dell-bios-fan-control

Turns BIOS fan control on Dell machines on and off from user space with the SMM calls 0x34a3 and 0x35a3 — the same pair the kernel's dell-smm-hwmon driver uses on whitelisted models.

With BIOS control disabled the firmware stops overriding the fans, so fancontrol or i8kctl can drive them. Enabling it hands them back.

Build

make

Nothing beyond a C compiler and libc.

Usage

Needs root and takes exactly one argument:

dell-bios-fan-control 0   # disable BIOS control, the fans are yours
dell-bios-fan-control 1   # enable BIOS control, the firmware takes over again

With the i8k module loaded you can then set a speed directly:

i8kctl fan 1 1

Machines

Written for the XPS 9560, but the SMM pair is not model specific and works on desktops too — verified on an OptiPlex 3080 (BIOS 2.35.0, kernel 6.1), where both calls return successfully.

Many models re-arm the BIOS control on their own, so a machine that goes quiet after 0 can start overriding the fans again later. That is why dellfan reapplies it from a systemd drop-in rather than calling this once at boot; it vendors the tool in helper/ and installs it when the kernel driver cannot turn the BIOS control off by itself.

Differences from the original

• Arguments are matched exactly. With atoi a typo parsed as 0 and silently disabled the BIOS control.
• The inline SMM assembly declares its memory clobber.
• A failed SMM call warns on stderr instead of passing unnoticed. The exit code is left at 0 on purpose — the error heuristic misfires on some BIOSes.
• The Makefile builds with -O2 -Wall -Wextra.

Credits and licence

SMM code lineage: i8k by Massimo Dal Zotto, dellfan by Carlos Alberto Lopez Perez, and dell-bios-fan-control by Tom Freudenberg, which this repository started from.

GPL-2.0-or-later, per the copyright headers in the source.