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

Documentation for those are missing at the EDAC description.

I guess we end by moving such descriptions in the past to the
ABI document (or only added it there), but it means that the
EDAC documentation is incomplete. So, add it there.
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/edac.txt

@@ -208,6 +208,126 @@ For a description of the sysfs API, please see:
 	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
 --------------------