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@@ -0,0 +1,94 @@
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+Overriding ACPI tables via initrd
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+=================================
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+
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+1) Introduction (What is this about)
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+2) What is this for
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+3) How does it work
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+4) References (Where to retrieve userspace tools)
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+
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+1) What is this about
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+---------------------
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+
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+If the ACPI_INITRD_TABLE_OVERRIDE compile option is true, it is possible to
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+override nearly any ACPI table provided by the BIOS with an instrumented,
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+modified one.
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+
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+For a full list of ACPI tables that can be overridden, take a look at
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+the char *table_sigs[MAX_ACPI_SIGNATURE]; definition in drivers/acpi/osl.c
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+All ACPI tables iasl (Intel's ACPI compiler and disassembler) knows should
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+be overridable, except:
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+ - ACPI_SIG_RSDP (has a signature of 6 bytes)
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+ - ACPI_SIG_FACS (does not have an ordinary ACPI table header)
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+Both could get implemented as well.
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+
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+
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+2) What is this for
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+-------------------
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+
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+Please keep in mind that this is a debug option.
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+ACPI tables should not get overridden for productive use.
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+If BIOS ACPI tables are overridden the kernel will get tainted with the
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+TAINT_OVERRIDDEN_ACPI_TABLE flag.
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+Complain to your platform/BIOS vendor if you find a bug which is so sever
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+that a workaround is not accepted in the Linux kernel.
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+
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+Still, it can and should be enabled in any kernel, because:
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+ - There is no functional change with not instrumented initrds
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+ - It provides a powerful feature to easily debug and test ACPI BIOS table
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+ compatibility with the Linux kernel.
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+
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+
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+3) How does it work
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+-------------------
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+
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+# Extract the machine's ACPI tables:
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+cd /tmp
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+acpidump >acpidump
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+acpixtract -a acpidump
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+# Disassemble, modify and recompile them:
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+iasl -d *.dat
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+# For example add this statement into a _PRT (PCI Routing Table) function
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+# of the DSDT:
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+Store("HELLO WORLD", debug)
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+iasl -sa dsdt.dsl
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+# Add the raw ACPI tables to an uncompressed cpio archive.
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+# They must be put into a /kernel/firmware/acpi directory inside the
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+# cpio archive.
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+# The uncompressed cpio archive must be the first.
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+# Other, typically compressed cpio archives, must be
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+# concatenated on top of the uncompressed one.
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+mkdir -p kernel/firmware/acpi
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+cp dsdt.aml kernel/firmware/acpi
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+# A maximum of: #define ACPI_OVERRIDE_TABLES 10
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+# tables are currently allowed (see osl.c):
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+iasl -sa facp.dsl
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+iasl -sa ssdt1.dsl
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+cp facp.aml kernel/firmware/acpi
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+cp ssdt1.aml kernel/firmware/acpi
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+# Create the uncompressed cpio archive and concatenate the original initrd
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+# on top:
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+find kernel | cpio -H newc --create > /boot/instrumented_initrd
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+cat /boot/initrd >>/boot/instrumented_initrd
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+# reboot with increased acpi debug level, e.g. boot params:
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+acpi.debug_level=0x2 acpi.debug_layer=0xFFFFFFFF
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+# and check your syslog:
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+[ 1.268089] ACPI: PCI Interrupt Routing Table [\_SB_.PCI0._PRT]
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+[ 1.272091] [ACPI Debug] String [0x0B] "HELLO WORLD"
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+
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+iasl is able to disassemble and recompile quite a lot different,
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+also static ACPI tables.
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+
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+
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+4) Where to retrieve userspace tools
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+------------------------------------
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+
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+iasl and acpixtract are part of Intel's ACPICA project:
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+http://acpica.org/
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+and should be packaged by distributions (for example in the acpica package
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+on SUSE).
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+
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+acpidump can be found in Len Browns pmtools:
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+ftp://kernel.org/pub/linux/kernel/people/lenb/acpi/utils/pmtools/acpidump
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+This tool is also part of the acpica package on SUSE.
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+Alternatively, used ACPI tables can be retrieved via sysfs in latest kernels:
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+/sys/firmware/acpi/tables
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Thomas Renninger