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114 lines
4.2 KiB
114 lines
4.2 KiB
What: /sys/firmware/dmi/entries/ |
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Date: February 2011 |
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Contact: Mike Waychison <[email protected]> |
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Description: |
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Many machines' firmware (x86 and ia64) export DMI / |
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SMBIOS tables to the operating system. Getting at this |
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information is often valuable to userland, especially in |
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cases where there are OEM extensions used. |
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The kernel itself does not rely on the majority of the |
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information in these tables being correct. It equally |
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cannot ensure that the data as exported to userland is |
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without error either. |
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DMI is structured as a large table of entries, where |
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each entry has a common header indicating the type and |
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length of the entry, as well as a firmware-provided |
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'handle' that is supposed to be unique amongst all |
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entries. |
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Some entries are required by the specification, but many |
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others are optional. In general though, users should |
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never expect to find a specific entry type on their |
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system unless they know for certain what their firmware |
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is doing. Machine to machine experiences will vary. |
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Multiple entries of the same type are allowed. In order |
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to handle these duplicate entry types, each entry is |
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assigned by the operating system an 'instance', which is |
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derived from an entry type's ordinal position. That is |
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to say, if there are 'N' multiple entries with the same type |
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'T' in the DMI tables (adjacent or spread apart, it |
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doesn't matter), they will be represented in sysfs as |
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entries "T-0" through "T-(N-1)": |
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Example entry directories:: |
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/sys/firmware/dmi/entries/17-0 |
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/sys/firmware/dmi/entries/17-1 |
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/sys/firmware/dmi/entries/17-2 |
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/sys/firmware/dmi/entries/17-3 |
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... |
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Instance numbers are used in lieu of the firmware |
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assigned entry handles as the kernel itself makes no |
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guarantees that handles as exported are unique, and |
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there are likely firmware images that get this wrong in |
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the wild. |
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Each DMI entry in sysfs has the common header values |
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exported as attributes: |
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======== ================================================= |
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handle The 16bit 'handle' that is assigned to this |
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entry by the firmware. This handle may be |
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referred to by other entries. |
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length The length of the entry, as presented in the |
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entry itself. Note that this is _not the |
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total count of bytes associated with the |
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entry. This value represents the length of |
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the "formatted" portion of the entry. This |
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"formatted" region is sometimes followed by |
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the "unformatted" region composed of nul |
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terminated strings, with termination signalled |
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by a two nul characters in series. |
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raw The raw bytes of the entry. This includes the |
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"formatted" portion of the entry, the |
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"unformatted" strings portion of the entry, |
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and the two terminating nul characters. |
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type The type of the entry. This value is the same |
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as found in the directory name. It indicates |
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how the rest of the entry should be interpreted. |
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instance The instance ordinal of the entry for the |
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given type. This value is the same as found |
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in the parent directory name. |
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position The ordinal position (zero-based) of the entry |
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within the entirety of the DMI entry table. |
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======== ================================================= |
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**Entry Specialization** |
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Some entry types may have other information available in |
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sysfs. Not all types are specialized. |
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**Type 15 - System Event Log** |
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This entry allows the firmware to export a log of |
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events the system has taken. This information is |
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typically backed by nvram, but the implementation |
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details are abstracted by this table. This entry's data |
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is exported in the directory:: |
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/sys/firmware/dmi/entries/15-0/system_event_log |
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and has the following attributes (documented in the |
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SMBIOS / DMI specification under "System Event Log (Type 15)": |
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- area_length |
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- header_start_offset |
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- data_start_offset |
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- access_method |
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- status |
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- change_token |
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- access_method_address |
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- header_format |
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- per_log_type_descriptor_length |
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- type_descriptors_supported_count |
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As well, the kernel exports the binary attribute: |
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============= ==================================== |
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raw_event_log The raw binary bits of the event log |
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as described by the DMI entry. |
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============= ====================================
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