Documentation: hwmon: Add OCC documentation

Document the hwmon interface for the OCC.
Signed-off-by: Eddie James <eajames@linux.ibm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/occ

+Kernel driver occ-hwmon
+=======================
+
+Supported chips:
+  * POWER8
+  * POWER9
+
+Author: Eddie James <eajames@linux.ibm.com>
+
+Description
+-----------
+
+This driver supports hardware monitoring for the On-Chip Controller (OCC)
+embedded on POWER processors. The OCC is a device that collects and aggregates
+sensor data from the processor and the system. The OCC can provide the raw
+sensor data as well as perform thermal and power management on the system.
+
+The P8 version of this driver is a client driver of I2C. It may be probed
+manually if an "ibm,p8-occ-hwmon" compatible device is found under the
+appropriate I2C bus node in the device-tree.
+
+The P9 version of this driver is a client driver of the FSI-based OCC driver.
+It will be probed automatically by the FSI-based OCC driver.
+
+Sysfs entries
+-------------
+
+The following attributes are supported. All attributes are read-only unless
+specified.
+
+The OCC sensor ID is an integer that represents the unique identifier of the
+sensor with respect to the OCC. For example, a temperature sensor for the third
+DIMM slot in the system may have a sensor ID of 7. This mapping is unavailable
+to the device driver, which must therefore export the sensor ID as-is.
+
+Some entries are only present with certain OCC sensor versions or only on
+certain OCCs in the system. The version number is not exported to the user
+but can be inferred.
+
+temp[1-n]_label		OCC sensor ID.
+[with temperature sensor version 1]
+    temp[1-n]_input	Measured temperature of the component in millidegrees
+			Celsius.
+[with temperature sensor version >= 2]
+    temp[1-n]_type		The FRU (Field Replaceable Unit) type
+				(represented by an integer) for the component
+				that this sensor measures.
+    temp[1-n]_fault		Temperature sensor fault boolean; 1 to indicate
+				that a fault is present or 0 to indicate that
+				no fault is present.
+    [with type == 3 (FRU type is VRM)]
+        temp[1-n]_alarm		VRM temperature alarm boolean; 1 to indicate
+				alarm, 0 to indicate no alarm
+    [else]
+        temp[1-n]_input		Measured temperature of the component in
+				millidegrees Celsius.
+
+freq[1-n]_label		OCC sensor ID.
+freq[1-n]_input		Measured frequency of the component in MHz.
+
+power[1-n]_input	Latest measured power reading of the component in
+			microwatts.
+power[1-n]_average	Average power of the component in microwatts.
+power[1-n]_average_interval	The amount of time over which the power average
+				was taken in microseconds.
+[with power sensor version < 2]
+    power[1-n]_label	OCC sensor ID.
+[with power sensor version >= 2]
+    power[1-n]_label	OCC sensor ID + function ID + channel in the form
+			of a string, delimited by underscores, i.e. "0_15_1".
+			Both the function ID and channel are integers that
+			further identify the power sensor.
+[with power sensor version 0xa0]
+    power[1-n]_label	OCC sensor ID + sensor type in the form of a string,
+			delimited by an underscore, i.e. "0_system". Sensor
+			type will be one of "system", "proc", "vdd" or "vdn".
+			For this sensor version, OCC sensor ID will be the same
+			for all power sensors.
+[present only on "master" OCC; represents the whole system power; only one of
+ this type of power sensor will be present]
+    power[1-n]_label		"system"
+    power[1-n]_input		Latest system output power in microwatts.
+    power[1-n]_cap		Current system power cap in microwatts.
+    power[1-n]_cap_not_redundant	System power cap in microwatts when
+					there is not redundant power.
+    power[1-n]_cap_max		Maximum power cap that the OCC can enforce in
+				microwatts.
+    power[1-n]_cap_min		Minimum power cap that the OCC can enforce in
+				microwatts.
+    power[1-n]_cap_user		The power cap set by the user, in microwatts.
+				This attribute will return 0 if no user power
+				cap has been set. This attribute is read-write,
+				but writing any precision below watts will be
+				ignored, i.e. requesting a power cap of
+				500900000 microwatts will result in a power cap
+				request of 500 watts.
+    [with caps sensor version > 1]
+        power[1-n]_cap_user_source	Indicates how the user power cap was
+					set. This is an integer that maps to
+					system or firmware components that can
+					set the user power cap.
+
+The following "extn" sensors are exported as a way for the OCC to provide data
+that doesn't fit anywhere else. The meaning of these sensors is entirely
+dependent on their data, and cannot be statically defined.
+
+extn[1-n]_label		ASCII ID or OCC sensor ID.
+extn[1-n]_flags		This is one byte hexadecimal value. Bit 7 indicates the
+			type of the label attribute; 1 for sensor ID, 0 for
+			ASCII ID. Other bits are reserved.
+extn[1-n]_input		6 bytes of hexadecimal data, with a meaning defined by
+			the sensor ID.