Documentation/hwmon/hwmon-kernel-api.rst - linux - Git at Google

 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.rst.

 For additional guidelines on how to write and improve hwmon drivers, please
 also read Documentation/hwmon/submitting-patches.rst.

 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_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);

   struct device *
   hwmon_device_register_with_info(struct device *dev,
 				  const char *name, void *drvdata,
 				  const struct hwmon_chip_info *info,
 				  const struct attribute_group **extra_groups);

   struct device *
   devm_hwmon_device_register_with_info(struct device *dev,
 				       const char *name,
 				       void *drvdata,
 				       const struct hwmon_chip_info *info,
 				       const struct attribute_group **extra_groups);

   void hwmon_device_unregister(struct device *dev);

   void devm_hwmon_device_unregister(struct device *dev);

   char *hwmon_sanitize_name(const char *name);

   char *devm_hwmon_sanitize_name(struct device *dev, const char *name);

 hwmon_device_register_with_groups registers a hardware monitoring device.
 The first parameter of this function is a pointer to the parent device.
 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.
 This function returns a pointer to the newly created hardware monitoring device
 or PTR_ERR for failure.

 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_register_with_info is the most comprehensive and preferred means
 to register a hardware monitoring device. It creates the standard sysfs
 attributes in the hardware monitoring core, letting the driver focus on reading
 from and writing to the chip instead of having to bother with sysfs attributes.
 The parent device parameter as well as the chip parameter must not be NULL. Its
 parameters are described in more detail below.

 devm_hwmon_device_register_with_info is similar to
 hwmon_device_register_with_info. 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_with_groups or hwmon_device_register_with_info.

 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 or
 hwmon_device_register_with_info and if the automatic (device managed)
 removal would be too late.

 All supported hwmon device registration functions only accept valid device
 names. Device names including invalid characters (whitespace, '*', or '-')
 will be rejected. The 'name' parameter is mandatory.

 If the driver doesn't use a static device name (for example it uses
 dev_name()), and therefore cannot make sure the name only contains valid
 characters, hwmon_sanitize_name can be used. This convenience function
 will duplicate the string and replace any invalid characters with an
 underscore. It will allocate memory for the new string and it is the
 responsibility of the caller to release the memory when the device is
 removed.

 devm_hwmon_sanitize_name is the resource managed version of
 hwmon_sanitize_name; the memory will be freed automatically on device
 removal.

 Using devm_hwmon_device_register_with_info()
 --------------------------------------------

 hwmon_device_register_with_info() registers a hardware monitoring device.
 The parameters to this function are

 =============================================== ===============================================
 `struct device *dev`				Pointer to parent device
 `const char *name`				Device name
 `void *drvdata`					Driver private data
 `const struct hwmon_chip_info *info`		Pointer to chip description.
 `const struct attribute_group **extra_groups` 	Null-terminated list of additional non-standard
 						sysfs attribute groups.
 =============================================== ===============================================

 This function returns a pointer to the created hardware monitoring device
 on success and a negative error code for failure.

 The hwmon_chip_info structure looks as follows::

 	struct hwmon_chip_info {
 		const struct hwmon_ops *ops;
 		const struct hwmon_channel_info **info;
 	};

 It contains the following fields:

 * ops:
 	Pointer to device operations.
 * info:
 	NULL-terminated list of device channel descriptors.

 The list of hwmon operations is defined as::

   struct hwmon_ops {
 	umode_t (*is_visible)(const void *, enum hwmon_sensor_types type,
 			      u32 attr, int);
 	int (*read)(struct device *, enum hwmon_sensor_types type,
 		    u32 attr, int, long *);
 	int (*write)(struct device *, enum hwmon_sensor_types type,
 		     u32 attr, int, long);
   };

 It defines the following operations.

 * is_visible:
     Pointer to a function to return the file mode for each supported
     attribute. This function is mandatory.

 * read:
     Pointer to a function for reading a value from the chip. This function
     is optional, but must be provided if any readable attributes exist.

 * write:
     Pointer to a function for writing a value to the chip. This function is
     optional, but must be provided if any writeable attributes exist.

