acpi: nfit: document sysfs interface

This is an attempt to document the nfit sysfs interface. The
descriptions have been collected from git commit logs and the ACPI
specification 6.2.
Signed-off-by: NAishwarya Pant <aishpant@gmail.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/ABI/testing/sysfs-bus-nfit

+For all of the nmem device attributes under nfit/*, see the 'NVDIMM Firmware
+Interface Table (NFIT)' section in the ACPI specification
+(http://www.uefi.org/specifications) for more details.
+
+What:		/sys/bus/nd/devices/nmemX/nfit/serial
+Date:		Jun, 2015
+KernelVersion:	v4.2
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) Serial number of the NVDIMM (non-volatile dual in-line
+		memory module), assigned by the module vendor.
+
+
+What:		/sys/bus/nd/devices/nmemX/nfit/handle
+Date:		Apr, 2015
+KernelVersion:	v4.2
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) The address (given by the _ADR object) of the device on its
+		parent bus of the NVDIMM device containing the NVDIMM region.
+
+
+What:		/sys/bus/nd/devices/nmemX/nfit/device
+Date:		Apr, 2015
+KernelVersion:	v4.1
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) Device id for the NVDIMM, assigned by the module vendor.
+
+
+What:		/sys/bus/nd/devices/nmemX/nfit/rev_id
+Date:		Jun, 2015
+KernelVersion:	v4.2
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) Revision of the NVDIMM, assigned by the module vendor.
+
+
+What:		/sys/bus/nd/devices/nmemX/nfit/phys_id
+Date:		Apr, 2015
+KernelVersion:	v4.2
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) Handle (i.e., instance number) for the SMBIOS (system
+		management BIOS) Memory Device structure describing the NVDIMM
+		containing the NVDIMM region.
+
+
+What:		/sys/bus/nd/devices/nmemX/nfit/flags
+Date:		Jun, 2015
+KernelVersion:	v4.2
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) The flags in the NFIT memory device sub-structure indicate
+		the state of the data on the nvdimm relative to its energy
+		source or last "flush to persistence".
+
+		The attribute is a translation of the 'NVDIMM State Flags' field
+		in section 5.2.25.3 'NVDIMM Region Mapping' Structure of the
+		ACPI specification 6.2.
+
+		The health states are "save_fail", "restore_fail", "flush_fail",
+		"not_armed", "smart_event", "map_fail" and "smart_notify".
+
+
+What:		/sys/bus/nd/devices/nmemX/nfit/format
+What:		/sys/bus/nd/devices/nmemX/nfit/format1
+What:		/sys/bus/nd/devices/nmemX/nfit/formats
+Date:		Apr, 2016
+KernelVersion:	v4.7
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) The interface codes indicate support for persistent memory
+		mapped directly into system physical address space and / or a
+		block aperture access mechanism to the NVDIMM media.
+		The 'formats' attribute displays the number of supported
+		interfaces.
+
+		This layout is compatible with existing libndctl binaries that
+		only expect one code per-dimm as they will ignore
+		nmemX/nfit/formats and nmemX/nfit/formatN.
+
+
+What:		/sys/bus/nd/devices/nmemX/nfit/vendor
+Date:		Apr, 2016
+KernelVersion:	v4.7
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) Vendor id of the NVDIMM.
+
+
+What:		/sys/bus/nd/devices/nmemX/nfit/dsm_mask
+Date:		May, 2016
+KernelVersion:	v4.7
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) The bitmask indicates the supported device specific control
+		functions relative to the NVDIMM command family supported by the
+		device
+
+
+What:		/sys/bus/nd/devices/nmemX/nfit/family
+Date:		Apr, 2016
+KernelVersion:	v4.7
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) Displays the NVDIMM family command sets. Values
+		0, 1, 2 and 3 correspond to NVDIMM_FAMILY_INTEL,
+		NVDIMM_FAMILY_HPE1, NVDIMM_FAMILY_HPE2 and NVDIMM_FAMILY_MSFT
+		respectively.
