WSL2-Linux-Kernel/Documentation/hwmon/sysfs-interface.rst

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.

`in[0-*]_rated_min`
		Minimum rated voltage.

		Unit: millivolt

		RO

`in[0-*]_rated_max`
		Maximum rated voltage.

		Unit: millivolt

		RO

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

`temp[1-*]_rated_min`
		Minimum rated temperature.

		Unit: millidegree Celsius

		RO

`temp[1-*]_rated_max`
		Maximum rated temperature.

		Unit: millidegree Celsius

		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
********

`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

`curr[1-*]_rated_min`
		Minimum rated current.

		Unit: milliampere

		RO

`curr[1-*]_rated_max`
		Maximum rated current.

		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

`power[1-*]_enable`
				Enable or disable the sensors.

				When disabled the sensor read will return
				-ENODATA.

				- 1: Enable
				- 0: Disable

				RW

`power[1-*]_rated_min`
				Minimum rated power.

				Unit: microWatt

				RO

`power[1-*]_rated_max`
				Maximum rated power.

				Unit: microWatt

				RO

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

`humidity[1-*]_rated_min`
				Minimum rated humidity.

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

				RO

`humidity[1-*]_rated_max`
				Maximum rated 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`,		| Channel alarm		|
| `curr[1-*]_alarm`,		|			|
| `power[1-*]_alarm`,		|   - 0: no alarm	|
| `fan[1-*]_alarm`,		|   - 1: alarm		|
| `temp[1-*]_alarm`**		|			|
|				|   RO			|
+-------------------------------+-----------------------+

**OR**

+-------------------------------+-----------------------+
| **`in[0-*]_min_alarm`,	| Limit alarm		|
| `in[0-*]_max_alarm`,		|			|
| `in[0-*]_lcrit_alarm`,	|   - 0: no alarm	|
| `in[0-*]_crit_alarm`,		|   - 1: alarm		|
| `curr[1-*]_min_alarm`,	|			|
| `curr[1-*]_max_alarm`,	| RO			|
| `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`**	|			|
+-------------------------------+-----------------------+

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   | Sets number of average samples for specific type of	       |
| power_samples| measurements.						       |
| curr_samples |							       |
| temp_samples | 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 */