 Each sensor channel is described with struct hwmon_channel_info, which is
 defined as follows::

 	struct hwmon_channel_info {
 		enum hwmon_sensor_types type;
 		u32 *config;
 	};

 It contains following fields:

 * type:
     The hardware monitoring sensor type.

     Supported sensor types are

      ================== ==================================================
      hwmon_chip		A virtual sensor type, used to describe attributes
 			which are not bound to a specific input or output
      hwmon_temp		Temperature sensor
      hwmon_in		Voltage sensor
      hwmon_curr		Current sensor
      hwmon_power		Power sensor
      hwmon_energy	Energy sensor
      hwmon_humidity	Humidity sensor
      hwmon_fan		Fan speed sensor
      hwmon_pwm		PWM control
      ================== ==================================================

 * config:
     Pointer to a 0-terminated list of configuration values for each
     sensor of the given type. Each value is a combination of bit values
     describing the attributes supposed by a single sensor.

 As an example, here is the complete description file for a LM75 compatible
 sensor chip. The chip has a single temperature sensor. The driver wants to
 register with the thermal subsystem (HWMON_C_REGISTER_TZ), and it supports
 the update_interval attribute (HWMON_C_UPDATE_INTERVAL). The chip supports
 reading the temperature (HWMON_T_INPUT), it has a maximum temperature
 register (HWMON_T_MAX) as well as a maximum temperature hysteresis register
 (HWMON_T_MAX_HYST)::

 	static const u32 lm75_chip_config[] = {
 		HWMON_C_REGISTER_TZ | HWMON_C_UPDATE_INTERVAL,
 		0
 	};

 	static const struct hwmon_channel_info lm75_chip = {
 		.type = hwmon_chip,
 		.config = lm75_chip_config,
 	};

 	static const u32 lm75_temp_config[] = {
 		HWMON_T_INPUT | HWMON_T_MAX | HWMON_T_MAX_HYST,
 		0
 	};

 	static const struct hwmon_channel_info lm75_temp = {
 		.type = hwmon_temp,
 		.config = lm75_temp_config,
 	};

 	static const struct hwmon_channel_info *lm75_info[] = {
 		&lm75_chip,
 		&lm75_temp,
 		NULL
 	};

 	The HWMON_CHANNEL_INFO() macro can and should be used when possible.
 	With this macro, the above example can be simplified to

 	static const struct hwmon_channel_info *lm75_info[] = {
 		HWMON_CHANNEL_INFO(chip,
 				HWMON_C_REGISTER_TZ | HWMON_C_UPDATE_INTERVAL),
 		HWMON_CHANNEL_INFO(temp,
 				HWMON_T_INPUT | HWMON_T_MAX | HWMON_T_MAX_HYST),
 		NULL
 	};

 	The remaining declarations are as follows.

 	static const struct hwmon_ops lm75_hwmon_ops = {
 		.is_visible = lm75_is_visible,
 		.read = lm75_read,
 		.write = lm75_write,
 	};

 	static const struct hwmon_chip_info lm75_chip_info = {
 		.ops = &lm75_hwmon_ops,
 		.info = lm75_info,
 	};

 A complete list of bit values indicating individual attribute support
 is defined in include/linux/hwmon.h. Definition prefixes are as follows.

 =============== =================================================
 HWMON_C_xxxx	Chip attributes, for use with hwmon_chip.
 HWMON_T_xxxx	Temperature attributes, for use with hwmon_temp.
 HWMON_I_xxxx	Voltage attributes, for use with hwmon_in.
 HWMON_C_xxxx	Current attributes, for use with hwmon_curr.
 		Notice the prefix overlap with chip attributes.
 HWMON_P_xxxx	Power attributes, for use with hwmon_power.
 HWMON_E_xxxx	Energy attributes, for use with hwmon_energy.
 HWMON_H_xxxx	Humidity attributes, for use with hwmon_humidity.
 HWMON_F_xxxx	Fan speed attributes, for use with hwmon_fan.
 HWMON_PWM_xxxx	PWM control attributes, for use with hwmon_pwm.
 =============== =================================================

 Driver callback functions
 -------------------------

 Each driver provides is_visible, read, and write functions. Parameters
 and return values for those functions are as follows::

   umode_t is_visible_func(const void *data, enum hwmon_sensor_types type,
 			  u32 attr, int channel)

 Parameters:
 	data:
 		Pointer to device private data structure.
 	type:
 		The sensor type.
 	attr:
 		Attribute identifier associated with a specific attribute.
 		For example, the attribute value for HWMON_T_INPUT would be
 		hwmon_temp_input. For complete mappings of bit fields to
 		attribute values please see include/linux/hwmon.h.
 	channel:
 		The sensor channel number.

