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 whitespace, dashes, or the wildcard character ‘*’. This attribute represents the chip name. It 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

in[0-*]_enable

Enable or disable the sensors.

When disabled the sensor read will return -ENODATA.

  • 1: Enable
  • 0: Disable

RW

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

fan[1-*]_enable

Enable or disable the sensors.

When disabled the sensor read will return -ENODATA.

  • 1: Enable
  • 0: Disable

RW

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-*]_min_hyst

Temperature hysteresis value for min limit. Unit: millidegree Celsius

Must be reported as an absolute temperature, NOT a delta from the min 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-*]_lcrit_hyst

Temperature hysteresis value for critical min limit.

Unit: millidegree Celsius

Must be reported as an absolute temperature, NOT a delta from the critical min value.

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

temp[1-*]_enable

Enable or disable the sensors.

When disabled the sensor read will return -ENODATA.

  • 1: Enable
  • 0: Disable

RW

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

curr[1-*]_enable

Enable or disable the sensors.

When disabled the sensor read will return -ENODATA.

  • 1: Enable
  • 0: Disable

RW

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

power[1-*]_enable

Enable or disable the sensors.

When disabled the sensor read will return -ENODATA.

  • 1: Enable
  • 0: Disable

RW

Also see the Alarms section for status flags associated with power readings.

Energy

energy[1-*]_input

Cumulative energy use

Unit: microJoule

RO

energy[1-*]_enable

Enable or disable the sensors.

When disabled the sensor read will return -ENODATA.

  • 1: Enable
  • 0: Disable

RW

Humidity

humidity[1-*]_input

Humidity

Unit: milli-percent (per cent mille, pcm)

RO

humidity[1-*]_enable

Enable or disable the sensors

When disabled the sensor read will return -ENODATA.

  • 1: Enable
  • 0: Disable

RW

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

Average sample configuration

Devices allowing for reading {in,power,curr,temp}_average values may export attributes for controlling number of samples used to compute average.

samples

Sets number of average samples for all types of measurements.

RW
in_samples power_samples curr_samples temp_samples

Sets number of average samples for specific type of measurements.

Note that on some devices it won’t be possible to set all of them to different values so changing one might also change some others.

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 */