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

[Qian Cai <cai@lca.pw>]

> 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.