linux-kernel - RE: [PATCH] ARM: delay: avoid clang -Wtautological-constant warning

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Date:   Wed, 24 Mar 2021 23:14:40 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arnd Bergmann' <arnd@...nel.org>,
        Russell King <linux@...linux.org.uk>
CC:     Arnd Bergmann <arnd@...db.de>,
        Nathan Chancellor <nathan@...nel.org>,
        "Nick Desaulniers" <ndesaulniers@...gle.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "clang-built-linux@...glegroups.com" 
        <clang-built-linux@...glegroups.com>
Subject: RE: [PATCH] ARM: delay: avoid clang -Wtautological-constant warning

From: Arnd Bergmann
> Sent: 23 March 2021 13:20
> Passing an 8-bit constant into delay() triggers a warning when building
> with 'make W=1' using clang:
> 
> drivers/clk/actions/owl-pll.c:182:2: error: result of comparison of constant 2000 with expression of
> type 'u8' (aka 'unsigned char') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
>         udelay(pll_hw->delay);
>         ^~~~~~~~~~~~~~~~~~~~~
> arch/arm/include/asm/delay.h:84:9: note: expanded from macro 'udelay'
>           ((n) > (MAX_UDELAY_MS * 1000) ? __bad_udelay() :              \
>            ~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~
> arch/arm/mach-omap2/wd_timer.c:89:3: error: result of comparison of constant 2000 with expression of
> type 'u8' (aka 'unsigned char') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
>                 udelay(oh->class->sysc->srst_udelay);
>                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Shut up the warning by adding a cast to a 64-bit number. A cast to 'int'
> would usually be sufficient, but would fail to cause a link-time error
> for large 64-bit constants.

Adding 0u probably has the desired effect - without any side effects of a cast.

	David


> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  arch/arm/include/asm/delay.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm/include/asm/delay.h b/arch/arm/include/asm/delay.h
> index 4f80b72372b4..1bb6417a3a83 100644
> --- a/arch/arm/include/asm/delay.h
> +++ b/arch/arm/include/asm/delay.h
> @@ -81,7 +81,7 @@ extern void __bad_udelay(void);
> 
>  #define udelay(n)							\
>  	(__builtin_constant_p(n) ?					\
> -	  ((n) > (MAX_UDELAY_MS * 1000) ? __bad_udelay() :		\
> +	  ((u64)(n) > (MAX_UDELAY_MS * 1000) ? __bad_udelay() :		\
>  			__const_udelay((n) * UDELAY_MULT)) :		\
>  	  __udelay(n))
> 
> --
> 2.29.2

-
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