linux-kernel - Re: [PATCH v1 1/2] bitmap: Use constants and macros from bits.h

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Date:   Fri, 18 Aug 2023 08:28:21 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Yury Norov <yury.norov@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/2] bitmap: Use constants and macros from bits.h

On 17/08/2023 18.54, Andy Shevchenko wrote:
> Use BITS_PER_LONG and BITS_PER_BYTE for BITMAP_MEM_ALIGNMENT.
> Calculate bytes from bits for memcmp() and memset() with BITS_TO_BYTES().
> 
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> ---
>  include/linux/bitmap.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> index 1516ff979315..2d5042d1b501 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -354,9 +354,9 @@ static inline void bitmap_complement(unsigned long *dst, const unsigned long *sr
>  }
>  
>  #ifdef __LITTLE_ENDIAN
> -#define BITMAP_MEM_ALIGNMENT 8
> +#define BITMAP_MEM_ALIGNMENT	BITS_PER_BYTE
>  #else
> -#define BITMAP_MEM_ALIGNMENT (8 * sizeof(unsigned long))
> +#define BITMAP_MEM_ALIGNMENT	BITS_PER_LONG
>  #endif
>  #define BITMAP_MEM_MASK (BITMAP_MEM_ALIGNMENT - 1)
>  
> @@ -367,7 +367,7 @@ static inline bool bitmap_equal(const unsigned long *src1,
>  		return !((*src1 ^ *src2) & BITMAP_LAST_WORD_MASK(nbits));
>  	if (__builtin_constant_p(nbits & BITMAP_MEM_MASK) &&
>  	    IS_ALIGNED(nbits, BITMAP_MEM_ALIGNMENT))
> -		return !memcmp(src1, src2, nbits / 8);
> +		return !memcmp(src1, src2, BITS_TO_BYTES(nbits));

Please no. Currently, I can verify the arithmetic directly. Using such a
"helper" I'd have to know whether it just does /8 or if it's more like
the bitmap_words() thing which rounds up to a whole number of words. And
BITS_PER_BYTE (and similarly CHAR_BITS) really is, IMO, much less
readable than 8.

Rasmus