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Subject[PATCH 37/50] firmware: Add documentation for /sys/firmware/dmi
From: Mike Waychison <>

Document the new ABI added by the dmi-sysfs module.

Signed-off-by: Mike Waychison <>
Signed-off-by: Greg Kroah-Hartman <>
Documentation/ABI/testing/sysfs-firmware-dmi | 110 ++++++++++++++++++++++++++
1 files changed, 110 insertions(+), 0 deletions(-)
create mode 100644 Documentation/ABI/testing/sysfs-firmware-dmi

diff --git a/Documentation/ABI/testing/sysfs-firmware-dmi b/Documentation/ABI/testing/sysfs-firmware-dmi
new file mode 100644
index 0000000..ba9da95
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-firmware-dmi
@@ -0,0 +1,110 @@
+What: /sys/firmware/dmi/
+Date: February 2011
+Contact: Mike Waychison <>
+ Many machines' firmware (x86 and ia64) export DMI /
+ SMBIOS tables to the operating system. Getting at this
+ information is often valuable to userland, especially in
+ cases where there are OEM extensions used.
+ The kernel itself does not rely on the majority of the
+ information in these tables being correct. It equally
+ cannot ensure that the data as exported to userland is
+ without error either.
+ DMI is structured as a large table of entries, where
+ each entry has a common header indicating the type and
+ length of the entry, as well as 'handle' that is
+ supposed to be unique amongst all entries.
+ Some entries are required by the specification, but many
+ others are optional. In general though, users should
+ never expect to find a specific entry type on their
+ system unless they know for certain what their firmware
+ is doing. Machine to machine will vary.
+ Multiple entries of the same type are allowed. In order
+ to handle these duplicate entry types, each entry is
+ assigned by the operating system an 'instance', which is
+ derived from an entry type's ordinal position. That is
+ to say, if there are 'N' multiple entries with the same type
+ 'T' in the DMI tables (adjacent or spread apart, it
+ doesn't matter), they will be represented in sysfs as
+ entries "T-0" through "T-(N-1)":
+ Example entry directories:
+ /sys/firmware/dmi/entries/17-0
+ /sys/firmware/dmi/entries/17-1
+ /sys/firmware/dmi/entries/17-2
+ /sys/firmware/dmi/entries/17-3
+ ...
+ Instance numbers are used in lieu of the firmware
+ assigned entry handles as the kernel itself makes no
+ guarantees that handles as exported are unique, and
+ there are likely firmware images that get this wrong in
+ the wild.
+ Each DMI entry in sysfs has the common header values
+ exported as attributes:
+ handle : The 16bit 'handle' that is assigned to this
+ entry by the firmware. This handle may be
+ referred to by other entries.
+ length : The length of the entry, as presented in the
+ entry itself. Note that this is _not the
+ total count of bytes associated with the
+ entry_. This value represents the length of
+ the "formatted" portion of the entry. This
+ "formatted" region is sometimes followed by
+ the "unformatted" region composed of nul
+ terminated strings, with termination signalled
+ by a two nul characters in series.
+ raw : The raw bytes of the entry. This includes the
+ "formatted" portion of the entry, the
+ "unformatted" strings portion of the entry,
+ and the two terminating nul characters.
+ type : The type of the entry. This value is the same
+ as found in the directory name. It indicates
+ how the rest of the entry should be
+ interpreted.
+ instance: The instance ordinal of the entry for the
+ given type. This value is the same as found
+ in the parent directory name.
+ position: The position of the entry within the entirety
+ of the entirety.
+ === Entry Specialization ===
+ Some entry types may have other information available in
+ sysfs.
+ --- Type 15 - System Event Log ---
+ This entry allows the firmware to export a log of
+ events the system has taken. This information is
+ typically backed by nvram, but the implementation
+ details are abstracted by this table. This entries data
+ is exported in the directory:
+ /sys/firmware/dmi/entries/15-0/system_event_log
+ and has the following attributes (documented in the
+ SMBIOS / DMI specification under "System Event Log (Type 15)":
+ area_length
+ header_start_offset
+ data_start_offset
+ access_method
+ status
+ change_token
+ access_method_address
+ header_format
+ per_log_type_descriptor_length
+ type_descriptors_supported_count
+ As well, the kernel exports the binary attribute:
+ raw_event_log : The raw binary bits of the event log
+ as described by the DMI entry.

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