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

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

[Miles Chen]

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@mediatek.com>
Cc: CK Hu <ck.hu@mediatek.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>

Signed-off-by: Miles Chen <miles.chen@mediatek.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,
-- 
2.18.0


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

[Matthias Brugger]

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@mediatek.com>
> Cc: CK Hu <ck.hu@mediatek.com>
> Cc: Matthias Brugger <matthias.bgg@gmail.com>
> 
> Signed-off-by: Miles Chen <miles.chen@mediatek.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@gmail.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,
> 

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

[Miles Chen]

Hi Matthias,

>> 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@gmail.com>

> thanks. I think you are right.
> I will change the subject to "drm/mediatek: Fix mtk_cec_mask()", remove the 
> clang part and submit patch v4.

I posted patch v4 https://lore.kernel.org/linux-mediatek/20220103054706.8072-1-miles.chen@mediatek.com/
without your reviewed-by tag.

Would you mind taking a look at the patch?

Miles

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

[miles.chen]

> 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@gmail.com>

thanks. I think you are right.
I will change the subject to "drm/mediatek: Fix mtk_cec_mask()", remove the 
clang part and submit patch v4.

e.g,
"""
drm/mediatek: Fix mtk_cec_mask()

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")
"""

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