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

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Date:   Thu, 14 Jul 2022 10:47:09 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     ChiaEn Wu <peterwu.pub@...il.com>,
        Lee Jones <lee.jones@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>, Pavel Machek <pavel@....cz>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Sebastian Reichel <sre@...nel.org>,
        Chunfeng Yun <chunfeng.yun@...iatek.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        "Krogerus, Heikki" <heikki.krogerus@...ux.intel.com>,
        Helge Deller <deller@....de>,
        ChiaEn Wu <chiaen_wu@...htek.com>,
        Alice Chen <alice_chen@...htek.com>,
        cy_huang <cy_huang@...htek.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux LED Subsystem <linux-leds@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        USB <linux-usb@...r.kernel.org>,
        linux-iio <linux-iio@...r.kernel.org>,
        "open list:FRAMEBUFFER LAYER" <linux-fbdev@...r.kernel.org>,
        szuni chen <szunichen@...il.com>
Subject: Re: [PATCH v4 13/13] video: backlight: mt6370: Add Mediatek MT6370
 support

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@...il.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.