Re: [PATCH v4 13/13] video: backlight: mt6370: Add Mediatek MT6370 support

[Andy Shevchenko]

On Thu, Jul 14, 2022 at 11:27 AM Andy Shevchenko
<andy.shevchenko@xxxxxxxxx> wrote:
>
> On Thu, Jul 14, 2022 at 9:13 AM ChiaEn Wu <peterwu.pub@xxxxxxxxx> wrote:
> > Andy Shevchenko <andy.shevchenko@xxxxxxxxx> 於 2022年7月13日 週三 晚上8:07寫道：

...

> > 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?

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


-- 
With Best Regards,
Andy Shevchenko