path: root/Documentation/hwmon/sysfs-interface

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-*]_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: PII/Celeron 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

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 SENSORS_LIMIT(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 = SENSORS_LIMIT(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 */