linux-kernel - RE: [PATCH] drm/ssd130x: Silence a `dubious: x & !y` warning

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Date:   Mon, 23 Jan 2023 09:26:17 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Javier Martinez Canillas' <javierm@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Thomas Zimmermann <tzimmermann@...e.de>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...il.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>
Subject: RE: [PATCH] drm/ssd130x: Silence a `dubious: x & !y` warning

From: Javier Martinez Canillas
> Sent: 21 January 2023 19:10
> 
> The sparse tool complains with the following warning:
> 
> $ make M=drivers/gpu/drm/solomon/ C=2
>   CC [M]  drivers/gpu/drm/solomon/ssd130x.o
>   CHECK   drivers/gpu/drm/solomon/ssd130x.c
> drivers/gpu/drm/solomon/ssd130x.c:363:21: warning: dubious: x & !y
> 
> This seems to be a false positive in my opinion but still we can silence
> the tool while making the code easier to read. Let's also add a comment,
> to explain why the "com_seq" logical not is used rather than its value.
> 
> Reported-by: Thomas Zimmermann <tzimmermann@...e.de>
> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
> Reviewed-by: Thomas Zimmermann <tzimmermann@...e.de>
> ---
> 
>  drivers/gpu/drm/solomon/ssd130x.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/solomon/ssd130x.c b/drivers/gpu/drm/solomon/ssd130x.c
> index c3bf3a18302e..b16330a8b624 100644
> --- a/drivers/gpu/drm/solomon/ssd130x.c
> +++ b/drivers/gpu/drm/solomon/ssd130x.c
> @@ -81,7 +81,7 @@
>  #define SSD130X_SET_PRECHARGE_PERIOD2_MASK	GENMASK(7, 4)
>  #define SSD130X_SET_PRECHARGE_PERIOD2_SET(val)	FIELD_PREP(SSD130X_SET_PRECHARGE_PERIOD2_MASK, (val))
>  #define SSD130X_SET_COM_PINS_CONFIG1_MASK	GENMASK(4, 4)
> -#define SSD130X_SET_COM_PINS_CONFIG1_SET(val)	FIELD_PREP(SSD130X_SET_COM_PINS_CONFIG1_MASK, !(val))
> +#define SSD130X_SET_COM_PINS_CONFIG1_SET(val)	FIELD_PREP(SSD130X_SET_COM_PINS_CONFIG1_MASK, (val))

How about just changing !(val) to (val) ? 0 : 1
It should shut the compiler up and is probably more descriptive.

	David

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