hwmon: (nct6775) Add support for automatic fan control

[mirror_ubuntu-artful-kernel.git] / Documentation / hwmon / nct6775

Note
====

This driver supersedes the NCT6775F and NCT6776F support in the W83627EHF
driver.

Kernel driver NCT6775
=====================

Supported chips:
  * Nuvoton NCT6775F/W83667HG-I
    Prefix: 'nct6775'
    Addresses scanned: ISA address retrieved from Super I/O registers
    Datasheet: Available from Nuvoton upon request
  * Nuvoton NCT6776F
    Prefix: 'nct6776'
    Addresses scanned: ISA address retrieved from Super I/O registers
    Datasheet: Available from Nuvoton upon request
  * Nuvoton NCT6779D
    Prefix: 'nct6779'
    Addresses scanned: ISA address retrieved from Super I/O registers
    Datasheet: Available from Nuvoton upon request

Authors:
        Guenter Roeck <linux@roeck-us.net>

Description
-----------

This driver implements support for the Nuvoton NCT6775F, NCT6776F, and NCT6779D
super I/O chips.

The chips support up to 25 temperature monitoring sources. Up to 6 of those are
direct temperature sensor inputs, the others are special sources such as PECI,
PCH, and SMBUS. Depending on the chip type, 2 to 6 of the temperature sources
can be monitored and compared against minimum, maximum, and critical
temperatures. The driver reports up to 10 of the temperatures to the user.
There are 4 to 5 fan rotation speed sensors, 8 to 15 analog voltage sensors,
one VID, alarms with beep warnings (control unimplemented), and some automatic
fan regulation strategies (plus manual fan control mode).

The temperature sensor sources on all chips are configurable. The configured
source for each of the temperature sensors is provided in tempX_label.

Temperatures are measured in degrees Celsius and measurement resolution is
either 1 degC or 0.5 degC, depending on the temperature source and
configuration. An alarm is triggered when the temperature gets higher than
the high limit; it stays on until the temperature falls below the hysteresis
value. Alarms are only supported for temp1 to temp6, depending on the chip type.

Fan rotation speeds are reported in RPM (rotations per minute). An alarm is
triggered if the rotation speed has dropped below a programmable limit. On
NCT6775F, fan readings can be divided by a programmable divider (1, 2, 4, 8,
16, 32, 64 or 128) to give the readings more range or accuracy; the other chips
do not have a fan speed divider. The driver sets the most suitable fan divisor
itself; specifically, it increases the divider value each time a fan speed
reading returns an invalid value, and it reduces it if the fan speed reading
is lower than optimal. Some fans might not be present because they share pins
with other functions.

Voltage sensors (also known as IN sensors) report their values in millivolts.
An alarm is triggered if the voltage has crossed a programmable minimum
or maximum limit.

The driver supports automatic fan control mode known as Thermal Cruise.
In this mode, the chip attempts to keep the measured temperature in a
predefined temperature range. If the temperature goes out of range, fan
is driven slower/faster to reach the predefined range again.

The mode works for fan1-fan5.

sysfs attributes
----------------

pwm[1-5] - this file stores PWM duty cycle or DC value (fan speed) in range:
	   0 (lowest speed) to 255 (full)

pwm[1-5]_enable - this file controls mode of fan/temperature control:
	* 0 Fan control disabled (fans set to maximum speed)
	* 1 Manual mode, write to pwm[0-5] any value 0-255
	* 2 "Thermal Cruise" mode
	* 3 "Fan Speed Cruise" mode
	* 4 "Smart Fan III" mode (NCT6775F only)
	* 5 "Smart Fan IV" mode

pwm[1-5]_mode - controls if output is PWM or DC level
        * 0 DC output
        * 1 PWM output

Common fan control attributes
-----------------------------

pwm[1-5]_temp_sel	Temperature source. Value is temperature sensor index.
			For example, select '1' for temp1_input.

Thermal Cruise mode (2)
-----------------------

If the temperature is in the range defined by:

pwm[1-5]_target_temp	Target temperature, unit millidegree Celsius
			(range 0 - 127000)
pwm[1-5]_temp_tolerance
			Target temperature tolerance, unit millidegree Celsius

there are no changes to fan speed. Once the temperature leaves the interval, fan
speed increases (if temperature is higher that desired) or decreases (if
temperature is lower than desired), using the following limits and time
intervals.

pwm[1-5]_start		fan pwm start value (range 1 - 255), to start fan
			when the temperature is above defined range.
pwm[1-5]_floor		lowest fan pwm (range 0 - 255) if temperature is below
			the defined range. If set to 0, the fan is expected to
			stop if the temperature is below the defined range.
pwm[1-5]_step_up_time	milliseconds before fan speed is increased
pwm[1-5]_step_down_time	milliseconds before fan speed is decreased
pwm[1-5]_stop_time	how many milliseconds must elapse to switch
			corresponding fan off (when the temperature was below
			defined range).

Speed Cruise mode (3)
---------------------

This modes tries to keep the fan speed constant.

fan[1-5]_target		Target fan speed
fan[1-5]_tolerance
			Target speed tolerance


Untested; use at your own risk.

Smart Fan IV mode (5)
---------------------

This mode offers multiple slopes to control the fan speed. The slopes can be
controlled by setting the pwm and temperature attributes. When the temperature
rises, the chip will calculate the DC/PWM output based on the current slope.
There are up to seven data points depending on the chip type. Subsequent data
points should be set to higher temperatures and higher pwm values to achieve
higher fan speeds with increasing temperature. The last data point reflects
critical temperature mode, in which the fans should run at full speed.

pwm[1-5]_auto_point[1-7]_pwm
			pwm value to be set if temperature reaches matching
			temperature range.
pwm[1-5]_auto_point[1-7]_temp
			Temperature over which the matching pwm is enabled.
pwm[1-5]_temp_tolerance
			Temperature tolerance, unit millidegree Celsius
pwm[1-5]_crit_temp_tolerance
			Temperature tolerance for critical temperature,
			unit millidegree Celsius

pwm[1-5]_step_up_time	milliseconds before fan speed is increased
pwm[1-5]_step_down_time	milliseconds before fan speed is decreased

Usage Notes
-----------

On various ASUS boards with NCT6776F, it appears that CPUTIN is not really
connected to anything and floats, or that it is connected to some non-standard
temperature measurement device. As a result, the temperature reported on CPUTIN
will not reflect a usable value. It often reports unreasonably high
temperatures, and in some cases the reported temperature declines if the actual
temperature increases (similar to the raw PECI temperature value - see PECI
specification for details). CPUTIN should therefore be be ignored on ASUS
boards. The CPU temperature on ASUS boards is reported from PECI 0.