linux-vybrid_public/Documentation/acpi/dsdt-initrd.txt

ACPI Custom DSDT read from initramfs

2003 by Markus Gaugusch < dsdt at gaugusch dot at >
Special thanks go to Thomas Renninger from SuSE, who updated the patch for
2.6.0 and later modified it to read inside initramfs
2004 - 2008 maintained by Eric Piel < eric dot piel at tremplin-utc dot net >

This option is intended for people who would like to hack their DSDT and don't
want to recompile their kernel after every change. It can also be useful to
distros which offers pre-compiled kernels and want to allow their users to use
a modified DSDT. In the Kernel config, enable the initial RAM filesystem
support (in General Setup) and enable ACPI_CUSTOM_DSDT_INITRD at the ACPI
options (General Setup|ACPI Support|Read Custom DSDT from initramfs).

A custom DSDT (Differentiated System Description Table) is useful when your
computer uses ACPI but problems occur due to broken implementation. Typically,
your computer works but there are some troubles with the hardware detection or
the power management. You can check that troubles come from errors in the DSDT by
activating the ACPI debug option and reading the logs. This table is provided
by the BIOS, therefore it might be a good idea to check for BIOS update on your
vendor website before going any further. Errors are often caused by vendors
testing their hardware only with Windows or because there is code which is
executed only on a specific OS with a specific version and Linux hasn't been
considered during the development.

Before you run away from customising your DSDT, you should note that already
corrected tables are available for a fair amount of computers on this web-page:
http://acpi.sf.net/dsdt . Be careful though, to work correctly a DSDT has to
match closely the hardware, including the amount of RAM, the frequency of the
processor and the PCI cards present! If you are part of the unluckies who
cannot find their hardware in this database, you can modify your DSDT by
yourself. This process is less painful than it sounds. Download the Intel ASL
compiler/decompiler at http://www.intel.com/technology/IAPC/acpi/downloads.htm .
As root, you then have to dump your DSDT and decompile it. By using the
compiler messages as well as the kernel ACPI debug messages and the reference
book (available at the Intel website and also at http://www.acpi.info), it is
quite easy to obtain a fully working table.

Once your new DSDT is ready you'll have to add it to an initramfs so that the
kernel can read the table at the very beginning of the boot. As the file has to
be accessed very early during the boot process the initramfs has to be an
initramfs. The file is contained into the initramfs under the name /DSDT.aml .
To obtain such an initramfs, you might have to modify your initramfs script or
you can add it later to the initramfs with the script appended to this
document. The command will look like:
initramfs-add-dsdt initramfs.img my-dsdt.aml

In case you don't use any initramfs, the possibilities you have are to either
start using one (try mkinitrd or yaird), or use the "Include Custom DSDT"
configure option to directly include your DSDT inside the kernel.

The message "Looking for DSDT in initramfs..." will tell you if the DSDT was
found or not. If you need to update your DSDT, generate a new initramfs and
perform the steps above. Don't forget that with Lilo, you'll have to re-run it.


====================== Here starts initramfs-add-dsdt ==========================
#!/bin/bash
# Adds a DSDT file to the initrd (if it's an initramfs)
# first argument is the name of archive
# second argument is the name of the file to add
# The file will be copied as /DSDT.aml

# 20060126: fix "Premature end of file" with some old cpio (Roland Robic)
# 20060205: this time it should really work

# check the arguments
if [ $# -ne 2 ]; then
	program_name=$(basename $0)
	echo "\
$program_name: too few arguments
Usage: $program_name initrd-name.img DSDT-to-add.aml
Adds a DSDT file to an initrd (in initramfs format)

  initrd-name.img: filename of the initrd in initramfs format
  DSDT-to-add.aml: filename of the DSDT file to add
  " 1>&2
    exit 1
fi

# we should check it's an initramfs

tempcpio=$(mktemp -d)
# cleanup on exit, hangup, interrupt, quit, termination
trap 'rm -rf $tempcpio' 0 1 2 3 15

# extract the archive
gunzip -c "$1" > "$tempcpio"/initramfs.cpio || exit 1

# copy the DSDT file at the root of the directory so that we can call it "/DSDT.aml"
cp -f "$2" "$tempcpio"/DSDT.aml

# add the file
cd "$tempcpio"
(echo DSDT.aml | cpio --quiet -H newc -o -A -O "$tempcpio"/initramfs.cpio) || exit 1
cd "$OLDPWD"

# re-compress the archive
gzip -c "$tempcpio"/initramfs.cpio > "$1"