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

[Pekka Paalanen]

On Wed, 19 Aug 2020 17:53:36 -0300
Melissa Wen <melissa.srw at gmail.com> 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 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 | 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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