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From:   Max Krummenacher <max.oss.09@gmail.com>
Date:   Wed, 11 May 2022 15:32:33 +0200
Message-ID: <CAEHkU3U=58qittoNazqXrkYKO9gs7VU3VW_67823fs+dt6r6eA@mail.gmail.com>
Subject: Re: [PATCH v2 3/4] ARM: dts: imx6qdl-colibri: backlight pwm: Simplify
 inverted backlight
To:     Fabio Estevam <festevam@gmail.com>
Cc:     Max Krummenacher <max.krummenacher@toradex.com>,
        Krzysztof Kozlowski <krzk+dt@kernel.org>,
        NXP Linux Team <linux-imx@nxp.com>,
        Pengutronix Kernel Team <kernel@pengutronix.de>,
        Rob Herring <robh+dt@kernel.org>,
        Sascha Hauer <s.hauer@pengutronix.de>,
        Shawn Guo <shawnguo@kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@lists.infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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Hi Fabio

On Wed, May 11, 2022 at 2:07 PM Fabio Estevam <festevam@gmail.com> wrote:
>
> On Wed, May 11, 2022 at 8:59 AM Max Krummenacher <max.oss.09@gmail.com> wrote:
>
> >         backlight: backlight {
> >                 compatible = "pwm-backlight";
> > -               brightness-levels = <0 127 191 223 239 247 251 255>;
> > -               default-brightness-level = <1>;
> > +               brightness-levels = <0 4 8 16 32 64 128 255>;
> > +               default-brightness-level = <6>;
>
> In this patch, you are still changing the brightness levels + passing
> the polarity.
>
> I would suggest that this patch only touches the PWM polarity.

I disagree. Just setting the invert without at the same changing the
brightness-levels does
change the user experience way more than when one adapts the available
duty cycles
at the same time.

With the change to use the PWM with inverted polarity the PWM signals
is inverted to
how it was before this patch. Keeping the brightness-levels will then
have a big brightness
jump from 0 to 127 duty cycle, the other 6 steps will then be barely noticable.

I.e. before the change the brightness for level [0..7] was
['off', 128/255, 64/255, 32/255, 16/255, 8/255, 4/255, 'off'],
if one only inverts the polarity it will be
['off', 128/255, 191/255, 223/255, 239/255, 247/255, 255/255].
With the proposed patch it will be
['off', 4/255, 8/255, 16/255, 32/255, 64/255, 128/255, 255/255].

Max

> The next patch could fix the brightness levels.