This adds a short description for the new RPM support of the pwm-fan
driver.

Signed-off-by: Stefan Wahren <stefan.wah...@i2se.com>
---
 Documentation/hwmon/pwm-fan | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Documentation/hwmon/pwm-fan b/Documentation/hwmon/pwm-fan
index 18529d2..a1ba34c 100644
--- a/Documentation/hwmon/pwm-fan
+++ b/Documentation/hwmon/pwm-fan
@@ -15,3 +15,7 @@ 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 measured fan rotation speed returned via 'fan1_input' is derived
+from the sampled interrupts from the tachometer signal by assuming
+a 2 pulse-per-revolution fan.
-- 
2.7.4

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