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Date:   Thu, 14 Jul 2022 10:47:09 +0100
From:   Daniel Thompson <daniel.thompson@linaro.org>
To:     Andy Shevchenko <andy.shevchenko@gmail.com>
Cc:     ChiaEn Wu <peterwu.pub@gmail.com>,
        Lee Jones <lee.jones@linaro.org>,
        Jingoo Han <jingoohan1@gmail.com>, Pavel Machek <pavel@ucw.cz>,
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Subject: Re: [PATCH v4 13/13] video: backlight: mt6370: Add Mediatek MT6370
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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.


Daniel.