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. 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
possible.

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
preserved.

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*.

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
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.


********
* Name *
********

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


************
* Voltages *
************

in[0-*]_min	Voltage min value.
		Unit: millivolt
		RW
		
in[0-*]_max	Voltage max value.
		Unit: millivolt
		RW
		
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-*]_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-*]_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

OR

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


****************
* 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 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-*]_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

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
		RW

curr[1-*]_min	Current min value.
		Unit: milliampere
		RW

curr[1-*]_input	Current input value
		Unit: milliampere
		RO

*********
* Power *
*********

power[1-*]_average		Average power use
				Unit: microWatt
				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-*]_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

**********
* 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
fan[1-*]_alarm
temp[1-*]_alarm
		Channel alarm
		0: no alarm
		1: alarm
		RO

OR

in[0-*]_min_alarm
in[0-*]_max_alarm
fan[1-*]_min_alarm
temp[1-*]_min_alarm
temp[1-*]_max_alarm
temp[1-*]_crit_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.

in[0-*]_fault
fan[1-*]_fault
temp[1-*]_fault
		Input fault condition
		0: no fault occured
		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
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


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 */