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 or its variants
DEVICE_ATTR_{RW,RO,WO} 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.

Simplified variants of SENSOR_DEVICE_ATTR and SENSOR_DEVICE_ATTR_2 are available
and should be used if standard attribute permissions and function names are
feasible. Standard permissions are 0644 for SENSOR_DEVICE_ATTR[_2]_RW,
0444 for SENSOR_DEVICE_ATTR[_2]_RO, and 0200 for SENSOR_DEVICE_ATTR[_2]_WO.
Standard functions, similar to DEVICE_ATTR_{RW,RO,WO}, have _show and _store
appended to the provided function name.

SENSOR_DEVICE_ATTR and its variants define 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 and its variants define 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.
