Overriding ACPI tables via initrd
Introduction (What is this about)
What is this for
How does it work
References (Where to retrieve userspace tools)
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.
- 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.
- 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.
- 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
