hwmon/f71805f: Document the fan control features

Document the different fan control methods, list their options, and give
some hints for best results.
Signed-off-by: NJean Delvare <khali@linux-fr.org>

Documentation/hwmon/f71805f

@@ -13,8 +13,8 @@ Thanks to Denis Kieft from Barracuda Networks for the donation of a
 test system (custom Jetway K8M8MS motherboard, with CPU and RAM) and
 for providing initial documentation.
 
-Thanks to Kris Chen from Fintek for answering technical questions and
-providing additional documentation.
+Thanks to Kris Chen and Aaron Huang from Fintek for answering technical
+questions and providing additional documentation.
 
 Thanks to Chris Lin from Jetway for providing wiring schematics and
 answering technical questions.
@@ -28,8 +28,7 @@ capabilities. It can monitor up to 9 voltages (counting its own power
 source), 3 fans and 3 temperature sensors.
 
 This chip also has fan controlling features, using either DC or PWM, in
-three different modes (one manual, two automatic). The driver doesn't
-support these features yet.
+three different modes (one manual, two automatic).
 
 The driver assumes that no more than one chip is present, which seems
 reasonable.
@@ -103,3 +102,38 @@ sensor. Each channel can be used for connecting either a thermal diode
 or a thermistor. The driver reports the currently selected mode, but
 doesn't allow changing it. In theory, the BIOS should have configured
 everything properly.
+
+
+Fan Control
+-----------
+
+Both PWM (pulse-width modulation) and DC fan speed control methods are
+supported. The right one to use depends on external circuitry on the
+motherboard, so the driver assumes that the BIOS set the method
+properly. The driver will report the method, but won't let you change
+it.
+
+When the PWM method is used, you can select the operating frequency,
+from 187.5 kHz (default) to 31 Hz. The best frequency depends on the
+fan model. As a rule of thumb, lower frequencies seem to give better
+control, but may generate annoying high-pitch noise. Fintek recommends
+not going below 1 kHz, as the fan tachometers get confused by lower
+frequencies as well.
+
+When the DC method is used, Fintek recommends not going below 5 V, which
+corresponds to a pwm value of 106 for the driver. The driver doesn't
+enforce this limit though.
+
+Three different fan control modes are supported:
+
+* Manual mode
+  You ask for a specific PWM duty cycle or DC voltage.
+
+* Fan speed mode
+  You ask for a specific fan speed. This mode assumes that pwm1
+  corresponds to fan1, pwm2 to fan2 and pwm3 to fan3.
+
+* Temperature mode
+  You define 3 temperature/fan speed trip points, and the fan speed is
+  adjusted depending on the measured temperature, using interpolation.
+  This mode is not yet supported by the driver.