edac.txt: add a section explaining the dimmX and rankX directories

	Documentation/ABI/testing/sysfs-devices-edac

``dimmX`` or ``rankX`` directories

----------------------------------

The recommended way to use the EDAC subsystem is to look at the information

provided by the ``dimmX`` or ``rankX`` directories [#f5]_.

A typical EDAC system has the following structure under

``/sys/devices/system/edac/``\ [#f6]_::

	/sys/devices/system/edac/

	├── mc

	│   ├── mc0

	│   │   ├── ce_count

	│   │   ├── ce_noinfo_count

	│   │   ├── dimm0

	│   │   │   ├── dimm_dev_type

	│   │   │   ├── dimm_edac_mode

	│   │   │   ├── dimm_label

	│   │   │   ├── dimm_location

	│   │   │   ├── dimm_mem_type

	│   │   │   ├── size

	│   │   │   └── uevent

	│   │   ├── max_location

	│   │   ├── mc_name

	│   │   ├── reset_counters

	│   │   ├── seconds_since_reset

	│   │   ├── size_mb

	│   │   ├── ue_count

	│   │   ├── ue_noinfo_count

	│   │   └── uevent

	│   ├── mc1

	│   │   ├── ce_count

	│   │   ├── ce_noinfo_count

	│   │   ├── dimm0

	│   │   │   ├── dimm_dev_type

	│   │   │   ├── dimm_edac_mode

	│   │   │   ├── dimm_label

	│   │   │   ├── dimm_location

	│   │   │   ├── dimm_mem_type

	│   │   │   ├── size

	│   │   │   └── uevent

	│   │   ├── max_location

	│   │   ├── mc_name

	│   │   ├── reset_counters

	│   │   ├── seconds_since_reset

	│   │   ├── size_mb

	│   │   ├── ue_count

	│   │   ├── ue_noinfo_count

	│   │   └── uevent

	│   └── uevent

	└── uevent

In the ``dimmX`` directories are EDAC control and attribute files for

this ``X`` memory module:

- ``size`` - Total memory managed by this csrow attribute file

	This attribute file displays, in count of megabytes, the memory

	that this csrow contains.

- ``dimm_dev_type``  - Device type attribute file

	This attribute file will display what type of DRAM device is

	being utilized on this DIMM.

	Examples:

		- x1

		- x2

		- x4

		- x8

- ``dimm_edac_mode`` - EDAC Mode of operation attribute file

	This attribute file will display what type of Error detection

	and correction is being utilized.

- ``dimm_label`` - memory module label control file

	This control file allows this DIMM to have a label assigned

	to it. With this label in the module, when errors occur

	the output can provide the DIMM label in the system log.

	This becomes vital for panic events to isolate the

	cause of the UE event.

	DIMM Labels must be assigned after booting, with information

	that correctly identifies the physical slot with its

	silk screen label. This information is currently very

	motherboard specific and determination of this information

	must occur in userland at this time.

- ``dimm_location`` - location of the memory module

	The location can have up to 3 levels, and describe how the

	memory controller identifies the location of a memory module.

	Depending on the type of memory and memory controller, it

	can be:

		- *csrow* and *channel* - used when the memory controller

		  doesn't identify a single DIMM - e. g. in ``rankX`` dir;

		- *branch*, *channel*, *slot* - typically used on FB-DIMM memory

		  controllers;

		- *channel*, *slot* - used on Nehalem and newer Intel drivers.

- ``dimm_mem_type`` - Memory Type attribute file

	This attribute file will display what type of memory is currently

	on this csrow. Normally, either buffered or unbuffered memory.

	Examples:

		- Registered-DDR

		- Unbuffered-DDR

.. [#f5] On some systems, the memory controller doesn't have any logic

  to identify the memory module. On such systems, the directory is called ``rankX`` and works on a similar way as the ``csrowX`` directories.

  On modern Intel memory controllers, the memory controller identifies the

  memory modules directly. On such systems, the directory is called ``dimmX``.

.. [#f6] There are also some ``power`` directories and ``subsystem``

  symlinks inside the sysfs mapping that are automatically created by

  the sysfs subsystem. Currently, they serve no purpose.

'csrowX' directories

--------------------