[PATCH v4 2/3] input: pwm-beeper: add documentation for volume devicetree bindings

[Frieder Schrempf <frieder.schrempf@exceet.de>]

This patch adds the documentation for the devicetree bindings to set
the volume levels.

Signed-off-by: Frieder Schrempf <frieder.schrempf@exceet.de>
Acked-by: Rob Herring <robh@kernel.org>
---
Changes in v4:
 - none

 .../devicetree/bindings/input/pwm-beeper.txt         | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/Documentation/devicetree/bindings/input/pwm-beeper.txt b/Documentation/devicetree/bindings/input/pwm-beeper.txt
index be332ae..496b68f 100644
--- a/Documentation/devicetree/bindings/input/pwm-beeper.txt
+++ b/Documentation/devicetree/bindings/input/pwm-beeper.txt
@@ -5,3 +5,23 @@ Registers a PWM device as beeper.
 Required properties:
 - compatible: should be "pwm-beeper"
 - pwms: phandle to the physical PWM device
+
+Optional properties:
+- volume-levels: Array of PWM duty cycle values that correspond to
+      linear volume levels. These need to be in the range of 0 to 500,
+      while 0 means 0% duty cycle (mute) and 500 means 50% duty cycle
+      (max volume).
+      Please note that the actual volume of most beepers is highly
+      non-linear, which means that low volume levels are probably somewhere
+      in the range of 1 to 30 (0.1-3% duty cycle).
+- default-volume-level: the default volume level (index into the
+      array defined by the "volume-levels" property)
+
+Example:
+
+	pwm-beeper {
+		compatible = "pwm-beeper";
+		pwms = <&pwm4 0 5000>;
+		volume-levels = <0 8 20 40 500>;
+		default-volume-level = <4>;
+	};
-- 
2.7.4