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Date:   Sun, 30 Aug 2020 09:16:05 -0400
From:   Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
To:     Melissa Wen <melissa.srw@gmail.com>
Cc:     Haneen Mohammed <hamohammed.sa@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        David Airlie <airlied@linux.ie>,
        Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>,
        twoerner@gmail.com, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org, kernel-usp@googlegroups.com
Subject: Re: [PATCH v2] drm/vkms: add alpha-premultiplied color blending
Message-ID: <20200830131605.bkleteqg4anaadk3@smtp.gmail.com>
References: <20200825114532.abzdooluny2ekzvm@smtp.gmail.com>
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Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo@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.
>=20
> This change removes the following cursor-alpha-transparent warning:
> "Suspicious CRC: All values are 0."
>=20
> --
>=20
> v2:
> - static for local functions
> - const for the read-only variable argb_src
> - replaces variable names
> - drops unnecessary comment
>=20
> --
>=20
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
> Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
>=20
> Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
> ---
>  drivers/gpu/drm/vkms/vkms_composer.c | 55 ++++++++++++++++++++--------
>  1 file changed, 39 insertions(+), 16 deletions(-)
>=20
> 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 vkm=
s_composer *composer)
>  			src_offset =3D composer->offset
>  				     + (i * composer->pitch)
>  				     + (j * composer->cpp);
> -			/* XRGB format ignores Alpha channel */
> -			bitmap_clear(vaddr_out + src_offset, 24, 8);
>  			crc =3D crc32_le(crc, vaddr_out + src_offset,
>  				       sizeof(u32));
>  		}
> @@ -42,27 +40,51 @@ static uint32_t compute_crc(void *vaddr_out, struct v=
kms_composer *composer)
>  	return crc;
>  }
> =20
> +static u8 blend_channel(u8 src, u8 dst, u8 alpha)
> +{
> +	u32 pre_blend;
> +	u8 new_color;
> +
> +	pre_blend =3D (src * 255 + dst * (255 - alpha));
> +
> +	/* Faster div by 255 */
> +	new_color =3D ((pre_blend + ((pre_blend + 257) >> 8)) >> 8);
> +
> +	return new_color;
> +}
> +
> +static void alpha_blending(const u8 *argb_src, u8 *argb_dst)
> +{
> +	u8 alpha;
> +
> +	alpha =3D argb_src[3];
> +	argb_dst[0] =3D blend_channel(argb_src[0], argb_dst[0], alpha);
> +	argb_dst[1] =3D blend_channel(argb_src[1], argb_dst[1], alpha);
> +	argb_dst[2] =3D blend_channel(argb_src[2], argb_dst[2], alpha);
> +	/* Opaque primary */
> +	argb_dst[3] =3D 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-mult=
iplied
> + * alpha blending equation, since DRM currently assumes that the pixel c=
olor
> + * values have already been pre-multiplied with the alpha channel values=
=2E See
> + * more drm_plane_create_blend_mode_property(). This function uses buffe=
r'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;
> =20
>  	int x_src =3D src_composer->src.x1 >> 16;
>  	int y_src =3D src_composer->src.y1 >> 16;
> @@ -77,15 +99,16 @@ static void blend(void *vaddr_dst, void *vaddr_src,
> =20
>  	for (i =3D y_src, i_dst =3D y_dst; i < y_limit; ++i) {
>  		for (j =3D x_src, j_dst =3D x_dst; j < x_limit; ++j) {
> -			offset_dst =3D dest_composer->offset
> -				     + (i_dst * dest_composer->pitch)
> -				     + (j_dst++ * dest_composer->cpp);
> +			offset_dst =3D dst_composer->offset
> +				     + (i_dst * dst_composer->pitch)
> +				     + (j_dst++ * dst_composer->cpp);
>  			offset_src =3D src_composer->offset
>  				     + (i * src_composer->pitch)
>  				     + (j * src_composer->cpp);
> =20
> -			memcpy(vaddr_dst + offset_dst,
> -			       vaddr_src + offset_src, sizeof(u32));
> +			pixel_src =3D (u8 *)(vaddr_src + offset_src);
> +			pixel_dst =3D (u8 *)(vaddr_dst + offset_dst);
> +			alpha_blending(pixel_src, pixel_dst);
>  		}
>  		i_dst++;
>  	}
> --=20
> 2.28.0
>=20

--=20
Rodrigo Siqueira
https://siqueira.tech

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