[wtallis]

On typical desktop motherboards, the Super IO chip handles all the temperature monitoring and fan control. Those chips usually have a few modes to configure some very simple control system for mapping temperature inputs to fan speed outputs (never anything as advanced as a PID controller).

The main problem is that the Super IO only has access to the temperature sensors on the motherboard itself, and on the CPU (these days, through PECI). There's no standard way for the Super IO to do out of band monitoring of temperatures on your GPU or storage drives, so if you want those to affect fan speeds you need to implement it in software.

Servers typically have BMCs controlling fans, and even Apple's x86 machines have their SMC; in both cases you typically see a more thorough monitoring of component temperatures, configured out of the box with a proper awareness of which fans are blowing across which components. But that stuff doesn't trickle down to the build your own desktop market.

[deathanatos]

Hmm. I guess I have no idea how this chip interfaces with the kernel then? The only knobs that seem to be documented to exist ever are direct RPM controls for manual control.

I know it's possible (despite the insistence to the contrary on the other thread), as basically every laptop does it. Only do desktops seem to struggle with this concept.

[wtallis]

How fan control is configured depends on the SuperIO chip, so it's different for eg. Fintek SuperIOs than for Winbond/Nuvoton SuperIOs.

On Linux, a supported SuperIO will be exposed as a directory under /sys/class/hwmon. On one of my systems, the SuperIO is a Nuvoton NCT6791, so the relevant driver documentation is https://www.kernel.org/doc/Documentation/hwmon/nct6775

Relevant sysfs files to note are pwm[1-7]_mode to toggle between DC voltage and PWM control, and pwm[1-7]_enable to switch between full speed, pure software speed control, and several Nuvoton-specific automatic speed control modes.