Re: [PATCH v2 2/3] input: pwm-beeper: add documentation for volume devicetree bindings

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

On 10.10.2016 17:20, Rob Herring wrote:
> On Fri, Oct 07, 2016 at 09:08:17AM +0000, Schrempf Frieder wrote:
>> This patch adds the documentation for the devicetree bindings to set
>> the volume levels.
>>
>> Signed-off-by: Frieder Schrempf <frieder.schrempf@exceet.de>
>> ---
>> Changes in v2:
>>  - split into 3 separate patches
>>  - make volume properties optional
>>
>>  .../devicetree/bindings/input/pwm-beeper.txt       | 22 ++++++++++++++++++++++
>>  1 file changed, 22 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/input/pwm-beeper.txt b/Documentation/devicetree/bindings/input/pwm-beeper.txt
>> index be332ae..6d8ba4e 100644
>> --- a/Documentation/devicetree/bindings/input/pwm-beeper.txt
>> +++ b/Documentation/devicetree/bindings/input/pwm-beeper.txt
>> @@ -5,3 +5,25 @@ 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 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).
>> +      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).
>
> What does the index correspond to? The linear volume?

In most cases users probably need linear volume levels (e.g. 0%, 25%, 
50%, 75%, 100%) and in this case the index would indeed correspond to 
the linear perceived volume.

But also non-linear relations are possible (e.g. 0%, 20%, 100%), if the 
user needs for example "mute", "low", "high" as volume levels.

The linearization (defining the corresponding duty cycle for each index) 
depends on the beeper and the perception of the user.

For the example array definition below, I tried different duty cycles 
and found values of 0.8%, 2%, 4%, 50% to be approximately correspondent 
to perceived volume levels of 25%, 50%, 75%, 100% in my case.

>
>> +- 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>;
>> +	};
>> --
>> 1.9.1