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Date:   Tue, 01 Aug 2023 22:48:02 +0200
From:   "Arnd Bergmann" <arnd@arndb.de>
To:     "Borislav Petkov" <bp@alien8.de>, "Arnd Bergmann" <arnd@kernel.org>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        "Thomas Gleixner" <tglx@linutronix.de>,
        "Ingo Molnar" <mingo@redhat.com>,
        "Dave Hansen" <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH 2/5] [RESEND] x86: avoid unneeded __div64_32 function definition
Content-Type: text/plain
Precedence: bulk

On Tue, Aug 1, 2023, at 19:03, Borislav Petkov wrote:
> On Tue, Jul 25, 2023 at 03:48:34PM +0200, Arnd Bergmann wrote:
>> From: Arnd Bergmann <arnd@arndb.de>
>> 
>> The __div64_32() function is provided for 32-bit architectures that
>> don't have a custom do_div() implementation. x86_32 has one, and
>> does not use the header file that declares the function prototype,
>> so the definition causes a W=1 warning:
>> 
>> lib/math/div64.c:31:32: error: no previous prototype for '__div64_32' [-Werror=missing-prototypes]
>> 
>> Define an empty macro to prevent the function definition from getting
>> built, which avoids the warning and saves a little .text space.
>> 
>> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
>> ---
>>  arch/x86/include/asm/div64.h | 2 ++
>>  1 file changed, 2 insertions(+)
>> 
>> diff --git a/arch/x86/include/asm/div64.h b/arch/x86/include/asm/div64.h
>> index b8f1dc0761e4b..9826d5fc12e34 100644
>> --- a/arch/x86/include/asm/div64.h
>> +++ b/arch/x86/include/asm/div64.h
>> @@ -71,6 +71,8 @@ static inline u64 mul_u32_u32(u32 a, u32 b)
>>  }
>>  #define mul_u32_u32 mul_u32_u32
>>  
>> +#define __div64_32 /* not needed */
>
> This comment, *after* having read the commit message makes sense.
>
> When you look at it alone, after having opened the file, makes me
> scratch my head and wonder what is that thing supposed to mean. Please
> extend it.
>
> And put the comment ontop, not sideways.

Right, makes sense. How about this version?

--- a/arch/x86/include/asm/div64.h
+++ b/arch/x86/include/asm/div64.h
@@ -71,6 +71,11 @@ static inline u64 mul_u32_u32(u32 a, u32 b)
 }
 #define mul_u32_u32 mul_u32_u32
 
+/*
+ * __div64_32() is never called on x86, so prevent the
+ * generic definition from getting built.
+ */
+#define __div64_32
 
 #else
 # include <asm-generic/div64.h>


       Arnd