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

Linux supports two methods of overriding the BIOS DSDT:

CONFIG_ACPI_CUSTOM_DSDT builds the image into the kernel.

CONFIG_ACPI_CUSTOM_DSDT_INITRD adds the image to the initrd.

When to use these methods is described in detail on the
Linux/ACPI home page:
http://www.lesswatts.org/projects/acpi/overridingDSDT.php

Note that if both options are used, the DSDT supplied
by the INITRD method takes precedence.

Documentation/initramfs-add-dsdt.sh is provided for convenience
for use with the CONFIG_ACPI_CUSTOM_DSDT_INITRD method.