Messages in this thread | | | Date | Tue, 21 Feb 2023 10:36:53 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH -next] arm64: Optimize the comparison of unsigned expressions to avoid compiling error |
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On Tue, Feb 21, 2023 at 09:27:40AM +0800, Lin Yujun wrote: > while compile arch/arm64/include/asm/cpufeature.h with > -Werror=type-limits enabled, errors shown as below: > > ./arch/arm64/include/asm/cpufeature.h: In function 'system_supports_4kb_granule': > ./arch/arm64/include/asm/cpufeature.h:653:14: error: comparison of unsigned expression >= 0 is always true [-Werror=type-limits] > return (val >= ID_AA64MMFR0_TGRAN4_SUPPORTED_MIN) && > ^~ > ./arch/arm64/include/asm/cpufeature.h: In function 'system_supports_64kb_granule': > ./arch/arm64/include/asm/cpufeature.h:666:14: error: comparison of unsigned expression >= 0 is always true [-Werror=type-limits] > return (val >= ID_AA64MMFR0_TGRAN64_SUPPORTED_MIN) && > ^~
When is the `-Werror=type-limits` flag enabled by the build system?
We have patterns like this all over the kernel, and I don't think this is indicative of a real problem, and I don't think that we need to change code to make this warning disappear.
> Modify the return judgment statement, use > "((val - min) < (val - max - 1))" to confirm that returns > true in “min <= val <= max” cases, false in other cases.
That expression is far less clear than the existing code, so I do not think that is a good idea.
> Fixes: 79d82cbcbb3d ("arm64/kexec: Test page size support with new TGRAN range values")
What functional error does this fix?
What configuration is broken?
Thanks, Mark.
> Signed-off-by: Lin Yujun <linyujun809@huawei.com> > --- > arch/arm64/include/asm/cpufeature.h | 18 ++++++++++++------ > 1 file changed, 12 insertions(+), 6 deletions(-) > > diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h > index 03d1c9d7af82..0a6bda025141 100644 > --- a/arch/arm64/include/asm/cpufeature.h > +++ b/arch/arm64/include/asm/cpufeature.h > @@ -54,6 +54,9 @@ enum ftr_type { > #define FTR_VISIBLE_IF_IS_ENABLED(config) \ > (IS_ENABLED(config) ? FTR_VISIBLE : FTR_HIDDEN) > > +#define IN_RANGE_INCLUSIVE(val, min, max) \ > + (((val) - (min)) < ((val) - (max) - 1)) > + > struct arm64_ftr_bits { > bool sign; /* Value is signed ? */ > bool visible; > @@ -693,8 +696,9 @@ static inline bool system_supports_4kb_granule(void) > val = cpuid_feature_extract_unsigned_field(mmfr0, > ID_AA64MMFR0_EL1_TGRAN4_SHIFT); > > - return (val >= ID_AA64MMFR0_EL1_TGRAN4_SUPPORTED_MIN) && > - (val <= ID_AA64MMFR0_EL1_TGRAN4_SUPPORTED_MAX); > + return IN_RANGE_INCLUSIVE(val, > + ID_AA64MMFR0_EL1_TGRAN4_SUPPORTED_MIN, > + ID_AA64MMFR0_EL1_TGRAN4_SUPPORTED_MAX); > } > > static inline bool system_supports_64kb_granule(void) > @@ -706,8 +710,9 @@ static inline bool system_supports_64kb_granule(void) > val = cpuid_feature_extract_unsigned_field(mmfr0, > ID_AA64MMFR0_EL1_TGRAN64_SHIFT); > > - return (val >= ID_AA64MMFR0_EL1_TGRAN64_SUPPORTED_MIN) && > - (val <= ID_AA64MMFR0_EL1_TGRAN64_SUPPORTED_MAX); > + return IN_RANGE_INCLUSIVE(val, > + ID_AA64MMFR0_EL1_TGRAN64_SUPPORTED_MIN, > + ID_AA64MMFR0_EL1_TGRAN64_SUPPORTED_MAX); > } > > static inline bool system_supports_16kb_granule(void) > @@ -719,8 +724,9 @@ static inline bool system_supports_16kb_granule(void) > val = cpuid_feature_extract_unsigned_field(mmfr0, > ID_AA64MMFR0_EL1_TGRAN16_SHIFT); > > - return (val >= ID_AA64MMFR0_EL1_TGRAN16_SUPPORTED_MIN) && > - (val <= ID_AA64MMFR0_EL1_TGRAN16_SUPPORTED_MAX); > + return IN_RANGE_INCLUSIVE(val, > + ID_AA64MMFR0_EL1_TGRAN16_SUPPORTED_MIN, > + ID_AA64MMFR0_EL1_TGRAN16_SUPPORTED_MAX); > } > > static inline bool system_supports_mixed_endian_el0(void) > -- > 2.34.1 >
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