[PATCH] drm/format-helper: Normalize BT.601 factors to 256

[Thomas Zimmermann]

BT.601 weights RGB components by certain factors to convert the
color to grayscale. Normalize the constants to 256 instead of 10.
Allows for slightly more precise rounding. The division by 256 can
be compiled as an 8-bit shift, which might be faster on some hardware.

Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
---
 drivers/gpu/drm/drm_format_internal.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_format_internal.h b/drivers/gpu/drm/drm_format_internal.h
index 9f857bfa368d..3020ff267513 100644
--- a/drivers/gpu/drm/drm_format_internal.h
+++ b/drivers/gpu/drm/drm_format_internal.h
@@ -42,7 +42,7 @@ static inline u32 drm_pixel_xrgb8888_to_r8_bt601(u32 pix)
 	u32 b =  pix & 0x000000ff;
 
 	/* ITU-R BT.601: Y = 0.299 R + 0.587 G + 0.114 B */
-	return (3 * r + 6 * g + b) / 10;
+	return (77 * r + 150 * g + 29 * b) / 256;
 }
 
 static inline u32 drm_pixel_xrgb8888_to_rgb332(u32 pix)
-- 
2.49.0