drm/format-helper: Comment on RGB888 byte order

RGB888 is different than the other formats as most of its pixels are
unaligned and therefore helper functions do not use endianness conversion
helpers. Comment on this in the source code.
Signed-off-by: NThomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: NMaíra Canal <mcanal@igalia.com>
Reviewed-by: NJosé Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230102112927.26565-3-tzimmermann@suse.de

drivers/gpu/drm/drm_format_helper.c

@@ -404,6 +404,7 @@ static void drm_fb_xrgb8888_to_rgb888_line(void *dbuf, const void *sbuf, unsigne
 
 	for (x = 0; x < pixels; x++) {
 		pix = le32_to_cpu(sbuf32[x]);
+		/* write blue-green-red to output in little endianness */
 		*dbuf8++ = (pix & 0x000000FF) >>  0;
 		*dbuf8++ = (pix & 0x0000FF00) >>  8;
 		*dbuf8++ = (pix & 0x00FF0000) >> 16;

drivers/gpu/drm/tests/drm_format_helper_test.c

@@ -407,6 +407,10 @@ static void drm_test_fb_xrgb8888_to_rgb888(struct kunit *test)
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, xrgb8888);
 	iosys_map_set_vaddr(&src, xrgb8888);
 
+	/*
+	 * RGB888 expected results are already in little-endian
+	 * order, so there's no need to convert the test output.
+	 */
 	drm_fb_xrgb8888_to_rgb888(&dst, &result->dst_pitch, &src, &fb, &params->clip);
 	KUNIT_EXPECT_EQ(test, memcmp(buf, result->expected, dst_size), 0);
 }