Fan Service

Summary

ASUS laptops generally have a feature that lets the user toggle the fan speed. Fn-F5 on some models, Fn-F on others. The direct effect is to limit the fan speed, from whisper mode to megablast, and indirectly control performance. But it doesn’t work in OpenBSD, so I needed to write an ASUS ACPI WMI driver.acpiACPI is a hardware abstraction layer that lets the computer tell the operating system how to do things. It provides functions in byte code which the OS interprets to perform certain tasks. So instead of having to write a driver for every CPU, you call the ACPI method _PSS and that tells you what to do.Of course, vendors want to include functionality that isn’t defined in the standard, so they add some extra methods, and then you end up writing some custom drivers anyway, for the little buttons that turn the microphone on and off, etc. OpenBSD (and other OSs) have lots of little drivers for ThinkPads and whatnot.ACPI method names are only four letters long, so what do we do if two vendors include a method called WMNB and how do we determine what it does? (Technically, the ACPI nodes are identified by seven letter names, and the methods live under them, so it’s not really a problem, but somebody got paid to make a solution anyway.)wmiThe solution is an extension to ACPI called WMI, which stands for Windows Multiplies Irritations or something. You ask ACPI for a buffer called _WDG, and this contains a table mapping GUIDs (globally unique, says so right there in the name) to local method names. Looking through this table, we come across DCBA-FE-10-23-456789, which we recognize as a GUID of interest, and this tells us the method to call is WMNB, but now we know it’s the right one.OpenBSD does not have a WMI driver, which is part of what I need to set the fan profile. There is a WMI driver in Linux, and it includes an ASUS support driver for a variety of functions I’m interested in.first stepsFirst, I wrote a little acpi driver that attaches to PNP0C14. Read the _WDG...