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: <bd6b2ae7-71e0-4c65-bae2-81240bd2515f@oss.qualcomm.com>
Date: Wed, 19 Nov 2025 10:14:41 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Dmitry Baryshkov <dmitry.baryshkov@....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 11/19/25 8:50 AM, Dmitry Baryshkov wrote:
> 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.

Perhaps we can create one.. there's include/linux/wordpart.h
with upper/lower_16/32_bits().. Maybe we could add

#define lower_n_bits(x, n) (FIELD_GET(GENMASK((n - 1), 0), x)

// register def
#define CONST_ALPHA_BG	GENMASK(,)
#define CONST_ALPHA_FG	GENMASK(,)

const_alpha = FIELD_GET(CONST_ALPHA_BG, lower_n_bits(bg_alpha, 8)) |
	      FIELD_GET(CONST_ALPHA_FG, lower_n_bits(fg_alpha, 8));

At which point it arguably becomes equally difficult to read.. but I
think the rule of thumb is that syntax sugar is better than raw bitops ;)

Konrad