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References: <20220605094018.9782-1-r.stratiienko@gmail.com> <5826286.lOV4Wx5bFT@jernej-laptop>
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From:   Roman Stratiienko <r.stratiienko@gmail.com>
Date:   Mon, 6 Jun 2022 13:16:06 +0300
Message-ID: <CAGphcdm=sOyppe8sAYMK6aeRXpxiHvj-h-Es6nigCNKBxdBNZQ@mail.gmail.com>
Subject: Re: [PATCH v2] drm/sun4i: Enable output signal premultiplication for DE2/DE3
To:     =?UTF-8?Q?Jernej_=C5=A0krabec?= <jernej.skrabec@gmail.com>
Cc:     mripard@kernel.org, wens@csie.org, airlied@linux.ie,
        Daniel Vetter <daniel@ffwll.ch>,
        Samuel Holland <samuel@sholland.org>,
        dri-devel@lists.freedesktop.org,
        linux-arm-kernel@lists.infradead.org, linux-sunxi@lists.linux.dev,
        linux-kernel@vger.kernel.org, megi@xff.cz
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Hi,

=D0=B2=D1=81, 5 =D0=B8=D1=8E=D0=BD. 2022 =D0=B3. =D0=B2 23:23, Jernej =C5=
=A0krabec <jernej.skrabec@gmail.com>:
>
> Dne nedelja, 05. junij 2022 ob 11:40:18 CEST je Roman Stratiienko napisal=
(a):
> > Otherwise alpha value is discarded, resulting incorrect pixel
> > apperance on the display.
> >
> > This also fixes missing transparency for the most bottom layer.
>
> Can you explain that a bit more?

Well... I would recommend reading Bartosz Ciechanowski's blog
https://ciechanow.ski/alpha-compositing/ or the Porter-Duff's 1984
whitepaper itself.

HINT: That magic numbers from sun8i_mixer.h ( 0x03010301 ) corresponds
to SOURCE OVER mode.

As you can see from the blending equation it outputs both pixel value
and alpha value (non-premultiplied data mode).

Also single-layer non-premultiplied buffers may have for example
(R255,G255,B255,A2) pixel value, which should be sent as {R2, G2, B2}
through the physical display interface.

When OUTCTL.PREMULTI disabled pixel, the RGB values passes as is, and
even 100% transparent data {R255, G255, B255, A0} will appear as 100%
opaque white.

>Also, BSP driver never enables this bit. What are we doing differently?

Are you sure the BSP does not have an issues with presenting
transparent buffers?
Does the sunxi even have a customer-feedback mechanism and publicly
available stable BSP with all the fixes?

Regards,
Roman

>
> >
> > Test applications and videos w/ w/o this patch are available at [1].
> >
> > [1]: https://github.com/GloDroid/glodroid_tests/issues/1
>
> As stated in other emails, commit messages should not contain external li=
nks
> (per patch rules).
>
> Best regards,
> Jernej
>
> > Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
> >
> > ---
> > Changelog:
> >
> > V2: Added code hunk missing in v1
> > ---
> >  drivers/gpu/drm/sun4i/sun8i_mixer.c | 5 +++--
> >  drivers/gpu/drm/sun4i/sun8i_mixer.h | 1 +
> >  2 files changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/sun4i/sun8i_mixer.c
> > b/drivers/gpu/drm/sun4i/sun8i_mixer.c index 6b1711a9a71f..ba2932aaed08
> > 100644
> > --- a/drivers/gpu/drm/sun4i/sun8i_mixer.c
> > +++ b/drivers/gpu/drm/sun4i/sun8i_mixer.c
> > @@ -320,8 +320,9 @@ static void sun8i_mixer_mode_set(struct sunxi_engin=
e
> > *engine, else
> >               val =3D 0;
> >
> > -     regmap_update_bits(engine->regs,
> SUN8I_MIXER_BLEND_OUTCTL(bld_base),
> > -                        SUN8I_MIXER_BLEND_OUTCTL_INTERLACED,
> val);
> > +     val |=3D SUN8I_MIXER_BLEND_OUTCTL_PREMULTIPLY;
> > +
> > +     regmap_write(engine->regs, SUN8I_MIXER_BLEND_OUTCTL(bld_base),
> val);
> >
> >       DRM_DEBUG_DRIVER("Switching display mixer interlaced mode %s\n",
> >                        interlaced ? "on" : "off");
> > diff --git a/drivers/gpu/drm/sun4i/sun8i_mixer.h
> > b/drivers/gpu/drm/sun4i/sun8i_mixer.h index ebfc276b2464..bc12c95af6f3
> > 100644
> > --- a/drivers/gpu/drm/sun4i/sun8i_mixer.h
> > +++ b/drivers/gpu/drm/sun4i/sun8i_mixer.h
> > @@ -65,6 +65,7 @@
> >  #define SUN8I_MIXER_BLEND_ROUTE_PIPE_MSK(n)  (0xf << ((n) << 2))
> >  #define SUN8I_MIXER_BLEND_ROUTE_PIPE_SHIFT(n)        ((n) << 2)
> >
> > +#define SUN8I_MIXER_BLEND_OUTCTL_PREMULTIPLY BIT(0)
> >  #define SUN8I_MIXER_BLEND_OUTCTL_INTERLACED  BIT(1)
> >
> >  #define SUN50I_MIXER_BLEND_CSC_CTL_EN(ch)    BIT(ch)
>
>
>
>