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