linux-kernel - Re: [PATCH RESEND 2/2] drm/msm/dpu: use full scale alpha in _dpu_crtc_setup_blend

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Message-ID: <ogiiqha5ak5z4atck7ykyvbzw4juegsn4z7nazkkstiep3duwe@6qc5ag3kobj3>
Date: Wed, 19 Nov 2025 09:50:13 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
Cc: Rob Clark <robin.clark@....qualcomm.com>,
        Dmitry Baryshkov <lumag@...nel.org>,
        Abhinav Kumar <abhinav.kumar@...ux.dev>,
        Jessica Zhang <jessica.zhang@....qualcomm.com>,
        Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND 2/2] drm/msm/dpu: use full scale alpha in
 _dpu_crtc_setup_blend_cfg()

On Tue, Nov 18, 2025 at 04:02:03PM +0100, Konrad Dybcio wrote:
> On 11/18/25 3:51 PM, Dmitry Baryshkov wrote:
> > Both _dpu_crtc_setup_blend_cfg() and setup_blend_config_alpha()
> > callbacks embed knowledge about platform's alpha range (8-bit or
> > 10-bit). Make _dpu_crtc_setup_blend_cfg() use full 16-bit values for
> > alpha and reduce alpha only in DPU-specific callbacks.
> > 
> > Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
> > ---
> 
> [...]
> 
> > -	const_alpha = (bg_alpha & 0xFF) | ((fg_alpha & 0xFF) << 16);
> > +	const_alpha = ((bg_alpha >> 8) & 0xff) |
> > +		(((fg_alpha >> 8) & 0xff) << 16);
> 
> This begs for some bitfield.h

Which one(s) would you recommend? Ideally it should be something like
'get N top bits', but I don't see one.

> Konrad
> 
> >  	DPU_REG_WRITE(c, LM_BLEND0_CONST_ALPHA + stage_off, const_alpha);
> >  	DPU_REG_WRITE(c, LM_BLEND0_OP + stage_off, blend_op);
> >  }
> > @@ -160,7 +161,8 @@ dpu_hw_lm_setup_blend_config_combined_alpha_v12(struct dpu_hw_mixer *ctx,
> >  	if (WARN_ON(stage_off < 0))
> >  		return;
> >  
> > -	const_alpha = (bg_alpha & 0x3ff) | ((fg_alpha & 0x3ff) << 16);
> > +	const_alpha = ((bg_alpha >> 6) & 0x3ff) |
> > +		(((fg_alpha >> 6) & 0x3ff) << 16);
> >  	DPU_REG_WRITE(c, LM_BLEND0_CONST_ALPHA_V12 + stage_off, const_alpha);
> >  	DPU_REG_WRITE(c, LM_BLEND0_OP + stage_off, blend_op);
> >  }
> > @@ -178,8 +180,8 @@ static void dpu_hw_lm_setup_blend_config(struct dpu_hw_mixer *ctx,
> >  	if (WARN_ON(stage_off < 0))
> >  		return;
> >  
> > -	DPU_REG_WRITE(c, LM_BLEND0_FG_ALPHA + stage_off, fg_alpha);
> > -	DPU_REG_WRITE(c, LM_BLEND0_BG_ALPHA + stage_off, bg_alpha);
> > +	DPU_REG_WRITE(c, LM_BLEND0_FG_ALPHA + stage_off, fg_alpha >> 8);
> > +	DPU_REG_WRITE(c, LM_BLEND0_BG_ALPHA + stage_off, bg_alpha >> 8);
> >  	DPU_REG_WRITE(c, LM_BLEND0_OP + stage_off, blend_op);
> >  }
> >  
> > 

-- 
With best wishes
Dmitry