On Fri, 11 Feb 2022, Thomas Zimmermann [off-list ref] wrote:

Hi

Am 11.02.22 um 12:12 schrieb Andy Shevchenko:

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On Fri, Feb 11, 2022 at 11:40:13AM +0100, Javier Martinez Canillas wrote:

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On 2/11/22 11:28, Andy Shevchenko wrote:

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On Fri, Feb 11, 2022 at 10:19:22AM +0100, Javier Martinez Canillas wrote:

...

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+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

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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.

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Feel free to post a patch if you want to change that while loop.

Perhaps some day :-)

-- 
Jani Nikula, Intel Open Source Graphics Center