DSDT

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DSDT (Differentiated System Description Table) is a part of the ACPI specification. It supplies information about supported power events in a given system. ACPI tables are provided in firmware from the manufacturer. A common Linux problem is missing ACPI functionality, such as: fans not running, screens not turning off when the lid is closed, etc. This can stem from DSDTs made with Windows specifically in mind, which can be patched after installation. The goal of this article is to analyze and rebuild a faulty DSDT, so that the kernel can override the default one.

Basically a DSDT table is the code run on ACPI (Power Management) events.

Note: The goal of the Linux ACPI project is that Linux should work on unmodified firmware. If you still find this type of workaround necessary on modern kernels then you should consider submitting a bug report.

Before you start...

It is possible that the hardware manufacturer has released an updated firmware which fixes ACPI related problems. Installing an updated firmware is often preferred over this method because it would avoid duplication of effort.
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 of different shortcuts:

Tell the kernel to report a version of Windows

Use the variable acpi_os_name as a kernel parameter. For example:

acpi_os_name="Microsoft Windows NT"

Or

acpi_osi="!Windows2012"

appended to the kernel line in grub legacy configuration

other strings to test:

"Microsoft Windows XP"
"Microsoft Windows 2000"
"Microsoft Windows 2000.1"
"Microsoft Windows ME: Millennium Edition"
"Windows 2001"
"Windows 2006"
"Windows 2009"
"Windows 2012"
when all that fails, you can even try "Linux"

Out of curiousity, you can follow the steps below to extract your DSDT and search the .dsl file. Just grep for "Windows" and see what pops up.

Find a 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 will 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 will most likely be a compressed .asl file. So you will need to unzip it and compile it. The upside to this is that you will not 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 are tech savvy -- someone there has produced a working DSDT and maybe even provides a precompiled version (again, use at your own risk). Search engines are your best tools. Try keeping it short: 'model name' + 'dsdt' will probably produce results.

Recompiling it yourself

Your best resources in this endeavor are going to be ACPI Spec homepage, and Linux ACPI Project which supercedes the activity that occurred at acpi.sourceforge.net. In a nutshell, you can use Intel's ASL compiler to turn your systems DSDT table into source code, locate/fix the errors, and recompile.

You will need to install acpica to modify code.

What 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, abbreviation 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.

Extract the binary ACPI tables:

# cat /sys/firmware/acpi/tables/DSDT > dsdt.dat

Disassemble the ACPI tables to a .dsl file:

$ iasl -d dsdt.dat

Attempt to create a hex AML table (in C) from the .dsl file:

$ iasl -tc dsdt.dsl

Examine any errors outputted from creating the hex AML table and fix them. For example:

dsdt.dsl   6727:                         Name (_PLD, Buffer (0x10)  
Error    4105 -          Invalid object type for reserved name ^  (found BUFFER, requires Package)

Amend the file at line 6727 where the error occurred:

(_PLD, Package(1) {Buffer (0x10)
{
    ...
}})

Increase the OEM version. Otherwise, the kernel will not apply the modified ACPI table. For example, before increasing the OEM version:

DefinitionBlock ("DSDT.aml", "DSDT", 2, "INTEL ", "TEMPLATE", 0x00000000)

After increasing the OEM version:

DefinitionBlock ("DSDT.aml", "DSDT", 2, "INTEL ", "TEMPLATE", 0x00000001)

Create the hex AML table again after fixing all errors and increasing the OEM version:

$ iasl -tc dsdt.dsl

(Might want to try option -ic for C include file to insert into kernel source.) If it says no errors and no warnings you should be good to go.

Using modified code

Warning: After each BIOS update you will need to fix DSDT again and repeat these steps!

There are at least two ways to use a custom DSDT:

creating a CPIO archive that is loaded by the bootloader
compiling it into the kernel

Using a CPIO archive

This method has the advantage that you do not need to recompile your kernel, and updating the kernel will not make it necessary to repeat these steps.

This method requires the ACPI_TABLE_UPGRADE=y kernel config to be enabled (true for the linux package). See [1] for details.

First, create the following folder structure:

$ mkdir -p kernel/firmware/acpi

Copy the fixed ACPI tables into the just created kernel/firmware/acpi folder, for example:

$ cp dsdt.aml ssdt1.aml kernel/firmware/acpi

Within the same folder where the newly created kernel/ folder resides, run:

$ find kernel | cpio -H newc --create > acpi_override

This creates the CPIO archive containing the fixed ACPI tables. Copy the archive to the boot directory.

# cp acpi_override /boot

Lastly, configure the bootloader to load your CPIO archive. For example, using Systemd-boot, /boot/loader/entries/arch.conf might look like this:

title	 Arch Linux
linux	 /vmlinuz-linux
initrd   /acpi_override
initrd	 /initramfs-linux.img
options  root=PARTUUID=ec9d5998-a9db-4bd8-8ea0-35a45df04701 resume=PARTUUID=58d0aa86-d39b-4fe1-81cf-45e7add275a0 ...

Now all that is left to do is to reboot and to verify the result.

Compiling into the kernel

You will want to be familiar with compiling your own kernel. The most straightforward way is with the "traditional" approach. After compiling DSDT, iasl produce two files: dsdt.hex and dsdt.aml.

Using menuconfig:

Disable "Select only drivers that do not need compile-time external firmware". Located in "Device Drivers -> Generic Driver Options".
Enable "Include Custom DSDT" and specify the absolute path of your fixed DSDT file (dsdt.hex, not dsdt.aml). Located in "Power management and ACPI options -> ACPI (Advanced Configuration and Power Interface) Support".

Verify successful override

Run dmesg | grep ACPI as root.
Look for clues that suggest an override, for example:

[    0.000000] ACPI: Override [DSDT-   A M I], this is unsafe: tainting kernel
[    0.000000] ACPI: DSDT 00000000be9b1190 Logical table override, new table: ffffffff81865af0
[    0.000000] ACPI: DSDT ffffffff81865af0 0BBA3 (v02 ALASKA    A M I 000000F3 INTL 20130517)