drm/vkms: add alpha-premultiplied color blending

The VKMS blend function was ignoring the alpha channel and just
overwriting vaddr_src with vaddr_dst. This XRGB approach triggers a
warning when running the kms_cursor_crc/cursor-alpha-transparent test
case. In IGT, cairo_format_argb32 uses premultiplied alpha (according to
documentation). Also current DRM assumption is that alpha is
premultiplied. Therefore, this patch considers premultiplied alpha
blending eq to compose vaddr_src with vaddr_dst.

This change removes the following cursor-alpha-transparent warning:

 "Suspicious CRC: All values are 0."

V2:
- static for local functions
- const for the read-only variable argb_src
- replaces variable names
- drops unnecessary comment

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>
Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200825114532.abzdooluny2ekzvm@smtp.gmail.com

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));
 		}
@@ -42,27 +40,51 @@ static uint32_t compute_crc(void *vaddr_out, struct vkms_composer *composer)
 	return crc;
 }
 
+static u8 blend_channel(u8 src, u8 dst, u8 alpha)
+{
+	u32 pre_blend;
+	u8 new_color;
+
+	pre_blend = (src * 255 + dst * (255 - alpha));
+
+	/* Faster div by 255 */
+	new_color = ((pre_blend + ((pre_blend + 257) >> 8)) >> 8);
+
+	return new_color;
+}
+
+static void alpha_blending(const u8 *argb_src, u8 *argb_dst)
+{
+	u8 alpha;
+
+	alpha = argb_src[3];
+	argb_dst[0] = blend_channel(argb_src[0], argb_dst[0], alpha);
+	argb_dst[1] = blend_channel(argb_src[1], argb_dst[1], alpha);
+	argb_dst[2] = blend_channel(argb_src[2], argb_dst[2], alpha);
+	/* Opaque primary */
+	argb_dst[3] = 0xFF;
+}
+
 /**
  * blend - blend value at vaddr_src with value at vaddr_dst
  * @vaddr_dst: destination address
  * @vaddr_src: source address
- * @dest_composer: destination framebuffer's metadata
+ * @dst_composer: destination framebuffer's metadata
  * @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.
+ * Blend the vaddr_src value with the vaddr_dst value using the pre-multiplied
+ * alpha blending equation, since DRM currently assumes that the pixel color
+ * values have already been pre-multiplied with the alpha channel values. See
+ * more drm_plane_create_blend_mode_property(). This function 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,
+		  struct vkms_composer *dst_composer,
 		  struct vkms_composer *src_composer)
 {
 	int i, j, j_dst, i_dst;
 	int offset_src, offset_dst;
+	u8 *pixel_dst, *pixel_src;
 
 	int x_src = src_composer->src.x1 >> 16;
 	int y_src = src_composer->src.y1 >> 16;
@@ -77,15 +99,16 @@ 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) {
-			offset_dst = dest_composer->offset
-				     + (i_dst * dest_composer->pitch)
-				     + (j_dst++ * dest_composer->cpp);
+			offset_dst = dst_composer->offset
+				     + (i_dst * dst_composer->pitch)
+				     + (j_dst++ * dst_composer->cpp);
 			offset_src = src_composer->offset
 				     + (i * src_composer->pitch)
 				     + (j * src_composer->cpp);
 
-			memcpy(vaddr_dst + offset_dst,
-			       vaddr_src + offset_src, sizeof(u32));
+			pixel_src = (u8 *)(vaddr_src + offset_src);
+			pixel_dst = (u8 *)(vaddr_dst + offset_dst);
+			alpha_blending(pixel_src, pixel_dst);
 		}
 		i_dst++;
 	}