Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20220704053901.728-1-peterwu.pub@gmail.com> <20220704053901.728-14-peterwu.pub@gmail.com>
 <CAHp75VdwEc9AW1w8ejsxkw+sBTF1dumd99QyzTY9BZaXiViRWQ@mail.gmail.com>
 <CABtFH5K-2+2hbpvpq2nPE5AsznkQxZF2r3MVC64Q39DJhVuUtA@mail.gmail.com>
 <CAHp75VevDwdAKLYEWJgnMDvzuPuFibLuVqH-GKazEOT76wM6_A@mail.gmail.com>
 <CABtFH5LT1Ct_9-B_XRrGwYFmL5kGS6KHR7dNVyUO5z4sTy_6oA@mail.gmail.com> <CAHp75VcU_9Ao2CoqiUDZHqhVOjEMZor+hctPp3YYP4HOjYLDUg@mail.gmail.com>
In-Reply-To: <CAHp75VcU_9Ao2CoqiUDZHqhVOjEMZor+hctPp3YYP4HOjYLDUg@mail.gmail.com>
From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Thu, 14 Jul 2022 11:43:46 +0200
Message-ID: <CAHp75VeMP4zDMmGfdya5Y1C0Hkb7=jeOQGE_HqyDdZcmd_Uyyg@mail.gmail.com>
Subject: Re: [PATCH v4 13/13] video: backlight: mt6370: Add Mediatek MT6370 support
To:     ChiaEn Wu <peterwu.pub@gmail.com>
Cc:     Lee Jones <lee.jones@linaro.org>,
        Daniel Thompson <daniel.thompson@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>,
        cy_huang <cy_huang@richtek.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Linux LED Subsystem <linux-leds@vger.kernel.org>,
        devicetree <devicetree@vger.kernel.org>,
        linux-arm Mailing List <linux-arm-kernel@lists.infradead.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        USB <linux-usb@vger.kernel.org>,
        linux-iio <linux-iio@vger.kernel.org>,
        "open list:FRAMEBUFFER LAYER" <linux-fbdev@vger.kernel.org>,
        szuni chen <szunichen@gmail.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Thu, Jul 14, 2022 at 11:27 AM Andy Shevchenko
<andy.shevchenko@gmail.com> wrote:
>
> On Thu, Jul 14, 2022 at 9:13 AM ChiaEn Wu <peterwu.pub@gmail.com> wrote:
> > Andy Shevchenko <andy.shevchenko@gmail.com> =E6=96=BC 2022=E5=B9=B47=E6=
=9C=8813=E6=97=A5 =E9=80=B1=E4=B8=89 =E6=99=9A=E4=B8=8A8:07=E5=AF=AB=E9=81=
=93=EF=BC=9A

...

> > I have tried two methods so far, as follows
> > -------------------------------------------------------------
> > /*
> >  * prop_val =3D  1      -->  1 steps --> b'00
> >  * prop_val =3D  2 ~  4 -->  4 steps --> b'01
> >  * prop_val =3D  5 ~ 16 --> 16 steps --> b'10
> >  * prop_val =3D 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 alread=
y)
>
> Since we take the lower limit, it means ffs():
>
>   y =3D (ffs(x - 1) + 1) / 2;
>
> Does it work for you?

It wouldn't, because we need to use fls() against it actually.

So,
0..1   (-1..0)   --> 0
2..4   (1..3)   --> 1
5..16  (4..15)  --> 2
17..64 (16..63) --> 3

y =3D x ? ((fls(x - 1) + 1) / 2 : 0;


--=20
With Best Regards,
Andy Shevchenko