xref: /openbmc/linux/Documentation/hwmon/g762.rst (revision 7ebd8b66)

Kernel driver g762
==================

The GMT G762 Fan Speed PWM Controller is connected directly to a fan
and performs closed-loop or open-loop control of the fan speed. Two
modes - PWM or DC - are supported by the device.

For additional information, a detailed datasheet is available at
http://natisbad.org/NAS/ref/GMT_EDS-762_763-080710-0.2.pdf. sysfs
bindings are described in Documentation/hwmon/sysfs-interface.rst.

The following entries are available to the user in a subdirectory of
/sys/bus/i2c/drivers/g762/ to control the operation of the device.
This can be done manually using the following entries but is usually
done via a userland daemon like fancontrol.

Note that those entries do not provide ways to setup the specific
hardware characteristics of the system (reference clock, pulses per
fan revolution, ...); Those can be modified via devicetree bindings
documented in Documentation/devicetree/bindings/hwmon/g762.txt or
using a specific platform_data structure in board initialization
file (see include/linux/platform_data/g762.h).

  fan1_target:
	    set desired fan speed. This only makes sense in closed-loop
	    fan speed control (i.e. when pwm1_enable is set to 2).

  fan1_input:
	    provide current fan rotation value in RPM as reported by
	    the fan to the device.

  fan1_div:
	    fan clock divisor. Supported value are 1, 2, 4 and 8.

  fan1_pulses:
	    number of pulses per fan revolution. Supported values
	    are 2 and 4.

  fan1_fault:
	    reports fan failure, i.e. no transition on fan gear pin for
	    about 0.7s (if the fan is not voluntarily set off).

  fan1_alarm:
	    in closed-loop control mode, if fan RPM value is 25% out
	    of the programmed value for over 6 seconds 'fan1_alarm' is
	    set to 1.

  pwm1_enable:
	    set current fan speed control mode i.e. 1 for manual fan
	    speed control (open-loop) via pwm1 described below, 2 for
	    automatic fan speed control (closed-loop) via fan1_target
	    above.

  pwm1_mode:
	    set or get fan driving mode: 1 for PWM mode, 0 for DC mode.

  pwm1:
	    get or set PWM fan control value in open-loop mode. This is an
	    integer value between 0 and 255. 0 stops the fan, 255 makes
	    it run at full speed.

Both in PWM mode ('pwm1_mode' set to 1) and DC mode ('pwm1_mode' set to 0),
when current fan speed control mode is open-loop ('pwm1_enable' set to 1),
the fan speed is programmed by setting a value between 0 and 255 via 'pwm1'
entry (0 stops the fan, 255 makes it run at full speed). In closed-loop mode
('pwm1_enable' set to 2), the expected rotation speed in RPM can be passed to
the chip via 'fan1_target'. In closed-loop mode, the target speed is compared
with current speed (available via 'fan1_input') by the device and a feedback
is performed to match that target value. The fan speed value is computed
based on the parameters associated with the physical characteristics of the
system: a reference clock source frequency, a number of pulses per fan
revolution, etc.

Note that the driver will update its values at most once per second.