linux-kernel - Re: [PATCH v4 4/4] RFC: drm/sun4i: Process alpha channel of most bottom layer

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Date:   Tue, 24 Mar 2020 16:36:08 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Roman Stratiienko <r.stratiienko@...il.com>
Cc:     jernej.skrabec@...l.net, wens@...e.org, airlied@...ux.ie,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 4/4] RFC: drm/sun4i: Process alpha channel of most
 bottom layer

On Mon, Mar 02, 2020 at 12:31:38PM +0200, Roman Stratiienko wrote:
> Allwinner display engine blender consists of 3 pipelined blending units.
>
> PIPE0->\
>         BLD0-\
> PIPE1->/      BLD1-\
> PIPE2->------/      BLD2->OUT
> PIPE3->------------/
>
> This pipeline produces incorrect composition if PIPE0 buffer has alpha.

Why? What happens in that case?

> Correct solution is to add one more blending step and mix PIPE0 with
> background, but it is not supported by the hardware.
>
> Use premultiplied alpha buffer of PIPE0 overlay channel as is.
> In this case we got same effect as mixing PIPE0 with black background.
>
> Signed-off-by: Roman Stratiienko <r.stratiienko@...il.com>
>
> ---
>
> v4:
> - Initial version, depends on other unmerged patches in the patchset.
> ---
>  drivers/gpu/drm/sun4i/sun8i_ui_layer.c | 2 +-
>  drivers/gpu/drm/sun4i/sun8i_vi_layer.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/sun4i/sun8i_ui_layer.c b/drivers/gpu/drm/sun4i/sun8i_ui_layer.c
> index dd6145f80c36..d94f4d8b9128 100644
> --- a/drivers/gpu/drm/sun4i/sun8i_ui_layer.c
> +++ b/drivers/gpu/drm/sun4i/sun8i_ui_layer.c
> @@ -106,7 +106,7 @@ static void sun8i_ui_layer_update_alpha(struct sun8i_mixer *mixer, int channel,
>  	regmap_update_bits(mixer->engine.regs,
>  			   SUN8I_MIXER_BLEND_PREMULTIPLY(bld_base),
>  			   SUN8I_MIXER_BLEND_PREMULTIPLY_EN(zpos),
> -			   SUN8I_MIXER_BLEND_PREMULTIPLY_EN(zpos));
> +			   zpos ? SUN8I_MIXER_BLEND_PREMULTIPLY_EN(zpos) : 0);

Can you really use the zpos here? What happens if the zpos doesn't
match the pipe?

Maxime

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