linux-kernel - Re: [PATCH v2] media: v4l2-device.h: Explicitly compare grpmask to zero in v4l2_device_mask_call

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Message-ID: <79cb93108abc1f0a11f41ad3ab02cc5535b3e784.camel@collabora.com>
Date:   Sat, 07 Dec 2019 07:41:27 -0300
From:   Ezequiel Garcia <ezequiel@...labora.com>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil@...all.nl>
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2] media: v4l2-device.h: Explicitly compare grpmask to
 zero in v4l2_device_mask_call_all

(Adding Hans)

Hi Nathan, Nick,

Thanks for the patch (and the patience).

On Mon, 2019-11-04 at 21:59 -0700, Nathan Chancellor wrote:
> When building with Clang + -Wtautological-constant-compare, several of
> the ivtv drivers warn along the lines of:
> 

Nitpick: s/ivtv/ivtv and cx18

>  drivers/media/pci/cx18/cx18-driver.c:1005:21: warning: converting the
>  result of '<<' to a boolean always evaluates to true
>  [-Wtautological-constant-compare]
>                          cx18_call_hw(cx, CX18_HW_GPIO_RESET_CTRL,
>                                          ^
>  drivers/media/pci/cx18/cx18-cards.h:18:37: note: expanded from macro
>  'CX18_HW_GPIO_RESET_CTRL'
>  #define CX18_HW_GPIO_RESET_CTRL         (1 << 6)
>                                            ^
>  1 warning generated.
> 
> This is because the shift operation is implicitly converted to a boolean
> in v4l2_device_mask_call_all before being negated. This can be solved by
> just comparing the mask result to 0 explicitly so that there is no
> boolean conversion.
> 

Perhaps it's interesting to mention the reason for this change,
which you mentioned as a comment before:

"""
 The ultimate goal is to get
-Wtautological-compare enabled because there are several subwarnings
that would be helpful to have and right now they are all disabled.
"""

> Link: https://github.com/ClangBuiltLinux/linux/issues/752
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---
> 
> v1 -> v2: https://lore.kernel.org/lkml/20191024201240.49063-1-natechancellor@gmail.com/
> 
> * Fix typo in commit message
> * Add Nick's Reviewed-by.
> 
>  include/media/v4l2-device.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/media/v4l2-device.h b/include/media/v4l2-device.h
> index e0b8f2602670..8564b3227887 100644
> --- a/include/media/v4l2-device.h
> +++ b/include/media/v4l2-device.h
> @@ -431,8 +431,8 @@ static inline bool v4l2_device_supports_requests(struct v4l2_device *v4l2_dev)
>  		struct v4l2_subdev *__sd;				\
>  									\
>  		__v4l2_device_call_subdevs_p(v4l2_dev, __sd,		\
> -			!(grpmsk) || (__sd->grp_id & (grpmsk)), o, f ,	\
> -			##args);					\
> +			(grpmsk) == 0 || (__sd->grp_id & (grpmsk)), o,	\
> +			f , ##args);					\
>  	} while (0)
>  
>  /**

The change is small enough and if it helps enabling helpful
warnings, then it seems a good idea.

Reviewed-by: Ezequiel Garcia <ezequiel@...labora.com>

For consistency, it would be good to also patch the other case(s).

Thanks,
Ezequiel