Thank you Rob for looking at my first kernel contribution.
I come back to it now, as I had some time to process your comments.

On 11.04.2016 17:21, Rob Herring wrote:

On Thu, Apr 07, 2016 at 03:58:40PM +0000, Schrempf Frieder wrote:

quoted

Make the driver accept different volume levels via sysfs.
This can be helpful if the beep/bell sound intensity needs
to be adapted to the environment of the device.

It is preferred that bindings be a separate patch. Also, this is 2
independent features: the sysfs interface and DT properties. So that is
probably 3 patches.

Also, sysfs interfaces are supposed to be documented.

I have splitted this into 3 patches and will send them soon as v2.
I also added documentation for the sysfs interface.

quoted

The number of volume levels available and their values can
be specified via device tree (similar to pwm-backlight).

This patch was tested with linux-imx 3.10.17 and was
applied to current mainline without any changes.

You need to test with something not 3 years old...

I tested the v2 patches with Linux 4.7.4 now.

quoted

Signed-off-by: Frieder Schrempf <frieder.schrempf-wPoT/lNZgHizQB+pC5nmwQ@public.gmane.org>
---
 .../devicetree/bindings/input/pwm-beeper.txt    |  20 ++++
 drivers/input/misc/pwm-beeper.c                 | 109 ++++++++++++++++++-
 2 files changed, 126 insertions(+), 3 deletions(-)

diff --git a/Documentation/devicetree/bindings/input/pwm-beeper.txt b/Documentation/devicetree/bindings/input/pwm-beeper.txt
index be332ae..153cd3f 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
+- volume-levels: Array of distinct 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).

Where does the scale of 0-500 come from? It is tenth of a percent?

I used tenth of a percent because this is needed to define low volume 
levels with pwm duty cycles in the range of <1%.
This is necessary because of the highly non-linear relation between duty 
cycle and actual perceived volume.

The upper limit of 500 therefore results from 50% being the duty cycle 
with maximum volume.

How many array entries?

The array size and therefore the number of volume levels is dynamic and 
only defined by the number of entries in the volume-levels property.

quoted

+      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)
+
+The volume level can be set via sysfs under /sys/class/input/inputX/volume.
+The maximum volume level index can be read from /sys/class/input/inputX/max_volume_level.
+
+Example:
+
+	pwm-beeper {
+		compatible = "pwm-beeper";
+		pwms = <&pwm4 0 5000>;
+		volume-levels = <0 8 20 40 500>;
+		default-volume-level = <4>;
+	};--

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