0001 What: /sys/firmware/dmi/entries/
0002 Date: February 2011
0003 Contact: Mike Waychison <mikew@google.com>
0004 Description:
0005 Many machines' firmware (x86 and ia64) export DMI /
0006 SMBIOS tables to the operating system. Getting at this
0007 information is often valuable to userland, especially in
0008 cases where there are OEM extensions used.
0009
0010 The kernel itself does not rely on the majority of the
0011 information in these tables being correct. It equally
0012 cannot ensure that the data as exported to userland is
0013 without error either.
0014
0015 DMI is structured as a large table of entries, where
0016 each entry has a common header indicating the type and
0017 length of the entry, as well as a firmware-provided
0018 'handle' that is supposed to be unique amongst all
0019 entries.
0020
0021 Some entries are required by the specification, but many
0022 others are optional. In general though, users should
0023 never expect to find a specific entry type on their
0024 system unless they know for certain what their firmware
0025 is doing. Machine to machine experiences will vary.
0026
0027 Multiple entries of the same type are allowed. In order
0028 to handle these duplicate entry types, each entry is
0029 assigned by the operating system an 'instance', which is
0030 derived from an entry type's ordinal position. That is
0031 to say, if there are 'N' multiple entries with the same type
0032 'T' in the DMI tables (adjacent or spread apart, it
0033 doesn't matter), they will be represented in sysfs as
0034 entries "T-0" through "T-(N-1)":
0035
0036 Example entry directories::
0037
0038 /sys/firmware/dmi/entries/17-0
0039 /sys/firmware/dmi/entries/17-1
0040 /sys/firmware/dmi/entries/17-2
0041 /sys/firmware/dmi/entries/17-3
0042 ...
0043
0044 Instance numbers are used in lieu of the firmware
0045 assigned entry handles as the kernel itself makes no
0046 guarantees that handles as exported are unique, and
0047 there are likely firmware images that get this wrong in
0048 the wild.
0049
0050 Each DMI entry in sysfs has the common header values
0051 exported as attributes:
0052
0053 ======== =================================================
0054 handle The 16bit 'handle' that is assigned to this
0055 entry by the firmware. This handle may be
0056 referred to by other entries.
0057 length The length of the entry, as presented in the
0058 entry itself. Note that this is _not the
0059 total count of bytes associated with the
0060 entry. This value represents the length of
0061 the "formatted" portion of the entry. This
0062 "formatted" region is sometimes followed by
0063 the "unformatted" region composed of nul
0064 terminated strings, with termination signalled
0065 by a two nul characters in series.
0066 raw The raw bytes of the entry. This includes the
0067 "formatted" portion of the entry, the
0068 "unformatted" strings portion of the entry,
0069 and the two terminating nul characters.
0070 type The type of the entry. This value is the same
0071 as found in the directory name. It indicates
0072 how the rest of the entry should be interpreted.
0073 instance The instance ordinal of the entry for the
0074 given type. This value is the same as found
0075 in the parent directory name.
0076 position The ordinal position (zero-based) of the entry
0077 within the entirety of the DMI entry table.
0078 ======== =================================================
0079
0080 **Entry Specialization**
0081
0082 Some entry types may have other information available in
0083 sysfs. Not all types are specialized.
0084
0085 **Type 15 - System Event Log**
0086
0087 This entry allows the firmware to export a log of
0088 events the system has taken. This information is
0089 typically backed by nvram, but the implementation
0090 details are abstracted by this table. This entry's data
0091 is exported in the directory::
0092
0093 /sys/firmware/dmi/entries/15-0/system_event_log
0094
0095 and has the following attributes (documented in the
0096 SMBIOS / DMI specification under "System Event Log (Type 15)":
0097
0098 - area_length
0099 - header_start_offset
0100 - data_start_offset
0101 - access_method
0102 - status
0103 - change_token
0104 - access_method_address
0105 - header_format
0106 - per_log_type_descriptor_length
0107 - type_descriptors_supported_count
0108
0109 As well, the kernel exports the binary attribute:
0110
0111 ============= ====================================
0112 raw_event_log The raw binary bits of the event log
0113 as described by the DMI entry.
0114 ============= ====================================
