Naming and data format standards for sysfs files
  ------------------------------------------------
  
  The libsensors library offers an interface to the raw sensors data
  through the sysfs interface. Since lm-sensors 3.0.0, libsensors is
  completely chip-independent. It assumes that all the kernel drivers
  implement the standard sysfs interface described in this document.
  This makes adding or updating support for any given chip very easy, as
  libsensors, and applications using it, do not need to be modified.
  This is a major improvement compared to lm-sensors 2.
  
  Note that motherboards vary widely in the connections to sensor chips.
  There is no standard that ensures, for example, that the second
  temperature sensor is connected to the CPU, or that the second fan is on
  the CPU. Also, some values reported by the chips need some computation
  before they make full sense. For example, most chips can only measure
  voltages between 0 and +4V. Other voltages are scaled back into that
  range using external resistors. Since the values of these resistors
  can change from motherboard to motherboard, the conversions cannot be
  hard coded into the driver and have to be done in user space.
  
  For this reason, even if we aim at a chip-independent libsensors, it will
  still require a configuration file (e.g. /etc/sensors.conf) for proper
  values conversion, labeling of inputs and hiding of unused inputs.
  
  An alternative method that some programs use is to access the sysfs
  files directly. This document briefly describes the standards that the
  drivers follow, so that an application program can scan for entries and
  access this data in a simple and consistent way. That said, such programs
  will have to implement conversion, labeling and hiding of inputs. For
  this reason, it is still not recommended to bypass the library.
  
  Each chip gets its own directory in the sysfs /sys/devices tree.  To
  find all sensor chips, it is easier to follow the device symlinks from
  /sys/class/hwmon/hwmon*.
  
  Up to lm-sensors 3.0.0, libsensors looks for hardware monitoring attributes
  in the "physical" device directory. Since lm-sensors 3.0.1, attributes found
  in the hwmon "class" device directory are also supported. Complex drivers
  (e.g. drivers for multifunction chips) may want to use this possibility to
  avoid namespace pollution. The only drawback will be that older versions of
  libsensors won't support the driver in question.
  
  All sysfs values are fixed point numbers.
  
  There is only one value per file, unlike the older /proc specification.
  The common scheme for files naming is: <type><number>_<item>. Usual
  types for sensor chips are "in" (voltage), "temp" (temperature) and
  "fan" (fan). Usual items are "input" (measured value), "max" (high
  threshold, "min" (low threshold). Numbering usually starts from 1,
  except for voltages which start from 0 (because most data sheets use
  this). A number is always used for elements that can be present more
  than once, even if there is a single element of the given type on the
  specific chip. Other files do not refer to a specific element, so
  they have a simple name, and no number.
  
  Alarms are direct indications read from the chips. The drivers do NOT
  make comparisons of readings to thresholds. This allows violations
  between readings to be caught and alarmed. The exact definition of an
  alarm (for example, whether a threshold must be met or must be exceeded
  to cause an alarm) is chip-dependent.
  
  When setting values of hwmon sysfs attributes, the string representation of
  the desired value must be written, note that strings which are not a number
  are interpreted as 0! For more on how written strings are interpreted see the
  "sysfs attribute writes interpretation" section at the end of this file.
  
  -------------------------------------------------------------------------
  
  [0-*]	denotes any positive number starting from 0
  [1-*]	denotes any positive number starting from 1
  RO	read only value
  WO	write only value
  RW	read/write value
  
  Read/write values may be read-only for some chips, depending on the
  hardware implementation.
  
  All entries (except name) are optional, and should only be created in a
  given driver if the chip has the feature.
  
  
  *********************
  * Global attributes *
  *********************
  
  name		The chip name.
  		This should be a short, lowercase string, not containing
  		spaces nor dashes, representing the chip name. This is
  		the only mandatory attribute.
  		I2C devices get this attribute created automatically.
  		RO
  
  update_interval	The interval at which the chip will update readings.
  		Unit: millisecond
  		RW
  		Some devices have a variable update rate or interval.
  		This attribute can be used to change it to the desired value.
  
