linux-kernel - Re: [PATCH] [v2] platform/surface: aggregator: avoid clang -Wconstant-conversion warning

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Date:   Fri, 14 May 2021 14:22:53 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Arnd Bergmann <arnd@...nel.org>,
        Maximilian Luz <luzmaximilian@...il.com>,
        Hans de Goede <hdegoede@...hat.com>
Cc:     Arnd Bergmann <arnd@...db.de>, platform-driver-x86@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] [v2] platform/surface: aggregator: avoid clang
 -Wconstant-conversion warning

On 5/14/2021 1:04 PM, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> Clang complains about the assignment of SSAM_ANY_IID to
> ssam_device_uid->instance:
> 
> drivers/platform/surface/surface_aggregator_registry.c:478:25: error: implicit conversion from 'int' to '__u8' (aka 'unsigned char') changes value from 65535 to 255 [-Werror,-Wconstant-conversion]
>          { SSAM_VDEV(HUB, 0x02, SSAM_ANY_IID, 0x00) },
>          ~                      ^~~~~~~~~~~~
> include/linux/surface_aggregator/device.h:71:23: note: expanded from macro 'SSAM_ANY_IID'
>   #define SSAM_ANY_IID            0xffff
>                                  ^~~~~~
> include/linux/surface_aggregator/device.h:126:63: note: expanded from macro 'SSAM_VDEV'
>          SSAM_DEVICE(SSAM_DOMAIN_VIRTUAL, SSAM_VIRTUAL_TC_##cat, tid, iid, fun)
>                                                                       ^~~
> include/linux/surface_aggregator/device.h:102:41: note: expanded from macro 'SSAM_DEVICE'
>          .instance = ((iid) != SSAM_ANY_IID) ? (iid) : 0,                        \
>                                                 ^~~
> 
> The assignment doesn't actually happen, but clang checks the type limits
> before checking whether this assignment is reached. Replace the ?:
> operator with a __builtin_choose_expr() invocation that avoids the
> warning for the untaken part.
> 
> Fixes: eb0e90a82098 ("platform/surface: aggregator: Add dedicated bus and device type")
> Cc: platform-driver-x86@...r.kernel.org
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
> v2: use __builtin_choose_expr() instead of a cast to shut up the warning
> ---
>   include/linux/surface_aggregator/device.h | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/surface_aggregator/device.h b/include/linux/surface_aggregator/device.h
> index 4441ad667c3f..6ff9c58b3e17 100644
> --- a/include/linux/surface_aggregator/device.h
> +++ b/include/linux/surface_aggregator/device.h
> @@ -98,9 +98,9 @@ struct ssam_device_uid {
>   		     | (((fun) != SSAM_ANY_FUN) ? SSAM_MATCH_FUNCTION : 0),	\
>   	.domain   = d,								\
>   	.category = cat,							\
> -	.target   = ((tid) != SSAM_ANY_TID) ? (tid) : 0,			\
> -	.instance = ((iid) != SSAM_ANY_IID) ? (iid) : 0,			\
> -	.function = ((fun) != SSAM_ANY_FUN) ? (fun) : 0				\
> +	.target   = __builtin_choose_expr((tid) != SSAM_ANY_TID, (tid), 0),	\
> +	.instance = __builtin_choose_expr((iid) != SSAM_ANY_IID, (iid), 0),	\
> +	.function = __builtin_choose_expr((fun) != SSAM_ANY_FUN, (fun), 0)
>   
>   /**
>    * SSAM_VDEV() - Initialize a &struct ssam_device_id as virtual device with
>