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Naming and data format standards for sysfs files
The libsensors library offers an interface to the raw sensors data
through the sysfs interface. See libsensors documentation and source for
further information. As of writing this document, libsensors
(from lm_sensors 2.8.3) is heavily chip-dependent. Adding or updating
support for any given chip requires modifying the library's code.
This is because libsensors was written for the procfs interface
older kernel modules were using, which wasn't standardized enough.
Recent versions of libsensors (from lm_sensors 2.8.2 and later) have
support for the sysfs interface, though.
The new sysfs interface was designed to be as chip-independent as
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.
If you are developing a userspace application please send us feedback on
this standard.
Note that this standard isn't completely established yet, so it is subject
to changes. If you are writing a new hardware monitoring driver those
features can't seem to fit in this interface, please contact us with your
extension proposal. Keep in mind that backward compatibility must be
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
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.
[0-*] denotes any positive number starting from 0
[1-*] denotes any positive number starting from 1
RO read only value
RW read/write value
Read/write values may be read-only for some chips, depending on the
hardware implementation.
All entries are optional, and should only be created in a given driver
if the chip has the feature.
* Voltages *
in[0-*]_min Voltage min value.
Unit: millivolt
in[0-*]_max Voltage max value.
Unit: millivolt
in[0-*]_input Voltage input value.
Unit: millivolt
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.
Typical usage:
in0_* CPU #1 voltage (not scaled)
in1_* CPU #2 voltage (not scaled)
in2_* 3.3V nominal (not scaled)
in3_* 5.0V nominal (scaled)
in4_* 12.0V nominal (scaled)
in5_* -12.0V nominal (scaled)
in6_* -5.0V nominal (scaled)
in7_* varies
in8_* varies
cpu[0-*]_vid CPU core reference voltage.
Unit: millivolt
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
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)
fan[1-*]_input Fan input value.
Unit: revolution/min (RPM)
fan[1-*]_div Fan divisor.
Integer value in powers of two (1, 2, 4, 8, 16, 32, 64, 128).
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.
Desired fan speed
Unit: revolution/min (RPM)
Only makes sense if the chip supports closed-loop fan speed
control based on the measured fan speed.
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
255 is max or 100%.
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
pwm[1-*]_mode 0: DC mode (direct current)
1: PWM mode (pulse-width modulation)
pwm[1-*]_freq Base PWM frequency in Hz.
Only possibly available when pwmN_mode is PWM, but not always
present even then.
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.
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.
Define the PWM vs temperature curve. Number of trip points is
chip-dependent. Use this for chips which associate trip points
to temperature channels.
* Temperatures *
temp[1-*]_type Sensor type selection.
Integers 1 to 6 or thermistor Beta value (typically 3435)
1: PII/Celeron Diode
2: 3904 transistor
3: thermal diode
4: thermistor (default/unknown Beta)
6: Intel PECI
Not all types are supported by all chips
temp[1-*]_max Temperature max value.
Unit: millidegree Celsius (or millivolt, see below)
temp[1-*]_min Temperature min value.
Unit: millidegree Celsius
Temperature hysteresis value for max limit.
Unit: millidegree Celsius
Must be reported as an absolute temperature, NOT a delta
from the max value.
temp[1-*]_input Temperature input value.
Unit: millidegree Celsius
temp[1-*]_crit Temperature critical value, typically greater than
corresponding temp_max values.
Unit: millidegree Celsius
Temperature hysteresis value for critical limit.
Unit: millidegree Celsius
Must be reported as an absolute temperature, NOT a delta
from the critical value.
Temperature offset which is added to the temperature reading
by the chip.
Unit: millidegree Celsius
Read/Write value.
If there are multiple temperature sensors, temp1_* is
generally the sensor inside the chip itself,
reported as "motherboard temperature". temp2_* to
temp4_* are generally sensors external to the chip
itself, for example the thermal diode inside the CPU or
a thermistor nearby.
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 *
Note that no known chip provides current measurements as of writing,
so this part is theoretical, so to say.
curr[1-*]_max Current max value
Unit: milliampere
curr[1-*]_min Current min value.
Unit: milliampere
curr[1-*]_input Current input value
Unit: milliampere
* 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
Channel alarm
0: no alarm
1: alarm
Limit alarm
0: no alarm
1: alarm
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.
Input fault condition
0: no fault occured
1: fault condition
Some chips also offer the possibility to get beeped when an alarm occurs:
beep_enable Master beep enable
0: no beeps
1: beeps
Channel beep
0: disable
1: enable
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.
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.
* Other *
eeprom Raw EEPROM data in binary form.
pec Enable or disable PEC (SMBus only)
0: disable
1: enable