[PATCH v2] drm/vkms: add alpha-premultiplied color blending

[Rodrigo Siqueira]

Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo at gmail.com>

On 08/25, Melissa Wen wrote:
> 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 at ffwll.ch>
> Cc: Rodrigo Siqueira <rodrigosiqueiramelo at gmail.com>
> Cc: Haneen Mohammed <hamohammed.sa at gmail.com>
> 
> Signed-off-by: Melissa Wen <melissa.srw at gmail.com>
> ---
>  drivers/gpu/drm/vkms/vkms_composer.c | 55 ++++++++++++++++++++--------
>  1 file changed, 39 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
> index 4f3b07a32b60..eaecc5a6c5db 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));
>  		}
> @@ -42,27 +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++;
>  	}
> -- 
> 2.28.0
> 

-- 
Rodrigo Siqueira
https://siqueira.tech
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