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

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Message-ID: <2979815.5fSG56mABF@jernej-laptop>
Date:   Tue, 24 Mar 2020 18:58:56 +0100
From:   Jernej Škrabec <jernej.skrabec@...l.net>
To:     mripard@...nel.org, wens@...e.org,
        Roman Stratiienko <r.stratiienko@...il.com>
Cc:     airlied@...ux.ie, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org,
        Roman Stratiienko <r.stratiienko@...il.com>
Subject: Re: [PATCH v4 4/4] RFC: drm/sun4i: Process alpha channel of most bottom layer

Hi!

Dne ponedeljek, 02. marec 2020 ob 11:31:38 CET je Roman Stratiienko 
napisal(a):
> 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.

I always thought that if bottom layer has alpha, it's blended with background 
color, which is set to opaque black. If that is not the case, can you solve 
this by changing blending formula located in BLD control registers (offsets 
0x90, 0x94, 0x98 and 0x9c)?

Best regards,
Jernej

> 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);
>  }
> 
>  static int sun8i_ui_layer_update_coord(struct sun8i_mixer *mixer, int
> channel, diff --git a/drivers/gpu/drm/sun4i/sun8i_vi_layer.c
> b/drivers/gpu/drm/sun4i/sun8i_vi_layer.c index e6d8a539614f..68a6843db4ab
> 100644
> --- a/drivers/gpu/drm/sun4i/sun8i_vi_layer.c
> +++ b/drivers/gpu/drm/sun4i/sun8i_vi_layer.c
> @@ -108,7 +108,7 @@ static void sun8i_vi_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),
> -			   (mixer->cfg->is_de3) ?
> +			   (zpos != 0 && mixer->cfg->is_de3) ?
>  				
SUN8I_MIXER_BLEND_PREMULTIPLY_EN(zpos) : 0);
> 
>  }