Tag Archives: kernel

Patching DSDT in recent Linux kernels without recompiling

Up until a year or two ago, the Linux kernel let you replace the ACPI DSDT by adding a customized version to the initrd. However, more recent versions disable that by default. If you’re using Grub2 as your bootloader though, the alternative is simple: just add acpi /boot/dsdt.aml to your Grub config. If you prefer a more elegant solution, just add the attached 01_acpi.txt to /etc/grub.d, renamed it to 01_acpi and chmod +x it; then run update-grub2 to rebuild your Grub config. It originally came from ubuntuforums.org and I removed the -e flag in the acpi line, which caused the new DSDT only to be visible to Grub, but not to the OS.

I believe the acpi command in Grub2 originally came from the Hackintosh community – messing around with DSDTs is a lot more common there because Mac OS X is rather picky.

For those of you who don’t know what I’m talking about: the DSDT describes certain hardware features your PC has, such as buttons, CPU power save modes, and lots of other things. Some mainboards have very poorly done BIOSes that have equally messed up DSDT tables. I won’t go into a lot of detail regarding how to fix those here though (it’s as simple as cat /proc/acpi/dsdt > dsdt.dat; iasl -d dsdt.dat; editing dsdt.dsl to your liking; iasl -tc dsdt.dsl (this last step will probably produce a number of errors that can be solved by googling for the error number and making the appropriate changes in dsdt.dsl)).

Another valuable hint for people messing around with DSDTs: the DSDT is not the only place that can contain this kind of information, the other place would be the SSDT and possibly additional SSDTs. You can find them in /sys/firmware/acpi/tables and decompile them just like the DSDT as described above. Instead of recompiling the SSDT by itself, you could probably also consider merging it into your custom DSDT at the appropriate places.