+
+		See the specifications for these command families here:
+		http://pmem.io/documents/NVDIMM_DSM_Interface-V1.6.pdf
+		https://github.com/HewlettPackard/hpe-nvm/blob/master/Documentation/
+		https://msdn.microsoft.com/library/windows/hardware/mt604741"
+
+
+What:		/sys/bus/nd/devices/nmemX/nfit/id
+Date:		Apr, 2016
+KernelVersion:	v4.7
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) ACPI specification 6.2 section 5.2.25.9, defines an
+		identifier for an NVDIMM, which refelects the id attribute.
+
+
+What:		/sys/bus/nd/devices/nmemX/nfit/subsystem_vendor
+Date:		Apr, 2016
+KernelVersion:	v4.7
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) Sub-system vendor id of the NVDIMM non-volatile memory
+		subsystem controller.
+
+
+What:		/sys/bus/nd/devices/nmemX/nfit/subsystem_rev_id
+Date:		Apr, 2016
+KernelVersion:	v4.7
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) Sub-system revision id of the NVDIMM non-volatile memory subsystem
+		controller, assigned by the non-volatile memory subsystem
+		controller vendor.
+
+
+What:		/sys/bus/nd/devices/nmemX/nfit/subsystem_device
+Date:		Apr, 2016
+KernelVersion:	v4.7
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) Sub-system device id for the NVDIMM non-volatile memory
+		subsystem controller, assigned by the non-volatile memory
+		subsystem controller vendor.
+
+
+What:		/sys/bus/nd/devices/ndbusX/nfit/revision
+Date:		Jun, 2015
+KernelVersion:	v4.2
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) ACPI NFIT table revision number.
+
+
+What:		/sys/bus/nd/devices/ndbusX/nfit/scrub
+Date:		Sep, 2016
+KernelVersion:	v4.9
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RW) This shows the number of full Address Range Scrubs (ARS)
+		that have been completed since driver load time. Userspace can
+		wait on this using select/poll etc. A '+' at the end indicates
+		an ARS is in progress
+
+		Writing a value of 1 triggers an ARS scan.
+
+
+What:		/sys/bus/nd/devices/ndbusX/nfit/hw_error_scrub
+Date:		Sep, 2016
+KernelVersion:	v4.9
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RW) Provides a way to toggle the behavior between just adding
+		the address (cache line) where the MCE happened to the poison
+		list and doing a full scrub. The former (selective insertion of
+		the address) is done unconditionally.
+
+		This attribute can have the following values written to it:
+
+		'0': Switch to the default mode where an exception will only
+		insert the address of the memory error into the poison and
+		badblocks lists.
+		'1': Enable a full scrub to happen if an exception for a memory
+		error is received.
+
+
+What:		/sys/bus/nd/devices/ndbusX/nfit/dsm_mask
+Date:		Jun, 2017
+KernelVersion:	v4.13
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) The bitmask indicates the supported bus specific control
+		functions. See the section named 'NVDIMM Root Device _DSMs' in
+		the ACPI specification.
+
+
+What:		/sys/bus/nd/devices/regionX/nfit/range_index
+Date:		Jun, 2015
+KernelVersion:	v4.2
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) A unique number provided by the BIOS to identify an address
+		range. Used by NVDIMM Region Mapping Structure to uniquely refer
+		to this structure. Value of 0 is reserved and not used as an
+		index.
+
+
+What:		/sys/bus/nd/devices/regionX/nfit/ecc_unit_size
+Date:		Aug, 2017
+KernelVersion:	v4.14
+Contact:	linux-nvdimm@lists.01.org
+Description:
+		(RO) Size of a write request to a DIMM that will not incur a
+		read-modify-write cycle at the memory controller.
+
+		When the nfit driver initializes it runs an ARS (Address Range
+		Scrub) operation across every pmem range. Part of that process
+		involves determining the ARS capabilities of a given address
+		range. One of the capabilities that is reported is the 'Clear
+		Uncorrectable Error Range Length Unit Size' (see: ACPI 6.2
+		section 9.20.7.4 Function Index 1 - Query ARS Capabilities).
+		This property indicates the boundary at which the NVDIMM may
+		need to perform read-modify-write cycles to maintain ECC (Error
+		Correcting Code) blocks.