On Wed, 19 Aug 2020 17:53:36 -0300
Melissa Wen <melissa.srw@xxxxxxxxx> wrote:
> The current VKMS blend function ignores alpha channel and just overwrites
> vaddr_src with vaddr_dst. This XRGB approach triggers a warning when
> running the kms_cursor_crc/cursor-alpha-transparent test case. In IGT
> tests, cairo_format_argb32 uses premultiplied alpha (according to
> documentation), so this patch considers premultiplied alpha colors to
> compose vaddr_src with vaddr_dst.
>
> This change removes the following cursor-alpha-transparent warning:
> Suspicious CRC: All values are 0.
>
> Cc: Daniel Vetter <daniel@xxxxxxxx>
> Cc: Rodrigo Siqueira <rodrigosiqueiramelo@xxxxxxxxx>
> Cc: Haneen Mohammed <hamohammed.sa@xxxxxxxxx>
>
> Signed-off-by: Melissa Wen <melissa.srw@xxxxxxxxx>
> ---
> drivers/gpu/drm/vkms/vkms_composer.c | 43 +++++++++++++++++++++-------
> 1 file changed, 33 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
> index 4f3b07a32b60..6aac962d3e2e 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));
> }
> @@ -42,6 +40,32 @@ static uint32_t compute_crc(void *vaddr_out, struct vkms_composer *composer)
> return crc;
> }
>
> +u8 blend_channel(u8 c_src, u8 c_dst, u8 a_src)
> +{
> + u32 pre_blend;
> + u8 new_color;
> +
> + /* Premultiplied alpha blending - IGT + cairo context */
> + pre_blend = (c_src * 255 + c_dst * (255 - a_src));
> +
> + /* Faster div by 255 */
> + new_color = ((pre_blend + ((pre_blend + 257) >> 8)) >> 8);
> +
> + return new_color;
> +}
> +
> +void alpha_blending(u8 *argb_src, u8 *argb_dst)
> +{
> + u8 a_src;
> +
> + a_src = argb_src[3];
Hi,
DRM pixel formats are often defined as "bits in a 32-bit word", but
here you are accessing it as an array of bytes. To me that looks
suspicious wrt. big-endian architectures.
Unfortunately I have again forgot how DRM pixel formats should be
interpreted on a big-endian machine, if I ever even understood it, so I
can't say if this is right or not.
Thanks,
pq
> + argb_dst[0] = blend_channel(argb_src[0], argb_dst[0], a_src);
> + argb_dst[1] = blend_channel(argb_src[1], argb_dst[1], a_src);
> + argb_dst[2] = blend_channel(argb_src[2], argb_dst[2], a_src);
> + /* Opaque primary */
> + argb_dst[3] = 0xFF;
> +}
> +
> /**
> * blend - blend value at vaddr_src with value at vaddr_dst
> * @vaddr_dst: destination address
> @@ -50,12 +74,9 @@ static uint32_t compute_crc(void *vaddr_out, struct vkms_composer *composer)
> * @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.
> + * Currently, this function considers premultiplied alpha for blending, as used
> + * by Cairo. It 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,
> @@ -63,6 +84,7 @@ static void blend(void *vaddr_dst, void *vaddr_src,
> {
> int i, j, j_dst, i_dst;
> int offset_src, offset_dst;
> + u8 *p_dst, *p_src;
>
> int x_src = src_composer->src.x1 >> 16;
> int y_src = src_composer->src.y1 >> 16;
> @@ -84,8 +106,9 @@ 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));
> + p_src = (u8 *)(vaddr_src + offset_src);
> + p_dst = (u8 *)(vaddr_dst + offset_dst);
> + alpha_blending(p_src, p_dst);
> }
> i_dst++;
> }
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