 Return value:
 	The file mode for this attribute. Typically, this will be 0 (the
 	attribute will not be created), S_IRUGO, or 'S_IRUGO | S_IWUSR'.

 ::

 	int read_func(struct device *dev, enum hwmon_sensor_types type,
 		      u32 attr, int channel, long *val)

 Parameters:
 	dev:
 		Pointer to the hardware monitoring device.
 	type:
 		The sensor type.
 	attr:
 		Attribute identifier associated with a specific attribute.
 		For example, the attribute value for HWMON_T_INPUT would be
 		hwmon_temp_input. For complete mappings please see
 		include/linux/hwmon.h.
 	channel:
 		The sensor channel number.
 	val:
 		Pointer to attribute value.

 Return value:
 	0 on success, a negative error number otherwise.

 ::

 	int write_func(struct device *dev, enum hwmon_sensor_types type,
 		       u32 attr, int channel, long val)

 Parameters:
 	dev:
 		Pointer to the hardware monitoring device.
 	type:
 		The sensor type.
 	attr:
 		Attribute identifier associated with a specific attribute.
 		For example, the attribute value for HWMON_T_INPUT would be
 		hwmon_temp_input. For complete mappings please see
 		include/linux/hwmon.h.
 	channel:
 		The sensor channel number.
 	val:
 		The value to write to the chip.

 Return value:
 	0 on success, a negative error number otherwise.


 Driver-provided sysfs attributes
 --------------------------------

 If the hardware monitoring device is registered with
 hwmon_device_register_with_info or devm_hwmon_device_register_with_info,
 it is most likely not necessary to provide sysfs attributes. Only additional
 non-standard sysfs attributes need to be provided when one of those registration
 functions is used.

 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.
	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.rst.

	For additional guidelines on how to write and improve hwmon drivers, please
	also read Documentation/hwmon/submitting-patches.rst.

	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_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);

	struct device *
	hwmon_device_register_with_info(struct device *dev,
	const char name, void drvdata,
	const struct hwmon_chip_info *info,
	const struct attribute_group **extra_groups);

	struct device *
	devm_hwmon_device_register_with_info(struct device *dev,
	const char *name,
	void *drvdata,
	const struct hwmon_chip_info *info,
	const struct attribute_group **extra_groups);

	void hwmon_device_unregister(struct device *dev);

	void devm_hwmon_device_unregister(struct device *dev);

	char hwmon_sanitize_name(const char name);

	char devm_hwmon_sanitize_name(struct device dev, const char *name);

	hwmon_device_register_with_groups registers a hardware monitoring device.
	The first parameter of this function is a pointer to the parent device.
	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.
	This function returns a pointer to the newly created hardware monitoring device
	or PTR_ERR for failure.

	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_register_with_info is the most comprehensive and preferred means
	to register a hardware monitoring device. It creates the standard sysfs
	attributes in the hardware monitoring core, letting the driver focus on reading
	from and writing to the chip instead of having to bother with sysfs attributes.
	The parent device parameter as well as the chip parameter must not be NULL. Its
	parameters are described in more detail below.

	devm_hwmon_device_register_with_info is similar to
	hwmon_device_register_with_info. 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_with_groups or hwmon_device_register_with_info.

	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 or
	hwmon_device_register_with_info and if the automatic (device managed)
	removal would be too late.

	All supported hwmon device registration functions only accept valid device
	names. Device names including invalid characters (whitespace, '*', or '-')
	will be rejected. The 'name' parameter is mandatory.

