1 What: /sys/firmware/dmi/
3 Contact: Mike Waychison <mikew@google.com>
5 Many machines' firmware (x86 and ia64) export DMI /
6 SMBIOS tables to the operating system. Getting at this
7 information is often valuable to userland, especially in
8 cases where there are OEM extensions used.
10 The kernel itself does not rely on the majority of the
11 information in these tables being correct. It equally
12 cannot ensure that the data as exported to userland is
15 DMI is structured as a large table of entries, where
16 each entry has a common header indicating the type and
17 length of the entry, as well as 'handle' that is
18 supposed to be unique amongst all entries.
20 Some entries are required by the specification, but many
21 others are optional. In general though, users should
22 never expect to find a specific entry type on their
23 system unless they know for certain what their firmware
24 is doing. Machine to machine will vary.
26 Multiple entries of the same type are allowed. In order
27 to handle these duplicate entry types, each entry is
28 assigned by the operating system an 'instance', which is
29 derived from an entry type's ordinal position. That is
30 to say, if there are 'N' multiple entries with the same type
31 'T' in the DMI tables (adjacent or spread apart, it
32 doesn't matter), they will be represented in sysfs as
33 entries "T-0" through "T-(N-1)":
35 Example entry directories:
37 /sys/firmware/dmi/entries/17-0
38 /sys/firmware/dmi/entries/17-1
39 /sys/firmware/dmi/entries/17-2
40 /sys/firmware/dmi/entries/17-3
43 Instance numbers are used in lieu of the firmware
44 assigned entry handles as the kernel itself makes no
45 guarantees that handles as exported are unique, and
46 there are likely firmware images that get this wrong in
49 Each DMI entry in sysfs has the common header values
50 exported as attributes:
52 handle : The 16bit 'handle' that is assigned to this
53 entry by the firmware. This handle may be
54 referred to by other entries.
55 length : The length of the entry, as presented in the
56 entry itself. Note that this is _not the
57 total count of bytes associated with the
58 entry_. This value represents the length of
59 the "formatted" portion of the entry. This
60 "formatted" region is sometimes followed by
61 the "unformatted" region composed of nul
62 terminated strings, with termination signalled
63 by a two nul characters in series.
64 raw : The raw bytes of the entry. This includes the
65 "formatted" portion of the entry, the
66 "unformatted" strings portion of the entry,
67 and the two terminating nul characters.
68 type : The type of the entry. This value is the same
69 as found in the directory name. It indicates
70 how the rest of the entry should be
72 instance: The instance ordinal of the entry for the
73 given type. This value is the same as found
74 in the parent directory name.
75 position: The position of the entry within the entirety
78 === Entry Specialization ===
80 Some entry types may have other information available in
83 --- Type 15 - System Event Log ---
85 This entry allows the firmware to export a log of
86 events the system has taken. This information is
87 typically backed by nvram, but the implementation
88 details are abstracted by this table. This entries data
89 is exported in the directory:
91 /sys/firmware/dmi/entries/15-0/system_event_log
93 and has the following attributes (documented in the
94 SMBIOS / DMI specification under "System Event Log (Type 15)":
102 access_method_address
104 per_log_type_descriptor_length
105 type_descriptors_supported_count
107 As well, the kernel exports the binary attribute:
109 raw_event_log : The raw binary bits of the event log
110 as described by the DMI entry.