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Message-ID: <20210323133005.GC1463@shell.armlinux.org.uk>
Date: Tue, 23 Mar 2021 13:30:05 +0000
From: Russell King - ARM Linux admin <linux@...linux.org.uk>
To: Arnd Bergmann <arnd@...nel.org>
Cc: Arnd Bergmann <arnd@...db.de>,
Nathan Chancellor <nathan@...nel.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
clang-built-linux@...glegroups.com
Subject: Re: [PATCH] ARM: delay: avoid clang -Wtautological-constant warning
On Tue, Mar 23, 2021 at 02:20:23PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
>
> 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.
What effect (if any) does this have on code generation when the argument
is not constant?
--
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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