1.. SPDX-License-Identifier: GPL-2.0
2
3================================
4Upgrading ACPI tables via initrd
5================================
6
7What is this about
8==================
9
10If the ACPI_TABLE_UPGRADE compile option is true, it is possible to
11upgrade the ACPI execution environment that is defined by the ACPI tables
12via upgrading the ACPI tables provided by the BIOS with an instrumented,
13modified, more recent version one, or installing brand new ACPI tables.
14
15When building initrd with kernel in a single image, option
16ACPI_TABLE_OVERRIDE_VIA_BUILTIN_INITRD should also be true for this
17feature to work.
18
19For a full list of ACPI tables that can be upgraded/installed, take a look
20at the char `*table_sigs[MAX_ACPI_SIGNATURE];` definition in
21drivers/acpi/tables.c.
22
23All ACPI tables iasl (Intel's ACPI compiler and disassembler) knows should
24be overridable, except:
25
26  - ACPI_SIG_RSDP (has a signature of 6 bytes)
27  - ACPI_SIG_FACS (does not have an ordinary ACPI table header)
28
29Both could get implemented as well.
30
31
32What is this for
33================
34
35Complain to your platform/BIOS vendor if you find a bug which is so severe
36that a workaround is not accepted in the Linux kernel. And this facility
37allows you to upgrade the buggy tables before your platform/BIOS vendor
38releases an upgraded BIOS binary.
39
40This facility can be used by platform/BIOS vendors to provide a Linux
41compatible environment without modifying the underlying platform firmware.
42
43This facility also provides a powerful feature to easily debug and test
44ACPI BIOS table compatibility with the Linux kernel by modifying old
45platform provided ACPI tables or inserting new ACPI tables.
46
47It can and should be enabled in any kernel because there is no functional
48change with not instrumented initrds.
49
50
51How does it work
52================
53::
54
55  # Extract the machine's ACPI tables:
56  cd /tmp
57  acpidump >acpidump
58  acpixtract -a acpidump
59  # Disassemble, modify and recompile them:
60  iasl -d *.dat
61  # For example add this statement into a _PRT (PCI Routing Table) function
62  # of the DSDT:
63  Store("HELLO WORLD", debug)
64  # And increase the OEM Revision. For example, before modification:
65  DefinitionBlock ("DSDT.aml", "DSDT", 2, "INTEL ", "TEMPLATE", 0x00000000)
66  # After modification:
67  DefinitionBlock ("DSDT.aml", "DSDT", 2, "INTEL ", "TEMPLATE", 0x00000001)
68  iasl -sa dsdt.dsl
69  # Add the raw ACPI tables to an uncompressed cpio archive.
70  # They must be put into a /kernel/firmware/acpi directory inside the cpio
71  # archive. Note that if the table put here matches a platform table
72  # (similar Table Signature, and similar OEMID, and similar OEM Table ID)
73  # with a more recent OEM Revision, the platform table will be upgraded by
74  # this table. If the table put here doesn't match a platform table
75  # (dissimilar Table Signature, or dissimilar OEMID, or dissimilar OEM Table
76  # ID), this table will be appended.
77  mkdir -p kernel/firmware/acpi
78  cp dsdt.aml kernel/firmware/acpi
79  # A maximum of "NR_ACPI_INITRD_TABLES (64)" tables are currently allowed
80  # (see osl.c):
81  iasl -sa facp.dsl
82  iasl -sa ssdt1.dsl
83  cp facp.aml kernel/firmware/acpi
84  cp ssdt1.aml kernel/firmware/acpi
85  # The uncompressed cpio archive must be the first. Other, typically
86  # compressed cpio archives, must be concatenated on top of the uncompressed
87  # one. Following command creates the uncompressed cpio archive and
88  # concatenates the original initrd on top:
89  find kernel | cpio -H newc --create > /boot/instrumented_initrd
90  cat /boot/initrd >>/boot/instrumented_initrd
91  # reboot with increased acpi debug level, e.g. boot params:
92  acpi.debug_level=0x2 acpi.debug_layer=0xFFFFFFFF
93  # and check your syslog:
94  [    1.268089] ACPI: PCI Interrupt Routing Table [\_SB_.PCI0._PRT]
95  [    1.272091] [ACPI Debug]  String [0x0B] "HELLO WORLD"
96
97iasl is able to disassemble and recompile quite a lot different,
98also static ACPI tables.
99
100
101Where to retrieve userspace tools
102=================================
103
104iasl and acpixtract are part of Intel's ACPICA project:
105https://acpica.org/
106
107and should be packaged by distributions (for example in the acpica package
108on SUSE).
109
110acpidump can be found in Len Browns pmtools:
111ftp://kernel.org/pub/linux/kernel/people/lenb/acpi/utils/pmtools/acpidump
112
113This tool is also part of the acpica package on SUSE.
114Alternatively, used ACPI tables can be retrieved via sysfs in latest kernels:
115/sys/firmware/acpi/tables
116