  
  ************
  * Voltages *
  ************
  
  in[0-*]_min	Voltage min value.
  		Unit: millivolt
  		RW
  		
  in[0-*]_lcrit	Voltage critical min value.
  		Unit: millivolt
  		RW
  		If voltage drops to or below this limit, the system may
  		take drastic action such as power down or reset. At the very
  		least, it should report a fault.
  
  in[0-*]_max	Voltage max value.
  		Unit: millivolt
  		RW
  		
  in[0-*]_crit	Voltage critical max value.
  		Unit: millivolt
  		RW
  		If voltage reaches or exceeds this limit, the system may
  		take drastic action such as power down or reset. At the very
  		least, it should report a fault.
  
  in[0-*]_input	Voltage input value.
  		Unit: millivolt
  		RO
  		Voltage measured on the chip pin.
  		Actual voltage depends on the scaling resistors on the
  		motherboard, as recommended in the chip datasheet.
  		This varies by chip and by motherboard.
  		Because of this variation, values are generally NOT scaled
  		by the chip driver, and must be done by the application.
  		However, some drivers (notably lm87 and via686a)
  		do scale, because of internal resistors built into a chip.
  		These drivers will output the actual voltage. Rule of
  		thumb: drivers should report the voltage values at the
  		"pins" of the chip.
  
  in[0-*]_average
  		Average voltage
  		Unit: millivolt
  		RO
  
  in[0-*]_lowest
  		Historical minimum voltage
  		Unit: millivolt
  		RO
  
  in[0-*]_highest
  		Historical maximum voltage
  		Unit: millivolt
  		RO
  
  in[0-*]_reset_history
  		Reset inX_lowest and inX_highest
  		WO
  
  in_reset_history
  		Reset inX_lowest and inX_highest for all sensors
  		WO
  
  in[0-*]_label	Suggested voltage channel label.
  		Text string
  		Should only be created if the driver has hints about what
  		this voltage channel is being used for, and user-space
  		doesn't. In all other cases, the label is provided by
  		user-space.
  		RO
  
  cpu[0-*]_vid	CPU core reference voltage.
  		Unit: millivolt
  		RO
  		Not always correct.
  
  vrm		Voltage Regulator Module version number. 
  		RW (but changing it should no more be necessary)
  		Originally the VRM standard version multiplied by 10, but now
  		an arbitrary number, as not all standards have a version
  		number.
  		Affects the way the driver calculates the CPU core reference
  		voltage from the vid pins.
  
  Also see the Alarms section for status flags associated with voltages.
  
  
  ********
  * Fans *
  ********
  
  fan[1-*]_min	Fan minimum value
  		Unit: revolution/min (RPM)
  		RW
  
  fan[1-*]_max	Fan maximum value
  		Unit: revolution/min (RPM)
  		Only rarely supported by the hardware.
  		RW
  
  fan[1-*]_input	Fan input value.
  		Unit: revolution/min (RPM)
  		RO
  
  fan[1-*]_div	Fan divisor.
  		Integer value in powers of two (1, 2, 4, 8, 16, 32, 64, 128).
  		RW
  		Some chips only support values 1, 2, 4 and 8.
  		Note that this is actually an internal clock divisor, which
  		affects the measurable speed range, not the read value.
  
  fan[1-*]_pulses	Number of tachometer pulses per fan revolution.
  		Integer value, typically between 1 and 4.
  		RW
  		This value is a characteristic of the fan connected to the
  		device's input, so it has to be set in accordance with the fan
  		model.
  		Should only be created if the chip has a register to configure
  		the number of pulses. In the absence of such a register (and
  		thus attribute) the value assumed by all devices is 2 pulses
  		per fan revolution.
  
  fan[1-*]_target
  		Desired fan speed
  		Unit: revolution/min (RPM)
  		RW
  		Only makes sense if the chip supports closed-loop fan speed
  		control based on the measured fan speed.
  
  fan[1-*]_label	Suggested fan channel label.
  		Text string
  		Should only be created if the driver has hints about what
  		this fan channel is being used for, and user-space doesn't.
  		In all other cases, the label is provided by user-space.
  		RO
  
  Also see the Alarms section for status flags associated with fans.
  
  
  *******
  * PWM *
  *******
  
  pwm[1-*]	Pulse width modulation fan control.
  		Integer value in the range 0 to 255
  		RW
  		255 is max or 100%.
  
  pwm[1-*]_enable
  		Fan speed control method:
  		0: no fan speed control (i.e. fan at full speed)
  		1: manual fan speed control enabled (using pwm[1-*])
  		2+: automatic fan speed control enabled
  		Check individual chip documentation files for automatic mode
  		details.
  		RW
  
  pwm[1-*]_mode	0: DC mode (direct current)
  		1: PWM mode (pulse-width modulation)
  		RW
  
  pwm[1-*]_freq	Base PWM frequency in Hz.
  		Only possibly available when pwmN_mode is PWM, but not always
  		present even then.
  		RW
  
  pwm[1-*]_auto_channels_temp
  		Select which temperature channels affect this PWM output in
  		auto mode. Bitfield, 1 is temp1, 2 is temp2, 4 is temp3 etc...
  		Which values are possible depend on the chip used.
  		RW
  
