Documentation / acpi / initrd_table_override.txt

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Based on kernel version 3.9. Page generated on 2013-05-02 22:54 EST.

	Overriding ACPI tables via initrd
	=================================
	
	1) Introduction (What is this about)
	2) What is this for
	3) How does it work
	4) References (Where to retrieve userspace tools)
	
	1) What is this about
	---------------------
	
	If the ACPI_INITRD_TABLE_OVERRIDE compile option is true, it is possible to
	override nearly any ACPI table provided by the BIOS with an instrumented,
	modified one.
	
	For a full list of ACPI tables that can be overridden, take a look at
	the char *table_sigs[MAX_ACPI_SIGNATURE]; definition in drivers/acpi/osl.c
	All ACPI tables iasl (Intel's ACPI compiler and disassembler) knows should
	be overridable, except:
	   - ACPI_SIG_RSDP (has a signature of 6 bytes)
	   - ACPI_SIG_FACS (does not have an ordinary ACPI table header)
	Both could get implemented as well.
	
	
	2) What is this for
	-------------------
	
	Please keep in mind that this is a debug option.
	ACPI tables should not get overridden for productive use.
	If BIOS ACPI tables are overridden the kernel will get tainted with the
	TAINT_OVERRIDDEN_ACPI_TABLE flag.
	Complain to your platform/BIOS vendor if you find a bug which is so sever
	that a workaround is not accepted in the Linux kernel.
	
	Still, it can and should be enabled in any kernel, because:
	  - There is no functional change with not instrumented initrds
	  - It provides a powerful feature to easily debug and test ACPI BIOS table
	    compatibility with the Linux kernel.
	
	
	3) How does it work
	-------------------
	
	# Extract the machine's ACPI tables:
	cd /tmp
	acpidump >acpidump
	acpixtract -a acpidump
	# Disassemble, modify and recompile them:
	iasl -d *.dat
	# For example add this statement into a _PRT (PCI Routing Table) function
	# of the DSDT:
	Store("HELLO WORLD", debug)
	iasl -sa dsdt.dsl
	# Add the raw ACPI tables to an uncompressed cpio archive.
	# They must be put into a /kernel/firmware/acpi directory inside the
	# cpio archive.
	# The uncompressed cpio archive must be the first.
	# Other, typically compressed cpio archives, must be
	# concatenated on top of the uncompressed one.
	mkdir -p kernel/firmware/acpi
	cp dsdt.aml kernel/firmware/acpi
	# A maximum of: #define ACPI_OVERRIDE_TABLES 10
	# tables are  currently allowed (see osl.c):
	iasl -sa facp.dsl
	iasl -sa ssdt1.dsl
	cp facp.aml kernel/firmware/acpi
	cp ssdt1.aml kernel/firmware/acpi
	# Create the uncompressed cpio archive and concatenate the original initrd
	# on top:
	find kernel | cpio -H newc --create > /boot/instrumented_initrd
	cat /boot/initrd >>/boot/instrumented_initrd
	# reboot with increased acpi debug level, e.g. boot params:
	acpi.debug_level=0x2 acpi.debug_layer=0xFFFFFFFF
	# and check your syslog:
	[    1.268089] ACPI: PCI Interrupt Routing Table [\_SB_.PCI0._PRT]
	[    1.272091] [ACPI Debug]  String [0x0B] "HELLO WORLD"
	
	iasl is able to disassemble and recompile quite a lot different,
	also static ACPI tables.
	
	
	4) Where to retrieve userspace tools
	------------------------------------
	
	iasl and acpixtract are part of Intel's ACPICA project:
	http://acpica.org/
	and should be packaged by distributions (for example in the acpica package
	on SUSE).
	
	acpidump can be found in Len Browns pmtools:
	ftp://kernel.org/pub/linux/kernel/people/lenb/acpi/utils/pmtools/acpidump
	This tool is also part of the acpica package on SUSE.
	Alternatively, used ACPI tables can be retrieved via sysfs in latest kernels:
	/sys/firmware/acpi/tables

• [ acpi ]
• [ apei ]
• debug.txt
• dsdt-override.txt
• enumeration.txt
• initrd_table_override.txt
• method-customizing.txt
• method-tracing.txt
• scan_handlers.txt
•