linux-kernel - Re: [PATCH v3] drm/mediatek: Fix unused-but-set variable warning

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Date:   Thu, 30 Dec 2021 13:29:54 +0100
From:   Matthias Brugger <matthias.bgg@...il.com>
To:     Miles Chen <miles.chen@...iatek.com>,
        Chun-Kuang Hu <chunkuang.hu@...nel.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jie Qiu <jie.qiu@...iatek.com>,
        Junzhi Zhao <junzhi.zhao@...iatek.com>
Cc:     Zhiqiang Lin <zhiqiang.lin@...iatek.com>,
        CK Hu <ck.hu@...iatek.com>, dri-devel@...ts.freedesktop.org,
        linux-mediatek@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v3] drm/mediatek: Fix unused-but-set variable warning



On 30/12/2021 10:36, Miles Chen wrote:
> clang reports a unused-but-set variable warning:
> drivers/gpu/drm/mediatek/mtk_cec.c:85:6: warning: variable 'tmp' set but not used [-Wunused-but-set-variable]
> 
> In current implementation, mtk_cec_mask() writes val into target register
> and ignores the mask. After talking to our hdmi experts, mtk_cec_mask()
> should read a register, clean only mask bits, and update (val | mask) bits
> to the register.
> 
> Fixes: 8f83f26891e1 ("drm/mediatek: Add HDMI support")
> 
> Cc: Zhiqiang Lin <zhiqiang.lin@...iatek.com>
> Cc: CK Hu <ck.hu@...iatek.com>
> Cc: Matthias Brugger <matthias.bgg@...il.com>
> 
> Signed-off-by: Miles Chen <miles.chen@...iatek.com>

I'm still not happy with the commit subject, I think it is misleading. Clang 
only helped to find the bug, but the we are fixing something else, that's not 
just a clang warning. But I don't want to nit-pick too much so:

Reviewed-by: Matthias Brugger <matthias.bgg@...il.com>

> 
> ---
> 
> Change since v1:
> add Fixes tag
> 
> Change since v2:
> add explanation of mtk_cec_mask()
> 
> Change-Id: Iec6402b0532942f4d0e8bc56817eb87b95c03fad
> ---
>   drivers/gpu/drm/mediatek/mtk_cec.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/mediatek/mtk_cec.c b/drivers/gpu/drm/mediatek/mtk_cec.c
> index e9cef5c0c8f7..cdfa648910b2 100644
> --- a/drivers/gpu/drm/mediatek/mtk_cec.c
> +++ b/drivers/gpu/drm/mediatek/mtk_cec.c
> @@ -85,7 +85,7 @@ static void mtk_cec_mask(struct mtk_cec *cec, unsigned int offset,
>   	u32 tmp = readl(cec->regs + offset) & ~mask;
>   
>   	tmp |= val & mask;
> -	writel(val, cec->regs + offset);
> +	writel(tmp, cec->regs + offset);
>   }
>   
>   void mtk_cec_set_hpd_event(struct device *dev,
>