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 14:05:56 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Javier Martinez Canillas <javierm@...hat.com>
Cc:     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, 11 Feb 2022, Thomas Zimmermann <tzimmermann@...e.de> wrote:
> Hi
>
> 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.

And yes, the do-while suggested above is buggy, and you actually need to
stop and think to see why.


BR,
Jani.



>
> Best regards
> Thomas
>
>> 
>>> In any case, I just pulled the line conversion logic as a separate
>>> function with minimal code changes since doing that should be in a
>>> separate patch.
>> 
>> 
>>> Feel free to post a patch if you want to change that while loop.
>> 
>> Perhaps some day :-)
>> 

-- 
Jani Nikula, Intel Open Source Graphics Center