hwmon: add driver for NZXT RGB&Fan Controller/Smart Device v2.

This driver implements monitoring and control of fans plugged into the
device. Besides typical speed monitoring and PWM duty cycle control,
voltage and current are reported for every fan.

The device also has 2 connectors for RGB LEDs, support for them isn't
implemented (mainly because there is no standardized sysfs interface).

Also, the device has a noise sensor, but the sensor seems to be completely
useless (and very imprecise), so support for it isn't implemented too.

The driver coexists with userspace tools that access the device through
hidraw interface with no known issues.

The driver has been tested on x86_64, built in and as a module.

Some changes/improvements were suggested by Jonas Malaco.
Signed-off-by: Aleksandr Mezin <mezin.alexander@gmail.com>
Link: https://lore.kernel.org/r/20211031033058.151014-1-mezin.alexander@gmail.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/index.rst

@@ -154,6 +154,7 @@ Hardware Monitoring Kernel Drivers
    nsa320
    ntc_thermistor
    nzxt-kraken2
+   nzxt-smart2
    occ
    pc87360
    pc87427

Documentation/hwmon/nzxt-smart2.rst

+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+Kernel driver nzxt-smart2
+=========================
+
+Supported devices:
+
+- NZXT RGB & Fan controller
+- NZXT Smart Device v2
+
+Description
+-----------
+
+This driver implements monitoring and control of fans plugged into the device.
+Besides typical speed monitoring and PWM duty cycle control, voltage and current
+is reported for every fan.
+
+The device also has two connectors for RGB LEDs; support for them isn't
+implemented (mainly because there is no standardized sysfs interface).
+
+Also, the device has a noise sensor, but the sensor seems to be completely
+useless (and very imprecise), so support for it isn't implemented too.
+
+Usage Notes
+-----------
+
+The device should be autodetected, and the driver should load automatically.
+
+If fans are plugged in/unplugged while the system is powered on, the driver
+must be reloaded to detect configuration changes; otherwise, new fans can't
+be controlled (`pwm*` changes will be ignored). It is necessary because the
+device has a dedicated "detect fans" command, and currently, it is executed only
+during initialization. Speed, voltage, current monitoring will work even without
+reload. As an alternative to reloading the module, a userspace tool (like
+`liquidctl`_) can be used to run "detect fans" command through hidraw interface.
+
+The driver coexists with userspace tools that access the device through hidraw
+interface with no known issues.
+
+.. _liquidctl: https://github.com/liquidctl/liquidctl
+
+Sysfs entries
+-------------
+
+=======================	========================================================
+fan[1-3]_input		Fan speed monitoring (in rpm).
+curr[1-3]_input		Current supplied to the fan (in milliamperes).
+in[0-2]_input		Voltage supplied to the fan (in millivolts).
+pwm[1-3]		Controls fan speed: PWM duty cycle for PWM-controlled
+			fans, voltage for other fans. Voltage can be changed in
+			9-12 V range, but the value of the sysfs attribute is
+			always in 0-255 range (1 = 9V, 255 = 12V). Setting the
+			attribute to 0 turns off the fan completely.
+pwm[1-3]_enable		1 if the fan can be controlled by writing to the
+			corresponding pwm* attribute, 0 otherwise. The device
+			can control only the fans it detected itself, so the
+			attribute is read-only.
+pwm[1-3]_mode		Read-only, 1 for PWM-controlled fans, 0 for other fans
+			(or if no fan connected).
+update_interval		The interval at which all inputs are updated (in
+			milliseconds). The default is 1000ms. Minimum is 250ms.
+=======================	========================================================

MAINTAINERS

@@ -13805,6 +13805,13 @@ S:	Maintained
 F:	Documentation/hwmon/nzxt-kraken2.rst
 F:	drivers/hwmon/nzxt-kraken2.c
 
+NZXT-SMART2 HARDWARE MONITORING DRIVER
+M:	Aleksandr Mezin <mezin.alexander@gmail.com>
+L:	linux-hwmon@vger.kernel.org
+S:	Maintained
+F:	Documentation/hwmon/nzxt-smart2.rst
+F:	drivers/hwmon/nzxt-smart2.c
+
 OBJAGG
 M:	Jiri Pirko <jiri@nvidia.com>
 L:	netdev@vger.kernel.org

drivers/hwmon/Kconfig

@@ -1513,6 +1513,16 @@ config SENSORS_NZXT_KRAKEN2
 	  This driver can also be built as a module. If so, the module
 	  will be called nzxt-kraken2.
 
+config SENSORS_NZXT_SMART2
+	tristate "NZXT RGB & Fan Controller/Smart Device v2"
+	depends on USB_HID
+	help
+	  If you say yes here you get support for hardware monitoring for the
+	  NZXT RGB & Fan Controller/Smart Device v2.
+
+	  This driver can also be built as a module. If so, the module
+	  will be called nzxt-smart2.
+
 source "drivers/hwmon/occ/Kconfig"
 
 config SENSORS_PCF8591

drivers/hwmon/Makefile

@@ -160,6 +160,7 @@ obj-$(CONFIG_SENSORS_NPCM7XX)	+= npcm750-pwm-fan.o
 obj-$(CONFIG_SENSORS_NSA320)	+= nsa320-hwmon.o
 obj-$(CONFIG_SENSORS_NTC_THERMISTOR)	+= ntc_thermistor.o
 obj-$(CONFIG_SENSORS_NZXT_KRAKEN2) += nzxt-kraken2.o
+obj-$(CONFIG_SENSORS_NZXT_SMART2) += nzxt-smart2.o
 obj-$(CONFIG_SENSORS_PC87360)	+= pc87360.o
 obj-$(CONFIG_SENSORS_PC87427)	+= pc87427.o
 obj-$(CONFIG_SENSORS_PCF8591)	+= pcf8591.o