firmware: Add documentation for /sys/firmware/dmi

What:		/sys/firmware/dmi/

Date:		February 2011

Contact:	Mike Waychison <mikew@google.com>

Description:

		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 '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 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 position of the entry within the entirety

			  of the entirety.

		=== Entry Specialization ===

		Some entry types may have other information available in

		sysfs.

		--- 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 entries 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.