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-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 a firmware-provided
- '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 experiences 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 ordinal position (zero-based) of the entry
- within the entirety of the DMI entry table.
- === Entry Specialization ===
- Some entry types may have other information available in
- sysfs. Not all types are specialized.
- --- 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 entry's 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.