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+.. SPDX-License-Identifier: GPL-2.0
+
+================================
+Upgrading ACPI tables via initrd
+================================
+
+What is this about
+==================
+
+If the ACPI_TABLE_UPGRADE compile option is true, it is possible to
+upgrade the ACPI execution environment that is defined by the ACPI tables
+via upgrading the ACPI tables provided by the BIOS with an instrumented,
+modified, more recent version one, or installing brand new ACPI tables.
+
+When building initrd with kernel in a single image, option
+ACPI_TABLE_OVERRIDE_VIA_BUILTIN_INITRD should also be true for this
+feature to work.
+
+For a full list of ACPI tables that can be upgraded/installed, take a look
+at the char `*table_sigs[MAX_ACPI_SIGNATURE];` definition in
+drivers/acpi/tables.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.
+
+
+What is this for
+================
+
+Complain to your platform/BIOS vendor if you find a bug which is so severe
+that a workaround is not accepted in the Linux kernel. And this facility
+allows you to upgrade the buggy tables before your platform/BIOS vendor
+releases an upgraded BIOS binary.
+
+This facility can be used by platform/BIOS vendors to provide a Linux
+compatible environment without modifying the underlying platform firmware.
+
+This facility also provides a powerful feature to easily debug and test
+ACPI BIOS table compatibility with the Linux kernel by modifying old
+platform provided ACPI tables or inserting new ACPI tables.
+
+It can and should be enabled in any kernel because there is no functional
+change with not instrumented initrds.
+
+
+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)
+  # And increase the OEM Revision. For example, before modification:
+  DefinitionBlock ("DSDT.aml", "DSDT", 2, "INTEL ", "TEMPLATE", 0x00000000)
+  # After modification:
+  DefinitionBlock ("DSDT.aml", "DSDT", 2, "INTEL ", "TEMPLATE", 0x00000001)
+  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. Note that if the table put here matches a platform table
+  # (similar Table Signature, and similar OEMID, and similar OEM Table ID)
+  # with a more recent OEM Revision, the platform table will be upgraded by
+  # this table. If the table put here doesn't match a platform table
+  # (dissimilar Table Signature, or dissimilar OEMID, or dissimilar OEM Table
+  # ID), this table will be appended.
+  mkdir -p kernel/firmware/acpi
+  cp dsdt.aml kernel/firmware/acpi
+  # A maximum of "NR_ACPI_INITRD_TABLES (64)" 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
+  # The uncompressed cpio archive must be the first. Other, typically
+  # compressed cpio archives, must be concatenated on top of the uncompressed
+  # one. Following command creates the uncompressed cpio archive and
+  # concatenates 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.
+
+
+Where to retrieve userspace tools
+=================================
+
+iasl and acpixtract are part of Intel's ACPICA project:
+https://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