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

[Frieder Schrempf]

On 11.10.2016 15:39, Rob Herring wrote:
> On Tue, Oct 11, 2016 at 3:17 AM, Schrempf Frieder
> <frieder.schrempf@xxxxxxxxx> wrote:
> > 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@xxxxxxxxx>
> > > > ---
> > > > 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.
>
> Exclude off/mute and this is still linear. Also, the user exposed
> levels could be a subset of the defined h/w levels. That should be
> independent of DT.

Ok
> > The linearization (defining the corresponding duty cycle for each index)
> > depends on the beeper and the perception of the user.
>
> This has to be a consistent interface across h/w to have a userspace
> that can work across h/w. For that, you have to define the binding as
> linear. Of course, it's all measured by perception and not completely
> accurate which is fine.

I see. I will resend and change the description to make clear, that the
volume-levels property is meant to specify linear volume levels.
> > 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.
>
> Also, drop this. Not relevant to the binding.

Ok, I will drop this.
> Rob