Based on kernel version 4.0. Page generated on 2015-04-14 21:19 EST.
1 What: /sys/firmware/dmi/ 2 Date: February 2011 3 Contact: Mike Waychison <email@example.com> 4 Description: 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 handle : The 16bit 'handle' that is assigned to this 54 entry by the firmware. This handle may be 55 referred to by other entries. 56 length : The length of the entry, as presented in the 57 entry itself. Note that this is _not the 58 total count of bytes associated with the 59 entry_. This value represents the length of 60 the "formatted" portion of the entry. This 61 "formatted" region is sometimes followed by 62 the "unformatted" region composed of nul 63 terminated strings, with termination signalled 64 by a two nul characters in series. 65 raw : The raw bytes of the entry. This includes the 66 "formatted" portion of the entry, the 67 "unformatted" strings portion of the entry, 68 and the two terminating nul characters. 69 type : The type of the entry. This value is the same 70 as found in the directory name. It indicates 71 how the rest of the entry should be interpreted. 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 ordinal position (zero-based) of the entry 76 within the entirety of the DMI entry table. 77 78 === Entry Specialization === 79 80 Some entry types may have other information available in 81 sysfs. Not all types are specialized. 82 83 --- Type 15 - System Event Log --- 84 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 entry's data 89 is exported in the directory: 90 91 /sys/firmware/dmi/entries/15-0/system_event_log 92 93 and has the following attributes (documented in the 94 SMBIOS / DMI specification under "System Event Log (Type 15)": 95 96 area_length 97 header_start_offset 98 data_start_offset 99 access_method 100 status 101 change_token 102 access_method_address 103 header_format 104 per_log_type_descriptor_length 105 type_descriptors_supported_count 106 107 As well, the kernel exports the binary attribute: 108 109 raw_event_log : The raw binary bits of the event log 110 as described by the DMI entry.