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From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Thu, 14 Jul 2022 11:49:30 +0200
Message-ID: <CAHp75VdVSvux3HRPOj=xWXZoBdn1e=nSmWi9+BZUX69XAPcZPg@mail.gmail.com>
Subject: Re: [PATCH v4 13/13] video: backlight: mt6370: Add Mediatek MT6370 support
To:     Daniel Thompson <daniel.thompson@linaro.org>
Cc:     ChiaEn Wu <peterwu.pub@gmail.com>,
        Lee Jones <lee.jones@linaro.org>,
        Jingoo Han <jingoohan1@gmail.com>, Pavel Machek <pavel@ucw.cz>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Sebastian Reichel <sre@kernel.org>,
        Chunfeng Yun <chunfeng.yun@mediatek.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jonathan Cameron <jic23@kernel.org>,
        Lars-Peter Clausen <lars@metafoo.de>,
        Liam Girdwood <lgirdwood@gmail.com>,
        Mark Brown <broonie@kernel.org>,
        Guenter Roeck <linux@roeck-us.net>,
        "Krogerus, Heikki" <heikki.krogerus@linux.intel.com>,
        Helge Deller <deller@gmx.de>,
        ChiaEn Wu <chiaen_wu@richtek.com>,
        Alice Chen <alice_chen@richtek.com>,
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On Thu, Jul 14, 2022 at 11:47 AM Daniel Thompson
<daniel.thompson@linaro.org> wrote:
>
> On Thu, Jul 14, 2022 at 11:27:07AM +0200, Andy Shevchenko wrote:
> > On Thu, Jul 14, 2022 at 9:13 AM ChiaEn Wu <peterwu.pub@gmail.com> wrote:
> > > I have tried two methods so far, as follows
> > > -------------------------------------------------------------
> > > /*
> > >  * prop_val =  1      -->  1 steps --> b'00
> > >  * prop_val =  2 ~  4 -->  4 steps --> b'01
> > >  * prop_val =  5 ~ 16 --> 16 steps --> b'10
> > >  * prop_val = 17 ~ 64 --> 64 steps --> b'11
> > > */
> >
> > So, for 1 --> 0, for 2 --> 1, for 5 --> 2, and for 17 --> 3.
> > Now, consider x - 1:
> > 0  ( 0 ) --> 0
> > 1  (2^0) --> 1
> > 4  (2^2) --> 2
> > 16 (2^4) --> 3
> > 64 (2^6) --> ? (but let's consider that the range has been checked already)
> >
> > Since we take the lower limit, it means ffs():
> >
> >   y = (ffs(x - 1) + 1) / 2;
> >
> > Does it work for you?
>
> To be honest, for this tiny table, writing code that *doesn't* require intricate
> deciphering together with a huge comment saying what is does would probably be
> better:
>
>                 prop_val = (prop_val <=  1 ? 0 :
>                             prop_val <=  4 ? 1 :
>                             prop_val <= 16 ? 2 :
>                                              3);
>
> This would be "obviously correct" and require no comment.

Agree. It will also limit checking (and whatever needed for that).

-- 
With Best Regards,
Andy Shevchenko