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Date:   Thu, 5 Oct 2023 13:58:24 +0100
From:   Daniel Thompson <daniel.thompson@linaro.org>
To:     Flavio Suligoi <f.suligoi@asem.it>
Cc:     Lee Jones <lee@kernel.org>, Jingoo Han <jingoohan1@gmail.com>,
        Helge Deller <deller@gmx.de>, Pavel Machek <pavel@ucw.cz>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Conor Dooley <conor+dt@kernel.org>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        "linux-leds@vger.kernel.org" <linux-leds@vger.kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        "linux-fbdev@vger.kernel.org" <linux-fbdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Rob Herring <robh@kernel.org>
Subject: Re: [PATCH v3 1/2] dt-bindings: backlight: Add MPS MP3309C
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On Wed, Oct 04, 2023 at 03:18:24PM +0000, Flavio Suligoi wrote:
> Hi Daniel,
> ...
> > ...
> > > > > > +required:
> > > > > > +  - compatible
> > > > > > +  - reg
> > > > > > +  - max-brightness
> > > > >
> > > > > Why is this mandatory?
> > > > >
> > > > > There's no point in setting max-brightness when running in I2C
> > > > > mode
> > > > > (max- brightness should default to 31 in that case).
> > > > >
> > > > >
> > > > > > +  - default-brightness
> > > > >
> > > > > Again. I'm not clear why this needs to be mandatory.
> > > > >
> > > > >
> > > >
> > > > Ok, you are right, I'll remove max-brightness and default-brightness
> > > > from required properties list. I think to change these properties,
> > > > for the pwm dimming, into a clearer:
> > > >
> > > > - brightness-levels (uint32)
> > > > - default-brightness-levels (uint32).
> > > >
> > > > For example:
> > > >
> > > >   brightness-levels:
> > > >     description:
> > > >       Number of brightness levels. The actual brightness
> > > >       level (PWM duty cycle) will be interpolated from 0 to this value.
> > > >       0 means a  0% duty cycle (darkest/off), while the
> > > > brightness-levels
> > > represents
> > > >       a 100% duty cycle (brightest).
> > > >     $ref: /schemas/types.yaml#/definitions/uint32
> > > >
> > > >   default-brightness-level:
> > > >     description:
> > > >       The default brightness level (from 0 to brightness-levels)
> > > >     $ref: /schemas/types.yaml#/definitions/uint32
> > > >
> > > > Example:
> > > > brightness-levels = <10>;
> > > > default-brightness-level = <6>;
> > > >
> > > > What do you think about this solution?
> > >
> > > If you want to introduce a brightness-levels property then I would
> > > expect it to be defined with the same meaning as pwm-backlight (it's
> > > not relevant to the bindings but ideally it would be implemented by
> > > refactoring and reusing the code from pwm_bl.c).
> >
> > ok, I'll use the brightness-levels property as used in pwm-backlight
> >
> > >
> > > Same with default-brightness-level although I'm not sure why one
> > > wouldn't just use default-brightness for new bindings (doesn't
> > > default-brightness-level simply do exactly the same thing as default-
> > brightness).
> >
> > ok for default-brightness instead of default-brightness-level
>
> Just a question: default-brightness-level is the index into the brightness-levels array.
> But, if I use default-brightness instead of default-brightness-level,
> should I consider default-brightness also as an index into brightness-levels array?

Yes.


> Or, in this case, have the default-brightness to be equal to one of the values inside the
> brightness-levels array?

When there is a brightness array (and there is no interpolation) then
it is indexed by brightness. The values in the array are not
brightness (e.g. the controlable value describing the output of the
hardware). The values in the table are merely the PWM duty cycle...

Main difference is, with a correct table the brightness can use an
appropriate logarithmic power scale (which matches how humans perceive
brightness) instead of the linear scale provided by the PWM duty cycle.


Daniel.


Brightness and "index into the brightness-levels array" should be one
and the same thing
>
> >
> > >
> > >
> > > Daniel.
> >
> > Thanks an best regards,
> > Flavio
>
> Thanks,
>
> Flavio