linux-kernel - RE: [PATCH v3 1/2] dt-bindings: backlight: Add MPS MP3309C

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Message-ID: <AM8PR01MB804533DE0885FFC6A46A7C97F9C9A@AM8PR01MB8045.eurprd01.prod.exchangelabs.com>
Date:   Fri, 6 Oct 2023 08:56:15 +0000
From:   Flavio Suligoi <f.suligoi@...m.it>
To:     Daniel Thompson <daniel.thompson@...aro.org>
CC:     Lee Jones <lee@...nel.org>, Jingoo Han <jingoohan1@...il.com>,
        Helge Deller <deller@....de>, Pavel Machek <pavel@....cz>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-leds@...r.kernel.org" <linux-leds@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Rob Herring <robh@...nel.org>
Subject: RE: [PATCH v3 1/2] dt-bindings: backlight: Add MPS MP3309C

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...

ok

> 
> 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

ok, I'll use default-brightness, thanks for the explanations!

> >
> > >
> > > >
> > > >
> > > > Daniel.
> > >
> > > Thanks an best regards,
> > > Flavio
> >
> > Thanks,
> >
> > Flavio

Flavio