Messages in this thread | | | Subject | Re: [PATCH v2] arm64/prefetch: fix a -Wtype-limits warning | From | Qian Cai <> | Date | Mon, 5 Aug 2019 08:03:10 -0400 |
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> On Aug 5, 2019, at 6:00 AM, Will Deacon <will@kernel.org> wrote: > > 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@lca.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?
Yes, it is useful to catch unintended developer mistakes or simply optimize wasted instructions of checking like in,
919aef44d73d (“x86/efi: fix a -Wtype-limits compilation warning”)
5a82bdb48f04 (“x86/cacheinfo: Fix a -Wtype-limits warning”)
It is normal to fix a false positive this way as in other mainline commits,
ec6335586953 (“x86/apic: Silence -Wtype-limits compiler warnings”)
Once those false-positives are under control, the warning flag could be then enabled by default in the future.
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