[PATCH] drm/vkms: add alpha-premultiplied color blending

[Melissa Wen <melissa.srw@gmail.com>]

The current VKMS blend function ignores alpha channel and just overwrites
vaddr_src with vaddr_dst. This XRGB approach triggers a warning when
running the kms_cursor_crc/cursor-alpha-transparent test case. In IGT
tests, cairo_format_argb32 uses premultiplied alpha (according to
documentation), so this patch considers premultiplied alpha colors to
compose vaddr_src with vaddr_dst.

This change removes the following cursor-alpha-transparent warning:
Suspicious CRC: All values are 0.

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>

Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
---
 drivers/gpu/drm/vkms/vkms_composer.c | 43 +++++++++++++++++++++-------
 1 file changed, 33 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
index 4f3b07a32b60..6aac962d3e2e 100644
--- a/drivers/gpu/drm/vkms/vkms_composer.c
+++ b/drivers/gpu/drm/vkms/vkms_composer.c
@@ -32,8 +32,6 @@ static uint32_t compute_crc(void *vaddr_out, struct vkms_composer *composer)
 			src_offset = composer->offset
 				     + (i * composer->pitch)
 				     + (j * composer->cpp);
-			/* XRGB format ignores Alpha channel */
-			bitmap_clear(vaddr_out + src_offset, 24, 8);
 			crc = crc32_le(crc, vaddr_out + src_offset,
 				       sizeof(u32));
 		}
@@ -42,6 +40,32 @@ static uint32_t compute_crc(void *vaddr_out, struct vkms_composer *composer)
 	return crc;
 }
 
+u8 blend_channel(u8 c_src, u8 c_dst, u8 a_src)
+{
+	u32 pre_blend;
+	u8 new_color;
+
+	/* Premultiplied alpha blending - IGT + cairo context */
+	pre_blend = (c_src * 255 + c_dst * (255 - a_src));
+
+	/* Faster div by 255 */
+	new_color = ((pre_blend + ((pre_blend + 257) >> 8)) >> 8);
+
+	return new_color;
+}
+
+void alpha_blending(u8 *argb_src, u8 *argb_dst)
+{
+	u8 a_src;
+
+	a_src = argb_src[3];
+	argb_dst[0] = blend_channel(argb_src[0], argb_dst[0], a_src);
+	argb_dst[1] = blend_channel(argb_src[1], argb_dst[1], a_src);
+	argb_dst[2] = blend_channel(argb_src[2], argb_dst[2], a_src);
+	/* Opaque primary */
+	argb_dst[3] = 0xFF;
+}
+
 /**
  * blend - blend value at vaddr_src with value at vaddr_dst
  * @vaddr_dst: destination address
@@ -50,12 +74,9 @@ static uint32_t compute_crc(void *vaddr_out, struct vkms_composer *composer)
  * @src_composer: source framebuffer's metadata
  *
  * Blend value at vaddr_src with value at vaddr_dst.
- * Currently, this function write value of vaddr_src on value
- * at vaddr_dst using buffer's metadata to locate the new values
- * from vaddr_src and their destination at vaddr_dst.
- *
- * TODO: Use the alpha value to blend vaddr_src with vaddr_dst
- *	 instead of overwriting it.
+ * Currently, this function considers premultiplied alpha for blending, as used
+ * by Cairo. It uses buffer's metadata to locate the new composite values at
+ * vaddr_dst.
  */
 static void blend(void *vaddr_dst, void *vaddr_src,
 		  struct vkms_composer *dest_composer,
@@ -63,6 +84,7 @@ static void blend(void *vaddr_dst, void *vaddr_src,
 {
 	int i, j, j_dst, i_dst;
 	int offset_src, offset_dst;
+	u8 *p_dst, *p_src;
 
 	int x_src = src_composer->src.x1 >> 16;
 	int y_src = src_composer->src.y1 >> 16;
@@ -84,8 +106,9 @@ static void blend(void *vaddr_dst, void *vaddr_src,
 				     + (i * src_composer->pitch)
 				     + (j * src_composer->cpp);
 
-			memcpy(vaddr_dst + offset_dst,
-			       vaddr_src + offset_src, sizeof(u32));
+			p_src = (u8 *)(vaddr_src + offset_src);
+			p_dst = (u8 *)(vaddr_dst + offset_dst);
+			alpha_blending(p_src, p_dst);
 		}
 		i_dst++;
 	}
-- 
2.28.0