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

[Melissa Wen]

Hi Rodrigo,

Thanks for the review!
I just sent a v2 applying your suggestions.

Best Regards,

Melissa Wen

Melissa Wen


On Tue, Aug 25, 2020 at 12:04 AM Rodrigo Siqueira
<rodrigosiqueiramelo at gmail.com> wrote:
>
> Hi Melissa,
>
> First of all, thanks a lot for your patch!
>
> Follows my inline comments.
>
> On 08/19, Melissa Wen 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)
>
> Use static here.
>
> Also, replace c_src to src, c_dst to dst, and a_src to alpha.
>
> > +{
> > +     u32 pre_blend;
> > +     u8 new_color;
> > +
> > +     /* Premultiplied alpha blending - IGT + cairo context */
>
> You can drop the part that says "IGT + cairo context", this explanation
> better suit the commit message.
>
> > +     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)
>
> Use static.
>
> Looks like that argb_src is a read-only variable, in this sense add
> const.
>
> > +{
> > +     u8 a_src;
> > +
> > +     a_src = argb_src[3];
>
> change a_src to alpha.
>
> > +     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;
>
> I suppose that p stands for "pixel", right? In this case how about use
> pixel?
>
> Best Regards
> Rodrigo Siqueira
>
> >
> >       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++;
> >       }
> > --
> > 2.28.0
> >
>
> --
> Rodrigo Siqueira
> https://siqueira.tech