driver-api: create an edac.rst file with EDAC documentation

Currently, there's no device driver documentation for the EDAC
subsystem at the driver-api book. Fill in the blanks for the
structures and functions that misses documentation, uniform
the word on the existing ones, and add a new edac.rst file at
driver-api, in order to document the EDAC subsystem.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/driver-api/edac.rst

+Error Detection And Correction (EDAC) Devices
+=============================================
+
+Memory Controllers
+------------------
+
+Most of the EDAC core is focused on doing Memory Controller error detection.
+The :c:func:`edac_mc_alloc`. It uses internally the struct ``mem_ctl_info``
+to describe the memory controllers, with is an opaque struct for the EDAC
+drivers. Only the EDAC core is allowed to touch it.
+
+.. kernel-doc:: include/linux/edac.h
+
+.. kernel-doc:: drivers/edac/edac_mc.h
+
+PCI Controllers
+---------------
+
+The EDAC subsystem provides a mechanism to handle PCI controllers by calling
+the :c:func:`edac_pci_alloc_ctl_info`. It will use the struct
+:c:type:`edac_pci_ctl_info` to describe the PCI controllers.
+
+.. kernel-doc:: drivers/edac/edac_pci.h
+
+EDAC Blocks
+-----------
+
+The EDAC subsystem also provides a generic mechanism to report errors on
+other parts of the hardware via :c:func:`edac_device_alloc_ctl_info` function.
+
+The structures :c:type:`edac_dev_sysfs_block_attribute`,
+:c:type:`edac_device_block`, :c:type:`edac_device_instance` and
+:c:type:`edac_device_ctl_info` provide a generic or abstract 'edac_device'
+representation at sysfs.
+
+This set of structures and the code that implements the APIs for the same, provide for registering EDAC type devices which are NOT standard memory or
+PCI, like:
+
+- CPU caches (L1 and L2)
+- DMA engines
+- Core CPU switches
+- Fabric switch units
+- PCIe interface controllers
+- other EDAC/ECC type devices that can be monitored for
+  errors, etc.
+
+It allows for a 2 level set of hierarchy.
+
+For example, a cache could be composed of L1, L2 and L3 levels of cache.
+Each CPU core would have its own L1 cache, while sharing L2 and maybe L3
+caches. On such case, those can be represented via the following sysfs
+nodes::
+
+	/sys/devices/system/edac/..
+
+	pci/		<existing pci directory (if available)>
+	mc/		<existing memory device directory>
+	cpu/cpu0/..	<L1 and L2 block directory>
+		/L1-cache/ce_count
+			 /ue_count
+		/L2-cache/ce_count
+			 /ue_count
+	cpu/cpu1/..	<L1 and L2 block directory>
+		/L1-cache/ce_count
+			 /ue_count
+		/L2-cache/ce_count
+			 /ue_count
+	...
+
+	the L1 and L2 directories would be "edac_device_block's"
+
+.. kernel-doc:: drivers/edac/edac_device.h

Documentation/driver-api/index.rst

@@ -26,6 +26,7 @@ available subsections can be seen below.
    spi
    i2c
    hsi
+   edac
    miscellaneous
    vme
    80211/index

drivers/edac/edac_mc.h

@@ -152,8 +152,6 @@ extern void edac_mc_free(struct mem_ctl_info *mci);
  *
  * If found, return a pointer to the structure.
  * Else return NULL.
- *
- * Caller must hold mem_ctls_mutex.
  */
 extern struct mem_ctl_info *edac_mc_find(int idx);