[Bug 203191] The fan speed reports to 65535, despite the fan is stopped

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=203191

--- Comment #7 from Matthias Schiffer (mschiffer@universe-factory.net) ---
The T470 (without the 's' suffix) is affected as well. Still reproducible with
the latest BIOS 1.65 and Linux 5.11.6.

I believe the issue is not directly related to suspend/resume, as I don't use
suspend. Instead, the fan control can break any time the fan is supposed to
spin up, which may be when waking up from suspend, or simply because of
increased CPU workload.

Whenever the systems gets into the broken state:

- the fan is not spinning at all
- hwmon and /proc/acpi/ibm/fan report 65535 RPM
- manual control via /proc/acpi/ibm/fan is ignored (Note: I normally don't use
this interface, but rely on the builtin fan control)
- only a full poweroff will fix the issue. When attempting to reboot without a
poweroff, the BIOS will refuse to boot with a "Fan error" message.

As far as I remember, the issue originally started when Lenovo fixed a BIOS
issue that caused the fan to hang at 100% after suspend occasionally a few
years ago.

I'd be happy to provide any information that may help fix this issue, as I'm
seeing it pretty much daily.

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