[PATCH v2] drm/vkms: Use alpha value to blend values.

[Sidong Yang <realwakka@gmail.com>]

Use alpha value to blend source value and destination value Instead of
just overwrite with source value.

Signed-off-by: Sidong Yang <realwakka@gmail.com>
---
v1 -> v2: 
 * Move variables to tighter scope.

 drivers/gpu/drm/vkms/vkms_composer.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
index d5585695c64d..181472efa08c 100644
--- a/drivers/gpu/drm/vkms/vkms_composer.c
+++ b/drivers/gpu/drm/vkms/vkms_composer.c
@@ -77,6 +77,9 @@ static void blend(void *vaddr_dst, void *vaddr_src,
 
 	for (i = y_src, i_dst = y_dst; i < y_limit; ++i) {
 		for (j = x_src, j_dst = x_dst; j < x_limit; ++j) {
+			u8 *src, *dst;
+			u32 alpha, inv_alpha;
+
 			offset_dst = dest_composer->offset
 				     + (i_dst * dest_composer->pitch)
 				     + (j_dst++ * dest_composer->cpp);
@@ -84,8 +87,14 @@ 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));
+			src = vaddr_src + offset_src;
+			dst = vaddr_dst + offset_dst;
+			alpha = src[3] + 1;
+			inv_alpha = 256 - src[3];
+			dst[0] = (alpha * src[0] + inv_alpha * dst[0]) >> 8;
+			dst[1] = (alpha * src[1] + inv_alpha * dst[1]) >> 8;
+			dst[2] = (alpha * src[2] + inv_alpha * dst[2]) >> 8;
+			dst[3] = 0xff;
 		}
 		i_dst++;
 	}
-- 
2.20.1