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