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

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Date:   Fri, 11 Feb 2022 17:41:03 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     Thomas Zimmermann <tzimmermann@...e.de>,
        Javier Martinez Canillas <javierm@...hat.com>,
        linux-fbdev@...r.kernel.org, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Noralf Trønnes <noralf@...nnes.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Maxime Ripard <maxime@...no.tech>,
        Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH v4 1/6] drm/format-helper: Add
 drm_fb_xrgb8888_to_gray8_line()

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 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.

-- 
With Best Regards,
Andy Shevchenko