linux-kernel - Re: [PATCH v4 1/6] drm/format-helper: Add drm_fb_xrgb8888_to_gray8

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Date:   Mon, 14 Feb 2022 12:26:10 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Pekka Paalanen <ppaalanen@...il.com>
Cc:     Jani Nikula <jani.nikula@...ux.intel.com>,
        linux-fbdev@...r.kernel.org, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Javier Martinez Canillas <javierm@...hat.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Noralf Trønnes <noralf@...nnes.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Maxime Ripard <maxime@...no.tech>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH v4 1/6] drm/format-helper: Add
 drm_fb_xrgb8888_to_gray8_line()

On Mon, Feb 14, 2022 at 11:17:11AM +0200, Pekka Paalanen wrote:
> On Fri, 11 Feb 2022 19:27:12 +0200
> Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:
> > On Fri, Feb 11, 2022 at 06:25:17PM +0200, Jani Nikula wrote:
> > > On Fri, 11 Feb 2022, Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:  
> > > > On Fri, Feb 11, 2022 at 02:05:56PM +0200, Jani Nikula wrote:  
> > > >> On Fri, 11 Feb 2022, Thomas Zimmermann <tzimmermann@...e.de> wrote:  
> > > >> > Am 11.02.22 um 12:12 schrieb Andy Shevchenko:  
> > > >> >> On Fri, Feb 11, 2022 at 11:40:13AM +0100, Javier Martinez Canillas wrote:  
> > > >> >>> On 2/11/22 11:28, Andy Shevchenko wrote:  
> > > >> >>>> On Fri, Feb 11, 2022 at 10:19:22AM +0100, Javier Martinez Canillas wrote:  

...

> > > >> >>>>> +static void drm_fb_xrgb8888_to_gray8_line(u8 *dst, const u32 *src, unsigned int pixels)
> > > >> >>>>> +{
> > > >> >>>>> +	unsigned int x;
> > > >> >>>>> +
> > > >> >>>>> +	for (x = 0; x < pixels; x++) {
> > > >> >>>>> +		u8 r = (*src & 0x00ff0000) >> 16;
> > > >> >>>>> +		u8 g = (*src & 0x0000ff00) >> 8;
> > > >> >>>>> +		u8 b =  *src & 0x000000ff;
> > > >> >>>>> +
> > > >> >>>>> +		/* ITU BT.601: Y = 0.299 R + 0.587 G + 0.114 B */
> > > >> >>>>> +		*dst++ = (3 * r + 6 * g + b) / 10;
> > > >> >>>>> +		src++;
> > > >> >>>>> +	}  
> > > >> >>>>
> > > >> >>>> Can be done as
> > > >> >>>>
> > > >> >>>> 	while (pixels--) {
> > > >> >>>> 		...
> > > >> >>>> 	}
> > > >> >>>>
> > > >> >>>> or
> > > >> >>>>
> > > >> >>>> 	do {
> > > >> >>>> 		...
> > > >> >>>> 	} while (--pixels);
> > > >> >>>>  
> > > >> >>>
> > > >> >>> I don't see why a while loop would be an improvement here TBH.  
> > > >> >> 
> > > >> >> Less letters to parse when reading the code.  
> > > >> >
> > > >> > It's a simple refactoring of code that has worked well so far. Let's 
> > > >> > leave it as-is for now.  
> > > >> 
> > > >> IMO *always* prefer a for loop over while or do-while.
> > > >> 
> > > >> The for (i = 0; i < N; i++) is such a strong paradigm in C. You
> > > >> instantly know how many times you're going to loop, at a glance. Not so
> > > >> with with the alternatives, which should be used sparingly.  
> > > >
> > > > while () {}  _is_ a paradigm, for-loop is syntax sugar on top of it.  
> > > 
> > > And while() is just syntax sugar for goto. :p
> > > 
> > > The for loop written as for (i = 0; i < N; i++) is hands down the most
> > > obvious counting loop pattern there is in C.
> > >   
> > > >> And yes, the do-while suggested above is buggy, and you actually need to
> > > >> stop and think to see why.  
> > > >
> > > > It depends if pixels can be 0 or not and if it's not, then does it contain last
> > > > or number.
> > > >
> > > > The do {} while (--pixels); might be buggy iff pixels may be 0.  
> > > 
> > > Yeah. And how long does it take to figure that out?  
> > 
> > Okay, I made a mistake to drop the explanation. So, I (mistakenly) assumed
> > that people know this difference between post-decrement and pre-decrement
> > (note, while-loop here is not what is problematic).
> 
> That was not the question.
> 
> The question was, how long does it take to figure out if pixels can or
> cannot be zero?

To me these patterns, while() {} and do {} while(), while being shorter,
also give a hint. So if one is familiar with C, the do {} while (--foo)
_gives a hint_ while being shorter. It requires _less_ brain power to get
this.

But I assume my brain is unique and not working as million of others.

> Code is styled for humans other than the author, not for compilers.
> 
> Having to stop to think about the difference between post- and
> pre-decrement to figure out when the while-loop runs does take me a few
> more brain cycles to understand, even though I know the rules very well.
> 
> I would call that brain cycle optimization, and leave the CPU cycle
> optimization for the compiler in these cases.



-- 
With Best Regards,
Andy Shevchenko