1What:		/sys/firmware/dmi/entries/
2Date:		February 2011
3Contact:	Mike Waychison <mikew@google.com>
4Description:
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.
9
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
13		without error either.
14
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 a firmware-provided
18		'handle' that is supposed to be unique amongst all
19		entries.
20
21		Some entries are required by the specification, but many
22		others are optional.  In general though, users should
23		never expect to find a specific entry type on their
24		system unless they know for certain what their firmware
25		is doing.  Machine to machine experiences will vary.
26
27		Multiple entries of the same type are allowed.  In order
28		to handle these duplicate entry types, each entry is
29		assigned by the operating system an 'instance', which is
30		derived from an entry type's ordinal position.  That is
31		to say, if there are 'N' multiple entries with the same type
32		'T' in the DMI tables (adjacent or spread apart, it
33		doesn't matter), they will be represented in sysfs as
34		entries "T-0" through "T-(N-1)":
35
36		Example entry directories::
37
38			/sys/firmware/dmi/entries/17-0
39			/sys/firmware/dmi/entries/17-1
40			/sys/firmware/dmi/entries/17-2
41			/sys/firmware/dmi/entries/17-3
42			...
43
44		Instance numbers are used in lieu of the firmware
45		assigned entry handles as the kernel itself makes no
46		guarantees that handles as exported are unique, and
47		there are likely firmware images that get this wrong in
48		the wild.
49
50		Each DMI entry in sysfs has the common header values
51		exported as attributes:
52
53		========  =================================================
54		handle	  The 16bit 'handle' that is assigned to this
55			  entry by the firmware.  This handle may be
56			  referred to by other entries.
57		length	  The length of the entry, as presented in the
58			  entry itself.  Note that this is _not the
59			  total count of bytes associated with the
60			  entry.  This value represents the length of
61			  the "formatted" portion of the entry.  This
62			  "formatted" region is sometimes followed by
63			  the "unformatted" region composed of nul
64			  terminated strings, with termination signalled
65			  by a two nul characters in series.
66		raw	  The raw bytes of the entry. This includes the
67			  "formatted" portion of the entry, the
68			  "unformatted" strings portion of the entry,
69			  and the two terminating nul characters.
70		type	  The type of the entry.  This value is the same
71			  as found in the directory name.  It indicates
72			  how the rest of the entry should be interpreted.
73		instance  The instance ordinal of the entry for the
74			  given type.  This value is the same as found
75			  in the parent directory name.
76		position  The ordinal position (zero-based) of the entry
77			  within the entirety of the DMI entry table.
78		========  =================================================
79
80		**Entry Specialization**
81
82		Some entry types may have other information available in
83		sysfs.  Not all types are specialized.
84
85		**Type 15 - System Event Log**
86
87		This entry allows the firmware to export a log of
88		events the system has taken.  This information is
89		typically backed by nvram, but the implementation
90		details are abstracted by this table.  This entry's data
91		is exported in the directory::
92
93		  /sys/firmware/dmi/entries/15-0/system_event_log
94
95		and has the following attributes (documented in the
96		SMBIOS / DMI specification under "System Event Log (Type 15)":
97
98		- area_length
99		- header_start_offset
100		- data_start_offset
101		- access_method
102		- status
103		- change_token
104		- access_method_address
105		- header_format
106		- per_log_type_descriptor_length
107		- type_descriptors_supported_count
108
109		As well, the kernel exports the binary attribute:
110
111		=============	  ====================================
112		raw_event_log	  The raw binary bits of the event log
113				  as described by the DMI entry.
114		=============	  ====================================
115