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

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Date:   Thu, 24 Oct 2019 13:12:41 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Mauro Carvalho Chehab <mchehab@...nel.org>
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>
Subject: [PATCH] media: v4l2-device.h: Explicitly compare grpmask to zero in v4l2_device_mask_call_all

When building with Clang + -Wtautological-constant-compare, several of
the ivtv drivers warn along the lines of:

 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 explicity so that there is no
boolean conversion.

Link: https://github.com/ClangBuiltLinux/linux/issues/752
Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
---

I am aware that there is suddenly a style mismatch (some macros using
!(grpmask) and this one using (grpmask) == 0) but I chose to display the
minimal fix. If you want me to update all the macros to use this style,
I'd be happy to in a followup patch.

There are 19 of these warnings in the drivers/media/pci folder, which
can be seen here:

https://github.com/ClangBuiltLinux/linux/issues/488#issuecomment-545218125

This is the simplest fix but if you all prefer an alternative one, I
would be happy to see/review/test it. 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:

https://github.com/ClangBuiltLinux/linux/issues/488

 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)
 
 /**
-- 
2.24.0.rc1