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

From: Andy Shevchenko
Date: Thu Jul 14 2022 - 05:28:53 EST

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：
> > On Wed, Jul 13, 2022 at 12:53 PM ChiaEn Wu <peterwu.pub@xxxxxxxxx> wrote:
> > > Andy Shevchenko <andy.shevchenko@xxxxxxxxx> 於 2022年7月5日 週二 清晨5:14寫道：
> > > > On Mon, Jul 4, 2022 at 7:43 AM ChiaEn Wu <peterwu.pub@xxxxxxxxx> wrote:

^^^^^^^

...

> > > > > + prop_val = (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 = 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?

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

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

--
With Best Regards,
Andy Shevchenko