linux/Documentation/hwmon/max31790.rst
Guenter Roeck 148c847c9e hwmon: (max31790) Fix pwmX_enable attributes
pwmX_enable supports three possible values:

0: Fan control disabled. Duty cycle is fixed to 0%
1: Fan control enabled, pwm mode. Duty cycle is determined by
   values written into Target Duty Cycle registers.
2: Fan control enabled, rpm mode
   Duty cycle is adjusted such that fan speed matches
   the values in Target Count registers

The current code does not do this; instead, it mixes pwm control
configuration with fan speed monitoring configuration. Worse, it
reports that pwm control would be disabled (pwmX_enable==0) when
it is in fact enabled in pwm mode. Part of the problem may be that
the chip sets the "TACH input enable" bit on its own whenever the
mode bit is set to RPM mode, but that doesn't mean that "TACH input
enable" accurately reflects the pwm mode.

Fix it up and only handle pwm control with the pwmX_enable attributes.
In the documentation, clarify that disabling pwm control (pwmX_enable=0)
sets the pwm duty cycle to 0%. In the code, explain why TACH_INPUT_EN
is set together with RPM_MODE.

While at it, only update the configuration register if the configuration
has changed, and only update the cached configuration if updating the
chip configuration was successful.

Cc: Jan Kundrát <jan.kundrat@cesnet.cz>
Cc: Václav Kubernát <kubernat@cesnet.cz>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Václav Kubernát <kubernat@cesnet.cz>
Reviewed-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Link: https://lore.kernel.org/r/20210526154022.3223012-4-linux@roeck-us.net
2021-06-17 04:21:46 -07:00

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Kernel driver max31790
======================
Supported chips:
* Maxim MAX31790
Prefix: 'max31790'
Addresses scanned: -
Datasheet: https://pdfserv.maximintegrated.com/en/ds/MAX31790.pdf
Author: Il Han <corone.il.han@gmail.com>
Description
-----------
This driver implements support for the Maxim MAX31790 chip.
The MAX31790 controls the speeds of up to six fans using six independent
PWM outputs. The desired fan speeds (or PWM duty cycles) are written
through the I2C interface. The outputs drive "4-wire" fans directly,
or can be used to modulate the fan's power terminals using an external
pass transistor.
Tachometer inputs monitor fan tachometer logic outputs for precise (+/-1%)
monitoring and control of fan RPM as well as detection of fan failure.
Six pins are dedicated tachometer inputs. Any of the six PWM outputs can
also be configured to serve as tachometer inputs.
Sysfs entries
-------------
================== === =======================================================
fan[1-12]_input RO fan tachometer speed in RPM
fan[1-12]_fault RO fan experienced fault
fan[1-6]_target RW desired fan speed in RPM
pwm[1-6]_enable RW regulator mode, 0=disabled (duty cycle=0%), 1=manual mode, 2=rpm mode
pwm[1-6] RW read: current pwm duty cycle,
write: target pwm duty cycle (0-255)
================== === =======================================================