	If the driver doesn't use a static device name (for example it uses
	dev_name()), and therefore cannot make sure the name only contains valid
	characters, hwmon_sanitize_name can be used. This convenience function
	will duplicate the string and replace any invalid characters with an
	underscore. It will allocate memory for the new string and it is the
	responsibility of the caller to release the memory when the device is
	removed.

	devm_hwmon_sanitize_name is the resource managed version of
	hwmon_sanitize_name; the memory will be freed automatically on device
	removal.

	Using devm_hwmon_device_register_with_info()
	--------------------------------------------

	hwmon_device_register_with_info() registers a hardware monitoring device.
	The parameters to this function are

	=============================================== ===============================================
	`struct device *dev` Pointer to parent device
	`const char *name` Device name
	`void *drvdata` Driver private data
	`const struct hwmon_chip_info *info` Pointer to chip description.
	`const struct attribute_group **extra_groups` Null-terminated list of additional non-standard
	sysfs attribute groups.
	=============================================== ===============================================

	This function returns a pointer to the created hardware monitoring device
	on success and a negative error code for failure.

	The hwmon_chip_info structure looks as follows::

	struct hwmon_chip_info {
	const struct hwmon_ops *ops;
	const struct hwmon_channel_info **info;
	};

	It contains the following fields:

	* ops:
	Pointer to device operations.
	* info:
	NULL-terminated list of device channel descriptors.

	The list of hwmon operations is defined as::

	struct hwmon_ops {
	umode_t (is_visible)(const void , enum hwmon_sensor_types type,
	u32 attr, int);
	int (read)(struct device , enum hwmon_sensor_types type,
	u32 attr, int, long *);
	int (write)(struct device , enum hwmon_sensor_types type,
	u32 attr, int, long);
	};

	It defines the following operations.

	* is_visible:
	Pointer to a function to return the file mode for each supported
	attribute. This function is mandatory.

	* read:
	Pointer to a function for reading a value from the chip. This function
	is optional, but must be provided if any readable attributes exist.

	* write:
	Pointer to a function for writing a value to the chip. This function is
	optional, but must be provided if any writeable attributes exist.

	Each sensor channel is described with struct hwmon_channel_info, which is
	defined as follows::

	struct hwmon_channel_info {
	enum hwmon_sensor_types type;
	u32 *config;
	};

	It contains following fields:

	* type:
	The hardware monitoring sensor type.

	Supported sensor types are

	================== ==================================================
	hwmon_chip A virtual sensor type, used to describe attributes
	which are not bound to a specific input or output
	hwmon_temp Temperature sensor
	hwmon_in Voltage sensor
	hwmon_curr Current sensor
	hwmon_power Power sensor
	hwmon_energy Energy sensor
	hwmon_humidity Humidity sensor
	hwmon_fan Fan speed sensor
	hwmon_pwm PWM control
	================== ==================================================

	* config:
	Pointer to a 0-terminated list of configuration values for each
	sensor of the given type. Each value is a combination of bit values
	describing the attributes supposed by a single sensor.

	As an example, here is the complete description file for a LM75 compatible
	sensor chip. The chip has a single temperature sensor. The driver wants to
	register with the thermal subsystem (HWMON_C_REGISTER_TZ), and it supports
	the update_interval attribute (HWMON_C_UPDATE_INTERVAL). The chip supports
	reading the temperature (HWMON_T_INPUT), it has a maximum temperature
	register (HWMON_T_MAX) as well as a maximum temperature hysteresis register
	(HWMON_T_MAX_HYST)::

	static const u32 lm75_chip_config[] = {
	HWMON_C_REGISTER_TZ \| HWMON_C_UPDATE_INTERVAL,
	0
	};

	static const struct hwmon_channel_info lm75_chip = {
	.type = hwmon_chip,
	.config = lm75_chip_config,
	};

	static const u32 lm75_temp_config[] = {
	HWMON_T_INPUT \| HWMON_T_MAX \| HWMON_T_MAX_HYST,
	0
	};

	static const struct hwmon_channel_info lm75_temp = {
	.type = hwmon_temp,
	.config = lm75_temp_config,
	};

	static const struct hwmon_channel_info *lm75_info[] = {
	&lm75_chip,
	&lm75_temp,
	NULL
	};

