-The thinkpad-acpi kernel driver can be programmed to revert the fan
-level to a safe setting if userspace does not issue one of the fan
-commands: "enable", "disable", "level" or "watchdog" within a
-configurable ammount of time. To do this, use the "watchdog" command.
-
- echo 'watchdog <interval>' > /proc/acpi/ibm/fan
-
-Interval is the ammount of time in seconds to wait for one of the
-above mentioned fan commands before reseting the fan level to a safe
-one. If set to zero, the watchdog is disabled (default). When the
-watchdog timer runs out, it does the exact equivalent of the "enable"
-fan command.
-
-Note that the watchdog timer stops after it enables the fan. It will
-be rearmed again automatically (using the same interval) when one of
-the above mentioned fan commands is received. The fan watchdog is,
-therefore, not suitable to protect against fan mode changes made
-through means other than the "enable", "disable", and "level" fan
-commands.
-
-EXPERIMENTAL: WAN -- /proc/acpi/ibm/wan
----------------------------------------
+ echo 'watchdog <interval in seconds>' > /proc/acpi/ibm/fan
+
+If you want to disable the watchdog, use 0 as the interval.
+
+Sysfs notes:
+
+The sysfs interface follows the hwmon subsystem guidelines for the most
+part, and the exception is the fan safety watchdog.
+
+Writes to any of the sysfs attributes may return the EINVAL error if
+that operation is not supported in a given ThinkPad or if the parameter
+is out-of-bounds, and EPERM if it is forbidden. They may also return
+EINTR (interrupted system call), and EIO (I/O error while trying to talk
+to the firmware).
+
+Features not yet implemented by the driver return ENOSYS.
+
+hwmon device attribute pwm1_enable:
+ 0: PWM offline (fan is set to full-speed mode)
+ 1: Manual PWM control (use pwm1 to set fan level)
+ 2: Hardware PWM control (EC "auto" mode)
+ 3: reserved (Software PWM control, not implemented yet)
+
+ Modes 0 and 2 are not supported by all ThinkPads, and the
+ driver is not always able to detect this. If it does know a
+ mode is unsupported, it will return -EINVAL.
+
+hwmon device attribute pwm1:
+ Fan level, scaled from the firmware values of 0-7 to the hwmon
+ scale of 0-255. 0 means fan stopped, 255 means highest normal
+ speed (level 7).
+
+ This attribute only commands the fan if pmw1_enable is set to 1
+ (manual PWM control).
+
+hwmon device attribute fan1_input:
+ Fan tachometer reading, in RPM. May go stale on certain
+ ThinkPads while the EC transitions the PWM to offline mode,
+ which can take up to two minutes. May return rubbish on older
+ ThinkPads.
+
+driver attribute fan_watchdog:
+ Fan safety watchdog timer interval, in seconds. Minimum is
+ 1 second, maximum is 120 seconds. 0 disables the watchdog.
+
+To stop the fan: set pwm1 to zero, and pwm1_enable to 1.
+
+To start the fan in a safe mode: set pwm1_enable to 2. If that fails
+with EINVAL, try to set pwm1_enable to 1 and pwm1 to at least 128 (255
+would be the safest choice, though).
+
+
+EXPERIMENTAL: WAN
+-----------------
+
+procfs: /proc/acpi/ibm/wan
+sysfs device attribute: wwan_enable