drm/format-helper: Add drm_fb_xrgb8888_to_gray8_line()

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.
Suggested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220214133710.3278506-2-javierm@redhat.com

drivers/gpu/drm/drm_format_helper.c

@@ -464,6 +464,21 @@ void drm_fb_xrgb8888_to_xrgb2101010_toio(void __iomem *dst,
 }
 EXPORT_SYMBOL(drm_fb_xrgb8888_to_xrgb2101010_toio);
 
+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++;
+	}
+}
+
 /**
  * drm_fb_xrgb8888_to_gray8 - Convert XRGB8888 to grayscale
  * @dst: 8-bit grayscale destination buffer
@@ -484,8 +499,9 @@ EXPORT_SYMBOL(drm_fb_xrgb8888_to_xrgb2101010_toio);
 void drm_fb_xrgb8888_to_gray8(void *dst, unsigned int dst_pitch, const void *vaddr,
 			      const struct drm_framebuffer *fb, const struct drm_rect *clip)
 {
-	unsigned int len = (clip->x2 - clip->x1) * sizeof(u32);
-	unsigned int x, y;
+	unsigned int linepixels = clip->x2 - clip->x1;
+	unsigned int len = linepixels * sizeof(u32);
+	unsigned int y;
 	void *buf;
 	u8 *dst8;
 	u32 *src32;
@@ -508,16 +524,7 @@ void drm_fb_xrgb8888_to_gray8(void *dst, unsigned int dst_pitch, const void *vad
 	for (y = clip->y1; y < clip->y2; y++) {
 		dst8 = dst;
 		src32 = memcpy(buf, vaddr, len);
-		for (x = clip->x1; x < clip->x2; x++) {
-			u8 r = (*src32 & 0x00ff0000) >> 16;
-			u8 g = (*src32 & 0x0000ff00) >> 8;
-			u8 b =  *src32 & 0x000000ff;
-
-			/* ITU BT.601: Y = 0.299 R + 0.587 G + 0.114 B */
-			*dst8++ = (3 * r + 6 * g + b) / 10;
-			src32++;
-		}
-
+		drm_fb_xrgb8888_to_gray8_line(dst8, src32, linepixels);
 		vaddr += fb->pitches[0];
 		dst += dst_pitch;
 	}