	The HWMON_CHANNEL_INFO() macro can and should be used when possible.
	With this macro, the above example can be simplified to

	static const struct hwmon_channel_info *lm75_info[] = {
	HWMON_CHANNEL_INFO(chip,
	HWMON_C_REGISTER_TZ \| HWMON_C_UPDATE_INTERVAL),
	HWMON_CHANNEL_INFO(temp,
	HWMON_T_INPUT \| HWMON_T_MAX \| HWMON_T_MAX_HYST),
	NULL
	};

	The remaining declarations are as follows.

	static const struct hwmon_ops lm75_hwmon_ops = {
	.is_visible = lm75_is_visible,
	.read = lm75_read,
	.write = lm75_write,
	};

	static const struct hwmon_chip_info lm75_chip_info = {
	.ops = &lm75_hwmon_ops,
	.info = lm75_info,
	};

	A complete list of bit values indicating individual attribute support
	is defined in include/linux/hwmon.h. Definition prefixes are as follows.

	=============== =================================================
	HWMON_C_xxxx Chip attributes, for use with hwmon_chip.
	HWMON_T_xxxx Temperature attributes, for use with hwmon_temp.
	HWMON_I_xxxx Voltage attributes, for use with hwmon_in.
	HWMON_C_xxxx Current attributes, for use with hwmon_curr.
	Notice the prefix overlap with chip attributes.
	HWMON_P_xxxx Power attributes, for use with hwmon_power.
	HWMON_E_xxxx Energy attributes, for use with hwmon_energy.
	HWMON_H_xxxx Humidity attributes, for use with hwmon_humidity.
	HWMON_F_xxxx Fan speed attributes, for use with hwmon_fan.
	HWMON_PWM_xxxx PWM control attributes, for use with hwmon_pwm.
	=============== =================================================

	Driver callback functions
	-------------------------

	Each driver provides is_visible, read, and write functions. Parameters
	and return values for those functions are as follows::

	umode_t is_visible_func(const void *data, enum hwmon_sensor_types type,
	u32 attr, int channel)

	Parameters:
	data:
	Pointer to device private data structure.
	type:
	The sensor type.
	attr:
	Attribute identifier associated with a specific attribute.
	For example, the attribute value for HWMON_T_INPUT would be
	hwmon_temp_input. For complete mappings of bit fields to
	attribute values please see include/linux/hwmon.h.
	channel:
	The sensor channel number.

	Return value:
	The file mode for this attribute. Typically, this will be 0 (the
	attribute will not be created), S_IRUGO, or 'S_IRUGO \| S_IWUSR'.

	::

	int read_func(struct device *dev, enum hwmon_sensor_types type,
	u32 attr, int channel, long *val)

	Parameters:
	dev:
	Pointer to the hardware monitoring device.
	type:
	The sensor type.
	attr:
	Attribute identifier associated with a specific attribute.
	For example, the attribute value for HWMON_T_INPUT would be
	hwmon_temp_input. For complete mappings please see
	include/linux/hwmon.h.
	channel:
	The sensor channel number.
	val:
	Pointer to attribute value.

	Return value:
	0 on success, a negative error number otherwise.

	::

	int write_func(struct device *dev, enum hwmon_sensor_types type,
	u32 attr, int channel, long val)

	Parameters:
	dev:
	Pointer to the hardware monitoring device.
	type:
	The sensor type.
	attr:
	Attribute identifier associated with a specific attribute.
	For example, the attribute value for HWMON_T_INPUT would be
	hwmon_temp_input. For complete mappings please see
	include/linux/hwmon.h.
	channel:
	The sensor channel number.
	val:
	The value to write to the chip.

	Return value:
	0 on success, a negative error number otherwise.


	Driver-provided sysfs attributes
	--------------------------------

	If the hardware monitoring device is registered with
	hwmon_device_register_with_info or devm_hwmon_device_register_with_info,
	it is most likely not necessary to provide sysfs attributes. Only additional
	non-standard sysfs attributes need to be provided when one of those registration
	functions is used.

	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.