hwmon: Document hwmon kernel API

The hwmon subsystem has been around for a while. Time to document
its kernel API.
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/hwmon-kernel-api.txt

+The Linux Hardware Monitoring kernel API.
+=========================================
+
+Guenter Roeck
+
+Introduction
+------------
+
+This document describes the API that can be used by hardware monitoring
+drivers that want to use the hardware monitoring framework.
+
+This document does not describe what a hardware monitoring (hwmon) Driver or
+Device is. It also does not describe the API which can be used by user space
+to communicate with a hardware monitoring device. If you want to know this
+then please read the following file: Documentation/hwmon/sysfs-interface.
+
+For additional guidelines on how to write and improve hwmon drivers, please
+also read Documentation/hwmon/submitting-patches.
+
+The API
+-------
+Each hardware monitoring driver must #include <linux/hwmon.h> and, in most
+cases, <linux/hwmon-sysfs.h>. linux/hwmon.h declares the following
+register/unregister functions:
+
+struct device *hwmon_device_register(struct device *dev);
+struct device *
+hwmon_device_register_with_groups(struct device *dev, const char *name,
+				  void *drvdata,
+				  const struct attribute_group **groups);
+
+struct device *
+devm_hwmon_device_register_with_groups(struct device *dev,
+				       const char *name, void *drvdata,
+				       const struct attribute_group **groups);
+
+void hwmon_device_unregister(struct device *dev);
+void devm_hwmon_device_unregister(struct device *dev);
+
+hwmon_device_register registers a hardware monitoring device. The parameter
+of this function is a pointer to the parent device.
+This function returns a pointer to the newly created hardware monitoring device
+or PTR_ERR for failure. If this registration function is used, hardware
+monitoring sysfs attributes are expected to have been created and attached to
+the parent device prior to calling hwmon_device_register. A name attribute must
+have been created by the caller.
+
+hwmon_device_register_with_groups is similar to hwmon_device_register. However,
+it has additional parameters. The name parameter is a pointer to the hwmon
+device name. The registration function wil create a name sysfs attribute
+pointing to this name. The drvdata parameter is the pointer to the local
+driver data.  hwmon_device_register_with_groups will attach this pointer
+to the newly allocated hwmon device. The pointer can be retrieved by the driver
+using dev_get_drvdata() on the hwmon device pointer. The groups parameter is
+a pointer to a list of sysfs attribute groups. The list must be NULL terminated.
+hwmon_device_register_with_groups creates the hwmon device with name attribute
+as well as all sysfs attributes attached to the hwmon device.
+
+devm_hwmon_device_register_with_groups is similar to
+hwmon_device_register_with_groups. However, it is device managed, meaning the
+hwmon device does not have to be removed explicitly by the removal function.
+
+hwmon_device_unregister deregisters a registered hardware monitoring device.
+The parameter of this function is the pointer to the registered hardware
+monitoring device structure. This function must be called from the driver
+remove function if the hardware monitoring device was registered with
+hwmon_device_register or with hwmon_device_register_with_groups.
+
+devm_hwmon_device_unregister does not normally have to be called. It is only
+needed for error handling, and only needed if the driver probe fails after
+the call to devm_hwmon_device_register_with_groups.
+
+The header file linux/hwmon-sysfs.h provides a number of useful macros to
+declare and use hardware monitoring sysfs attributes.
+
+In many cases, you can use the exsting define DEVICE_ATTR to declare such
+attributes. This is feasible if an attribute has no additional context. However,
+in many cases there will be additional information such as a sensor index which
+will need to be passed to the sysfs attribute handling function.
+
+SENSOR_DEVICE_ATTR and SENSOR_DEVICE_ATTR_2 can be used to define attributes
+which need such additional context information. SENSOR_DEVICE_ATTR requires
+one additional argument, SENSOR_DEVICE_ATTR_2 requires two.
+
+SENSOR_DEVICE_ATTR defines a struct sensor_device_attribute variable.
+This structure has the following fields.
+
+struct sensor_device_attribute {
+	struct device_attribute dev_attr;
+	int index;
+};
+
+You can use to_sensor_dev_attr to get the pointer to this structure from the
+attribute read or write function. Its parameter is the device to which the
+attribute is attached.
+
+SENSOR_DEVICE_ATTR_2 defines a struct sensor_device_attribute_2 variable,
+which is defined as follows.
+
+struct sensor_device_attribute_2 {
+	struct device_attribute dev_attr;
+	u8 index;
+	u8 nr;
+};
+
+Use to_sensor_dev_attr_2 to get the pointer to this structure. Its parameter
+is the device to which the attribute is attached.