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 11:40:13 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, linux-fbdev@...r.kernel.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Maxime Ripard <maxime@...no.tech>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        dri-devel@...ts.freedesktop.org,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Sam Ravnborg <sam@...nborg.org>,
        Noralf Trønnes <noralf@...nnes.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>
Subject: Re: [PATCH v4 1/6] drm/format-helper: Add
 drm_fb_xrgb8888_to_gray8_line()

Hello Andy,

On 2/11/22 11:28, Andy Shevchenko wrote:
> On Fri, Feb 11, 2022 at 10:19:22AM +0100, Javier Martinez Canillas wrote:
>> Pull the per-line conversion logic into a separate helper function.
>>
>> This will allow to do line-by-line conversion in other helpers that
>> convert to a gray8 format.
> 
> ...
> 
>> +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.

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.

Best regards,
-- 
Javier Martinez Canillas
Linux Engineering
Red Hat