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From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Thu, 14 Jul 2022 11:27:07 +0200
Message-ID: <CAHp75VcU_9Ao2CoqiUDZHqhVOjEMZor+hctPp3YYP4HOjYLDUg@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>,
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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
> > On Wed, Jul 13, 2022 at 12:53 PM ChiaEn Wu <peterwu.pub@gmail.com> wrot=
e:
> > > Andy Shevchenko <andy.shevchenko@gmail.com> =E6=96=BC 2022=E5=B9=B47=
=E6=9C=885=E6=97=A5 =E9=80=B1=E4=BA=8C =E6=B8=85=E6=99=A85:14=E5=AF=AB=E9=
=81=93=EF=BC=9A
> > > > On Mon, Jul 4, 2022 at 7:43 AM ChiaEn Wu <peterwu.pub@gmail.com> wr=
ote:

Please, once again, remove unneeded context when replying!
^^^^^^^

...

> > > > > +               prop_val =3D (ilog2(roundup_pow_of_two(prop_val))=
 + 1) >> 1;
> > > >
> > > > Isn't something closer to get_order() or fls()?
> > >
> > > I will revise it to "(get_order(prop_va * PAGE_SIZE) + 1) / 2" and
> > > this change is meet your expectations??
> >
> > Nope. Try again. What about fls()?
>
> 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 already)

Since we take the lower limit, it means ffs():

  y =3D (ffs(x - 1) + 1) / 2;

Does it work for you?

> // 1. use fls() and ffs() combination
> prop_val =3D ffs(prop_val) =3D=3D fls(prop_val) ? fls(prop_val) >> 1 :
> (fls(prop_val) + 1) >> 1;
>
> // 2. use one line for-loop, but without fls()
> for (i =3D --prop_val, prop_val =3D 0; i >> 2 * prop_val !=3D 0; prop_val=
++);
> -------------------------------------------------------------
> Do these changes meet your expectations??

No, this is ugly. Yes, I understand that a bit arithmetics is hard...

--=20
With Best Regards,
Andy Shevchenko