What does the kernel boot parameter "set acpi_osi=Linux" do?

I think you mean =Windows, not Linux. The argument tells the kernel to lie to the ACPI BIOS and tell it that it is something other than Linux (which is why specifying Linux is silly).

I'm sure he means what he wrote. Yes, BIOS's usually disable functionality if Windows is not detected, but specifying Linux is not silly because by default the kernel responds false when asked if Linux is running. acpi_osi=Linux tells the kernel to respond true.(see drivers/acpi/osl.c) It seems that you need to tell the BIOS the truth.

It's either a driver problem or incorrect ACPI implementation. You'd have to take out your dsdt and disassemble it to review the code and get a better understanding of what is broke or disabled under the Linux OSI.

Reference

• Why Linux ACPI driver pretends to be Windows and disables OSI(Linux) by default
• ACPI/Fix common problems - Gentoo Linux Wiki
• How do I add a kernel boot parameter?

The parameter adds "Linux" to the list of supported operating systems that the kernel will get when it asks the BIOS "Hey, what do you support". By adding that string, later, things that use ACPI will say "hey, I have a feature that needs ACPI, and I'm running Linux - what platforms are supported?" and they'll get back a list which will include Linux.

By setting this to Linux, you basically disable any checking to see if the BIOS says that certain ACPI features will work with certain operating systems. And that often makes things works which were previously hidden. But it could also break stuff that used to work, in [hopefully] rare cases. :)

I think you mean =Windows, not Linux. The argument tells the kernel to lie to the ACPI BIOS and tell it that it is something other than Linux (which is why specifying Linux is silly). Certain BIOS are known to check to see if they are being run under Windows, or a specific version of Windows, and disable functionality if they are not. This parameter lets you work around that idiocy.