  pwm[1-*]_auto_point[1-*]_pwm
  pwm[1-*]_auto_point[1-*]_temp
  pwm[1-*]_auto_point[1-*]_temp_hyst
  		Define the PWM vs temperature curve. Number of trip points is
  		chip-dependent. Use this for chips which associate trip points
  		to PWM output channels.
  		RW
  
  temp[1-*]_auto_point[1-*]_pwm
  temp[1-*]_auto_point[1-*]_temp
  temp[1-*]_auto_point[1-*]_temp_hyst
  		Define the PWM vs temperature curve. Number of trip points is
  		chip-dependent. Use this for chips which associate trip points
  		to temperature channels.
  		RW
  
  There is a third case where trip points are associated to both PWM output
  channels and temperature channels: the PWM values are associated to PWM
  output channels while the temperature values are associated to temperature
  channels. In that case, the result is determined by the mapping between
  temperature inputs and PWM outputs. When several temperature inputs are
  mapped to a given PWM output, this leads to several candidate PWM values.
  The actual result is up to the chip, but in general the highest candidate
  value (fastest fan speed) wins.
  
  
  ****************
  * Temperatures *
  ****************
  
  temp[1-*]_type	Sensor type selection.
  		Integers 1 to 6
  		RW
  		1: CPU embedded diode
  		2: 3904 transistor
  		3: thermal diode
  		4: thermistor
  		5: AMD AMDSI
  		6: Intel PECI
  		Not all types are supported by all chips
  
  temp[1-*]_max	Temperature max value.
  		Unit: millidegree Celsius (or millivolt, see below)
  		RW
  
  temp[1-*]_min	Temperature min value.
  		Unit: millidegree Celsius
  		RW
  
  temp[1-*]_max_hyst
  		Temperature hysteresis value for max limit.
  		Unit: millidegree Celsius
  		Must be reported as an absolute temperature, NOT a delta
  		from the max value.
  		RW
  
  temp[1-*]_input Temperature input value.
  		Unit: millidegree Celsius
  		RO
  
  temp[1-*]_crit	Temperature critical max value, typically greater than
  		corresponding temp_max values.
  		Unit: millidegree Celsius
  		RW
  
  temp[1-*]_crit_hyst
  		Temperature hysteresis value for critical limit.
  		Unit: millidegree Celsius
  		Must be reported as an absolute temperature, NOT a delta
  		from the critical value.
  		RW
  
  temp[1-*]_emergency
  		Temperature emergency max value, for chips supporting more than
  		two upper temperature limits. Must be equal or greater than
  		corresponding temp_crit values.
  		Unit: millidegree Celsius
  		RW
  
  temp[1-*]_emergency_hyst
  		Temperature hysteresis value for emergency limit.
  		Unit: millidegree Celsius
  		Must be reported as an absolute temperature, NOT a delta
  		from the emergency value.
  		RW
  
  temp[1-*]_lcrit	Temperature critical min value, typically lower than
  		corresponding temp_min values.
  		Unit: millidegree Celsius
  		RW
  
  temp[1-*]_offset
  		Temperature offset which is added to the temperature reading
  		by the chip.
  		Unit: millidegree Celsius
  		Read/Write value.
  
  temp[1-*]_label	Suggested temperature channel label.
  		Text string
  		Should only be created if the driver has hints about what
  		this temperature channel is being used for, and user-space
  		doesn't. In all other cases, the label is provided by
  		user-space.
  		RO
  
  temp[1-*]_lowest
  		Historical minimum temperature
  		Unit: millidegree Celsius
  		RO
  
  temp[1-*]_highest
  		Historical maximum temperature
  		Unit: millidegree Celsius
  		RO
  
  temp[1-*]_reset_history
  		Reset temp_lowest and temp_highest
  		WO
  
  temp_reset_history
  		Reset temp_lowest and temp_highest for all sensors
  		WO
  
  Some chips measure temperature using external thermistors and an ADC, and
  report the temperature measurement as a voltage. Converting this voltage
  back to a temperature (or the other way around for limits) requires
  mathematical functions not available in the kernel, so the conversion
  must occur in user space. For these chips, all temp* files described
  above should contain values expressed in millivolt instead of millidegree
  Celsius. In other words, such temperature channels are handled as voltage
  channels by the driver.
  
  Also see the Alarms section for status flags associated with temperatures.
  
