path: root/Documentation/acpi
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authorLinus Torvalds <>2012-12-14 10:03:23 -0800
committerLinus Torvalds <>2012-12-14 10:03:23 -0800
commit18dd0bf22b6f0c1bd5e4e813a42245ed86ec57b6 (patch)
treea4794b6041b44fa94f5d9438cdb5bbfc58b35f05 /Documentation/acpi
parent2d9c8b5d6a5f5f7a6111cc68a050b5b44729376b (diff)
parent385ddeac7ed99cf7dc62d76274d55fbd7cae1b5a (diff)
Merge branch 'x86-acpi-for-linus' of git://
Pull x86 ACPI update from Peter Anvin: "This is a patchset which didn't make the last merge window. It adds a debugging capability to feed ACPI tables via the initramfs. On a grander scope, it formalizes using the initramfs protocol for feeding arbitrary blobs which need to be accessed early to the kernel: they are fed first in the initramfs blob (lots of bootloaders can concatenate this at boot time, others can use a single file) in an uncompressed cpio archive using filenames starting with "kernel/". The ACPI maintainers requested that this patchset be fed via the x86 tree rather than the ACPI tree as the footprint in the general x86 code is much bigger than in the ACPI code proper." * 'x86-acpi-for-linus' of git:// X86 ACPI: Use #ifdef not #if for CONFIG_X86 check ACPI: Fix build when disabled ACPI: Document ACPI table overriding via initrd ACPI: Create acpi_table_taint() function to avoid code duplication ACPI: Implement physical address table override ACPI: Store valid ACPI tables passed via early initrd in reserved memblock areas x86, acpi: Introduce x86 arch specific arch_reserve_mem_area() for e820 handling lib: Add early cpio decoder
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+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
+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:
+and should be packaged by distributions (for example in the acpica package
+on SUSE).
+acpidump can be found in Len Browns pmtools:
+This tool is also part of the acpica package on SUSE.
+Alternatively, used ACPI tables can be retrieved via sysfs in latest kernels: