, select the Thunderbolt (TM) 3 item.
• Thunderbolt BIOS Assist Mode is set to Enabled.
• Wake by Thunderbolt(TM) 3 is set to Disabled.
• Security Level is set to Display Port and USB.
  Setting the Security Level to No Security was not enough to solve the fan blowing issue.

So far, the fan has been behaving properly: only blowing when the CPU is really used. The USB-C cable that connects the ThinkPad to a Dell 4K monitor also provides power, display port and USB hub. Exactly what I need!

That’s All Folks!