linux-kernel - Re: [PATCH v2] arm64/prefetch: fix a -Wtype-limits warning

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Date:   Mon, 5 Aug 2019 11:00:59 +0100
From:   Will Deacon <will@...nel.org>
To:     Qian Cai <cai@....pw>
Cc:     catalin.marinas@....com, rrichter@...ium.com, robin.murphy@....com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arm64/prefetch: fix a -Wtype-limits warning

On Fri, Aug 02, 2019 at 08:33:58PM -0400, Qian Cai wrote:
> The commit d5370f754875 ("arm64: prefetch: add alternative pattern for
> CPUs without a prefetcher") introduced MIDR_IS_CPU_MODEL_RANGE() to be
> used in has_no_hw_prefetch() with rv_min=0 which generates a compilation
> warning from GCC,
> 
> In file included from ./arch/arm64/include/asm/cache.h:8,
>                 from ./include/linux/cache.h:6,
>                 from ./include/linux/printk.h:9,
>                 from ./include/linux/kernel.h:15,
>                 from ./include/linux/cpumask.h:10,
>                 from arch/arm64/kernel/cpufeature.c:11:
> arch/arm64/kernel/cpufeature.c: In function 'has_no_hw_prefetch':
> ./arch/arm64/include/asm/cputype.h:59:26: warning: comparison of
> unsigned expression >= 0 is always true [-Wtype-limits]
>  _model == (model) && rv >= (rv_min) && rv <= (rv_max);  \
>                          ^~
> arch/arm64/kernel/cpufeature.c:889:9: note: in expansion of macro
> 'MIDR_IS_CPU_MODEL_RANGE'
>  return MIDR_IS_CPU_MODEL_RANGE(midr, MIDR_THUNDERX,
>         ^~~~~~~~~~~~~~~~~~~~~~~
> 
> Fix it by making "rv" a "s32".
> 
> Signed-off-by: Qian Cai <cai@....pw>
> ---
> 
> v2: Use "s32" for "rv", so "variant 0/revision 0" can be covered.
> 
>  arch/arm64/include/asm/cputype.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/cputype.h b/arch/arm64/include/asm/cputype.h
> index e7d46631cc42..d52fe8651c2d 100644
> --- a/arch/arm64/include/asm/cputype.h
> +++ b/arch/arm64/include/asm/cputype.h
> @@ -54,7 +54,7 @@
>  #define MIDR_IS_CPU_MODEL_RANGE(midr, model, rv_min, rv_max)		\
>  ({									\
>  	u32 _model = (midr) & MIDR_CPU_MODEL_MASK;			\
> -	u32 rv = (midr) & (MIDR_REVISION_MASK | MIDR_VARIANT_MASK);	\
> +	s32 rv = (midr) & (MIDR_REVISION_MASK | MIDR_VARIANT_MASK);	\

Hmm, but this really isn't a signed quantity: it's two fields extracted
from an ID register. I think the code is fine. Are you explicitly enabling
-Wtype-limits somehow?

Will