DSDT (Differentiated System Description Table) is a part of the ACPI specification and it supplies configuration information about a base system. ACPI capable computers come with a DSDT in firmware from the manufacturer. A common Linux problem is missing ACPI functionality (fans not running, screens not turning off when the lid is closed, etc.) stemming from DSDTs made with Windows specifically in mind. The solution that this article will try to detail is replacing the default DSDT with a 'fixed' version. Note that this fix can also be accomplished during installation ("Will you need support for custom DSDT?") but requires you to have a custom DSDT ready at hand.
Before you start...
This process does tamper with some fairly fundamental code on your installation. You will want to be absolutely sure of the changes you make. You might also wish to clone your disk beforehand.
Even before attempting to fix your DSDT yourself, you can attempt a couple different venues of attack:
Tell the kernel to report a version of Windows. The trick is to use the variable acpi_os_name as kernel parameter. For example:
acpi_os_name="Microsoft Windows NT"
in the kernel line in grub legacy configuration
other strings to test for:
- "Microsoft Windows XP"
- "Microsoft Windows 2000"
- "Microsoft Windows 2000.1"
- "Microsoft Windows ME: Millennium Edition"
- "Windows 2001"
- "Windows 2006"
- when all that fails, you can even try "Linux"
If that does not work, you can follow the steps below not to recompile or fix your DSDT but just to find out what test is made on the OS inside it and find the string to use as acpi_os_name. Just search for "Microsoft" string in the decompiled DSDT and look at the test made.
Get hold of fixed DSDT. A DSDT file is originally written in ACPI Source language (an .asl/.dsl file). Using a compiler this can produce an 'ACPI Machine Language' file (.aml) or a hex table (.hex). To incorporate the file in your Arch install, you'll need to get hold of a compiled .aml file. - whether this means compiling it yourself or trusting some stranger on the Internet is at your discretion. If you do download a file from the world wide web, it'll most likely be a compressed .asl file, so you'll need to unzip it and compile it. You should really read the Gentoo wiki for that but the upshot is that you won't have to research specific code fixes yourself.
- Arch users with the same laptop as you: are a minority of a minority of a minority. Try browsing other distro/linux forums for talk about the same model. Likelihood is that they have the same problems and either because there is a lot of them, or because they're tech wizards -- someone there has produced a working DSDT and maybe even provides a precompiled version (again, use at your own risk).
- Google Is Your Friend: Try keeping it sweet and short - model name and dsdt will probably produce results.
Recompiling it yourself
In a nutshell, you can use Intel's ASL compiler to turn your systems DSDT table into source code, locate and fix the errors (the compiled files will typically have been made using Microsoft's compiler rather than Intel's more stringent one... need we say more?), and recompile. This process is detailed more comprehensively at the Gentoo wiki.
Who compiled the original code?
Check if your system's DSDT was compiled using Intel or Microsoft compiler:
$ dmesg|grep DSDT ACPI: DSDT 00000000bf7e5000 0A35F (v02 Intel CALPELLA 06040000 INTL 20060912) ACPI: EC: Look up EC in DSDT
In case Microsoft's compiler had been used, words INTL would instead be MSFT. In the example, there were 5 errors on decompiling/recompiling the DSDT. Two of them were easy to fix after a bit of googling and delving into the ACPI specification. Three of them were due to different versions of compiler used and are, as later discovered, handled by the ACPICA at boot-time. The ACPICA component of the kernel can handle most of the trivial errors you get while compiling the DSDT. So do not fret yourself over compile errors if your system is working the way it should.
Extracting ACPI tables
iasl -tc ''.asl/.dsl file''
on it and if it says no errors and no warnings you should be good to go.