linux-kernel - Re: [PATCH] platform/surface: aggregator: shut up clang -Wconstantn-conversion warning

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Message-ID: <219848ed-e0ce-634a-29c2-caca813b054c@gmail.com>
Date:   Fri, 14 May 2021 16:21:43 +0200
From:   Maximilian Luz <luzmaximilian@...il.com>
To:     Arnd Bergmann <arnd@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Hans de Goede <hdegoede@...hat.com>
Cc:     Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] platform/surface: aggregator: shut up clang
 -Wconstantn-conversion warning

On 5/14/21 4:05 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. Shut up the warning
> using an explicit type cast.

I'm not too happy about this fix as (I believe) it will also shut up any
valid GCC error message in case those macros are used with non-u8 (and
non-SSAM_ANY_xxx) values.

I'll let others judge and decide what's preferred, however.

In case you're deciding to apply this, feel free to add:

Reviewed-by: Maximilian Luz <luzmaximilian@...il.com>

Thanks,
Max

> Fixes: eb0e90a82098 ("platform/surface: aggregator: Add dedicated bus and device type")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>   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..90df092ed565 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   = ((tid) != SSAM_ANY_TID) ? (u8)(tid) : 0,			\
> +	.instance = ((iid) != SSAM_ANY_IID) ? (u8)(iid) : 0,			\
> +	.function = ((fun) != SSAM_ANY_FUN) ? (u8)(fun) : 0			\
>   
>   /**
>    * SSAM_VDEV() - Initialize a &struct ssam_device_id as virtual device with
>