  
  ************
  * Currents *
  ************
  
  curr[1-*]_max	Current max value
  		Unit: milliampere
  		RW
  
  curr[1-*]_min	Current min value.
  		Unit: milliampere
  		RW
  
  curr[1-*]_lcrit	Current critical low value
  		Unit: milliampere
  		RW
  
  curr[1-*]_crit	Current critical high value.
  		Unit: milliampere
  		RW
  
  curr[1-*]_input	Current input value
  		Unit: milliampere
  		RO
  
  curr[1-*]_average
  		Average current use
  		Unit: milliampere
  		RO
  
  curr[1-*]_lowest
  		Historical minimum current
  		Unit: milliampere
  		RO
  
  curr[1-*]_highest
  		Historical maximum current
  		Unit: milliampere
  		RO
  
  curr[1-*]_reset_history
  		Reset currX_lowest and currX_highest
  		WO
  
  curr_reset_history
  		Reset currX_lowest and currX_highest for all sensors
  		WO
  
  Also see the Alarms section for status flags associated with currents.
  
  *********
  * Power *
  *********
  
  power[1-*]_average		Average power use
  				Unit: microWatt
  				RO
  
  power[1-*]_average_interval	Power use averaging interval.  A poll
  				notification is sent to this file if the
  				hardware changes the averaging interval.
  				Unit: milliseconds
  				RW
  
  power[1-*]_average_interval_max	Maximum power use averaging interval
  				Unit: milliseconds
  				RO
  
  power[1-*]_average_interval_min	Minimum power use averaging interval
  				Unit: milliseconds
  				RO
  
  power[1-*]_average_highest	Historical average maximum power use
  				Unit: microWatt
  				RO
  
  power[1-*]_average_lowest	Historical average minimum power use
  				Unit: microWatt
  				RO
  
  power[1-*]_average_max		A poll notification is sent to
  				power[1-*]_average when power use
  				rises above this value.
  				Unit: microWatt
  				RW
  
  power[1-*]_average_min		A poll notification is sent to
  				power[1-*]_average when power use
  				sinks below this value.
  				Unit: microWatt
  				RW
  
  power[1-*]_input		Instantaneous power use
  				Unit: microWatt
  				RO
  
  power[1-*]_input_highest	Historical maximum power use
  				Unit: microWatt
  				RO
  
  power[1-*]_input_lowest		Historical minimum power use
  				Unit: microWatt
  				RO
  
  power[1-*]_reset_history	Reset input_highest, input_lowest,
  				average_highest and average_lowest.
  				WO
  
  power[1-*]_accuracy		Accuracy of the power meter.
  				Unit: Percent
  				RO
  
  power[1-*]_cap			If power use rises above this limit, the
  				system should take action to reduce power use.
  				A poll notification is sent to this file if the
  				cap is changed by the hardware.  The *_cap
  				files only appear if the cap is known to be
  				enforced by hardware.
  				Unit: microWatt
  				RW
  
  power[1-*]_cap_hyst		Margin of hysteresis built around capping and
  				notification.
  				Unit: microWatt
  				RW
  
  power[1-*]_cap_max		Maximum cap that can be set.
  				Unit: microWatt
  				RO
  
  power[1-*]_cap_min		Minimum cap that can be set.
  				Unit: microWatt
  				RO
  
  power[1-*]_max			Maximum power.
  				Unit: microWatt
  				RW
  
  power[1-*]_crit			Critical maximum power.
  				If power rises to or above this limit, the
  				system is expected take drastic action to reduce
  				power consumption, such as a system shutdown or
  				a forced powerdown of some devices.
  				Unit: microWatt
  				RW
  
  Also see the Alarms section for status flags associated with power readings.
  
  **********
  * Energy *
  **********
  
  energy[1-*]_input		Cumulative energy use
  				Unit: microJoule
  				RO
  
  
  ************
  * Humidity *
  ************
  
  humidity[1-*]_input		Humidity
  				Unit: milli-percent (per cent mille, pcm)
  				RO
  
  
  **********
  * Alarms *
  **********
  
  Each channel or limit may have an associated alarm file, containing a
  boolean value. 1 means than an alarm condition exists, 0 means no alarm.
  
  Usually a given chip will either use channel-related alarms, or
  limit-related alarms, not both. The driver should just reflect the hardware
  implementation.
  
  in[0-*]_alarm
  curr[1-*]_alarm
  power[1-*]_alarm
  fan[1-*]_alarm
  temp[1-*]_alarm
  		Channel alarm
  		0: no alarm
  		1: alarm
  		RO
  
