diff --git a/Documentation/hwmon/pwm-fan b/Documentation/hwmon/pwm-fan
index 18529d2e3bcfa711e9a7e2ddc13da36b7dc98000..82fe96742feea339372b40c7be4cf06497bd8dd5 100644
--- a/Documentation/hwmon/pwm-fan
+++ b/Documentation/hwmon/pwm-fan
@@ -15,3 +15,6 @@ The driver implements a simple interface for driving a fan connected to
 a PWM output. It uses the generic PWM interface, thus it can be used with
 a range of SoCs. The driver exposes the fan to the user space through
 the hwmon's sysfs interface.
+
+The fan rotation speed returned via the optional 'fan1_input' is extrapolated
+from the sampled interrupts from the tachometer signal within 1 second.