hwmon: (pc87427) Add support for manual fan speed control

Add initial support for PWM outputs of the PC87427 Super-I/O chip.
Only mode change and manual fan speed control are supported. Automatic
mode configuration isn't supported, and won't be until at least one
board is known, which makes uses of the PWM outputs.
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Guenter Roeck <guenter.roeck@ericsson.com>

Documentation/hwmon/pc87427

@@ -20,8 +20,8 @@ The National Semiconductor Super I/O chip includes complete hardware
 monitoring capabilities. It can monitor up to 18 voltages, 8 fans and
 6 temperature sensors. Only the fans are supported at the moment.
 
-This chip also has fan controlling features, which are not yet supported
-by this driver either.
+This chip also has fan controlling features (up to 4 PWM outputs),
+which are partly supported by this driver.
 
 The driver assumes that no more than one chip is present, which seems
 reasonable.
@@ -36,3 +36,12 @@ signal. Speeds down to 83 RPM can be measured.
 An alarm is triggered if the rotation speed drops below a programmable
 limit. Another alarm is triggered if the speed is too low to be measured
 (including stalled or missing fan).
+
+
+Fan Speed Control
+-----------------
+
+Fan speed can be controlled by PWM outputs. There are 4 possible modes:
+always off, always on, manual and automatic. The latter isn't supported
+by the driver: you can only return to that mode if it was the original
+setting, and the configuration interface is missing.

drivers/hwmon/Kconfig

@@ -711,7 +711,8 @@ config SENSORS_PC87427
 	  functions of the National Semiconductor PC87427 Super-I/O chip.
 	  The chip has two distinct logical devices, one for fan speed
 	  monitoring and control, and one for voltage and temperature
-	  monitoring. Only fan speed monitoring is supported right now.
+	  monitoring. Only fan speed monitoring and control is supported
+	  right now.
 
 	  This driver can also be built as a module.  If so, the module
 	  will be called pc87427.