  OR
  
  in[0-*]_min_alarm
  in[0-*]_max_alarm
  in[0-*]_lcrit_alarm
  in[0-*]_crit_alarm
  curr[1-*]_min_alarm
  curr[1-*]_max_alarm
  curr[1-*]_lcrit_alarm
  curr[1-*]_crit_alarm
  power[1-*]_cap_alarm
  power[1-*]_max_alarm
  power[1-*]_crit_alarm
  fan[1-*]_min_alarm
  fan[1-*]_max_alarm
  temp[1-*]_min_alarm
  temp[1-*]_max_alarm
  temp[1-*]_lcrit_alarm
  temp[1-*]_crit_alarm
  temp[1-*]_emergency_alarm
  		Limit alarm
  		0: no alarm
  		1: alarm
  		RO
  
  Each input channel may have an associated fault file. This can be used
  to notify open diodes, unconnected fans etc. where the hardware
  supports it. When this boolean has value 1, the measurement for that
  channel should not be trusted.
  
  fan[1-*]_fault
  temp[1-*]_fault
  		Input fault condition
  		0: no fault occurred
  		1: fault condition
  		RO
  
  Some chips also offer the possibility to get beeped when an alarm occurs:
  
  beep_enable	Master beep enable
  		0: no beeps
  		1: beeps
  		RW
  
  in[0-*]_beep
  curr[1-*]_beep
  fan[1-*]_beep
  temp[1-*]_beep
  		Channel beep
  		0: disable
  		1: enable
  		RW
  
  In theory, a chip could provide per-limit beep masking, but no such chip
  was seen so far.
  
  Old drivers provided a different, non-standard interface to alarms and
  beeps. These interface files are deprecated, but will be kept around
  for compatibility reasons:
  
  alarms		Alarm bitmask.
  		RO
  		Integer representation of one to four bytes.
  		A '1' bit means an alarm.
  		Chips should be programmed for 'comparator' mode so that
  		the alarm will 'come back' after you read the register
  		if it is still valid.
  		Generally a direct representation of a chip's internal
  		alarm registers; there is no standard for the position
  		of individual bits. For this reason, the use of this
  		interface file for new drivers is discouraged. Use
  		individual *_alarm and *_fault files instead.
  		Bits are defined in kernel/include/sensors.h.
  
  beep_mask	Bitmask for beep.
  		Same format as 'alarms' with the same bit locations,
  		use discouraged for the same reason. Use individual
  		*_beep files instead.
  		RW
  
  
  ***********************
  * Intrusion detection *
  ***********************
  
  intrusion[0-*]_alarm
  		Chassis intrusion detection
  		0: OK
  		1: intrusion detected
  		RW
  		Contrary to regular alarm flags which clear themselves
  		automatically when read, this one sticks until cleared by
  		the user. This is done by writing 0 to the file. Writing
  		other values is unsupported.
  
  intrusion[0-*]_beep
  		Chassis intrusion beep
  		0: disable
  		1: enable
  		RW
  
  
  sysfs attribute writes interpretation
  -------------------------------------
  
  hwmon sysfs attributes always contain numbers, so the first thing to do is to
  convert the input to a number, there are 2 ways todo this depending whether
  the number can be negative or not:
  unsigned long u = simple_strtoul(buf, NULL, 10);
  long s = simple_strtol(buf, NULL, 10);
  
  With buf being the buffer with the user input being passed by the kernel.
  Notice that we do not use the second argument of strto[u]l, and thus cannot
  tell when 0 is returned, if this was really 0 or is caused by invalid input.
  This is done deliberately as checking this everywhere would add a lot of
  code to the kernel.
  
  Notice that it is important to always store the converted value in an
  unsigned long or long, so that no wrap around can happen before any further
  checking.
  
  After the input string is converted to an (unsigned) long, the value should be
  checked if its acceptable. Be careful with further conversions on the value
  before checking it for validity, as these conversions could still cause a wrap
  around before the check. For example do not multiply the result, and only
  add/subtract if it has been divided before the add/subtract.
  
  What to do if a value is found to be invalid, depends on the type of the
  sysfs attribute that is being set. If it is a continuous setting like a
  tempX_max or inX_max attribute, then the value should be clamped to its
  limits using clamp_val(value, min_limit, max_limit). If it is not continuous
  like for example a tempX_type, then when an invalid value is written,
  -EINVAL should be returned.
  
  Example1, temp1_max, register is a signed 8 bit value (-128 - 127 degrees):
  
  	long v = simple_strtol(buf, NULL, 10) / 1000;
  	v = clamp_val(v, -128, 127);
  	/* write v to register */
  
  Example2, fan divider setting, valid values 2, 4 and 8:
  
  	unsigned long v = simple_strtoul(buf, NULL, 10);
  
  	switch (v) {
  	case 2: v = 1; break;
  	case 4: v = 2; break;
  	case 8: v = 3; break;
  	default:
  		return -EINVAL;
  	}
  	/* write v to register */