hwmon: (it87) Add support for old automatic fan speed control

Add support for the automatic fan speed control interface as
implemented by IT8705F chips up to revision F and IT8712F chips up to
revision G. This implementation fits very well in our standard sysfs
interface.

I implemented the old and not the new interface because the only chip
I have at hand is an old one, and the new interface is more difficult
to map to the standard sysfs interface. Adding support later should be
possible though, if someone with a supported chip is interested.

Signed-off-by: Jean Delvare <khali@linux-fr.org>

1 files changed, 25 insertions, 2 deletions

diff --git a/Documentation/hwmon/it87 b/Documentation/hwmon/it87
index a96a70f..0083bd3 100644
--- a/Documentation/hwmon/it87
+++ b/Documentation/hwmon/it87
@@ -146,11 +146,34 @@ Fan speed control
 -----------------
 
 The fan speed control features are limited to manual PWM mode. Automatic
-"Smart Guardian" mode control handling is not implemented. However
-if you want to go for "manual mode" just write 1 to pwmN_enable.
+"Smart Guardian" mode control handling is only implemented for older chips
+(see below.) However if you want to go for "manual mode" just write 1 to
+pwmN_enable.
 
 If you are only able to control the fan speed with very small PWM values,
 try lowering the PWM base frequency (pwm1_freq). Depending on the fan,
 it may give you a somewhat greater control range. The same frequency is
 used to drive all fan outputs, which is why pwm2_freq and pwm3_freq are
 read-only.
+
+
+Automatic fan speed control (old interface)
+-------------------------------------------
+
+The driver supports the old interface to automatic fan speed control
+which is implemented by IT8705F chips up to revision F and IT8712F
+chips up to revision G.
+
+This interface implements 4 temperature vs. PWM output trip points.
+The PWM output of trip point 4 is always the maximum value (fan running
+at full speed) while the PWM output of the other 3 trip points can be
+freely chosen. The temperature of all 4 trip points can be freely chosen.
+Additionally, trip point 1 has an hysteresis temperature attached, to
+prevent fast switching between fan on and off.
+
+The chip automatically computes the PWM output value based on the input
+temperature, based on this simple rule: if the temperature value is
+between trip point N and trip point N+1 then the PWM output value is
+the one of trip point N. The automatic control mode is less flexible
+than the manual control mode, but it reacts faster, is more robust and
+doesn't use CPU cycles.