dt-bindings: hwmon: pwm-fan: Document start from stopped state properties

Delta AFC0612DB-F00 fan has to be set to at least 30% PWM duty cycle
to spin up from a stopped state, and can be afterward throttled down to
lower PWM duty cycle. Introduce support for operating such fans which
need to start at higher PWM duty cycle first and can slow down next.

Document two new DT properties, "fan-stop-to-start-percent" and
"fan-stop-to-start-usec". The former describes the minimum percent
of fan RPM at which it will surely spin up from stopped state. This
value can be found in the fan datasheet and can be converted to PWM
duty cycle easily. The "fan-stop-to-start-usec" describes the minimum
time in microseconds for which the fan has to be set to stopped state
start RPM for the fan to surely spin up.

Signed-off-by: Marek Vasut <[email protected]>
Acked-by: Conor Dooley <[email protected]>
Message-ID: <[email protected]>
Signed-off-by: Guenter Roeck <[email protected]>

Author: Marek Vasut

Documentation/devicetree/bindings/hwmon/pwm-fan.yaml

@@ -31,6 +31,16 @@ properties:
       it must be self resetting edge interrupts.
     maxItems: 1
 
+  fan-stop-to-start-percent:
+    description:
+      Minimum fan RPM in percent to start when stopped.
+    minimum: 0
+    maximum: 100
+
+  fan-stop-to-start-us:
+    description:
+      Time to wait in microseconds after start when stopped.
+
   pulses-per-revolution:
     description:
       Define the number of pulses per fan revolution for each tachometer