linux-kernel - Re: [PATCH v3 1/5] lib/bitmap: add bitmap_{set,get}

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Date:   Mon, 17 Jul 2023 08:50:59 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     catalin.marinas@....com, will@...nel.org, pcc@...gle.com,
        andreyknvl@...il.com, andriy.shevchenko@...ux.intel.com,
        linux@...musvillemoes.dk, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, eugenis@...gle.com,
        syednwaris@...il.com, william.gray@...aro.org,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v3 1/5] lib/bitmap: add bitmap_{set,get}_value()

On Mon, Jul 17, 2023 at 01:37:04PM +0200, Alexander Potapenko wrote:
> The two new functions allow setting/getting values of length up to
> BITS_PER_LONG bits at arbitrary position in the bitmap.
> 
> The code was taken from "bitops: Introduce the for_each_set_clump macro"
> by Syed Nayyar Waris with a couple of minor changes:
>  - instead of using roundup(), which adds an unnecessary dependency
>    on <linux/math.h>, we calculate space as BITS_PER_LONG-offset;
>  - indentation is reduced by not using else-clauses (suggested by
>    checkpatch for bitmap_get_value())

Please preserve Syed's authorship ('From' field in git log).

> Cc: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Syed Nayyar Waris <syednwaris@...il.com>
> Signed-off-by: William Breathitt Gray <william.gray@...aro.org>
> Link: https://lore.kernel.org/lkml/fe12eedf3666f4af5138de0e70b67a07c7f40338.1592224129.git.syednwaris@gmail.com/
> Suggested-by: Yury Norov <yury.norov@...il.com>
> Signed-off-by: Alexander Potapenko <glider@...gle.com>
> ---
>  include/linux/bitmap.h | 57 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 57 insertions(+)
> 
> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> index 03644237e1efb..4559366084988 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -76,7 +76,11 @@ struct device;
>   *  bitmap_to_arr32(buf, src, nbits)            Copy nbits from buf to u32[] dst
>   *  bitmap_to_arr64(buf, src, nbits)            Copy nbits from buf to u64[] dst
>   *  bitmap_get_value8(map, start)               Get 8bit value from map at start
> + *  bitmap_get_value(map, start, nbits)         Get bit value of size 'nbits'
> + *                                              from map at start
>   *  bitmap_set_value8(map, value, start)        Set 8bit value to map at start
> + *  bitmap_set_value(map, value, start, nbits)  Set bit value of size 'nbits'
> + *                                              of map at start

The 'bit value of size' sounds more confusing than it should. The size
of bit is actually a bit... Can you rephrase? Moreover, 'set bits' has
a meaning of actually setting them, i.e. switching to '1'. Maybe:
"Copy 'nbits' to bitmap starting at 'start'"?

>   *
>   * Note, bitmap_zero() and bitmap_fill() operate over the region of
>   * unsigned longs, that is, bits behind bitmap till the unsigned long
> @@ -583,6 +587,31 @@ static inline unsigned long bitmap_get_value8(const unsigned long *map,
>  	return (map[index] >> offset) & 0xFF;
>  }
>  
> +/**
> + * bitmap_get_value - get a value of n-bits from the memory region
> + * @map: address to the bitmap memory region
> + * @start: bit offset of the n-bit value
> + * @nbits: size of value in bits


 * @nbits: size of value in bits, up to BITS_PER_LONG

> + *
> + * Returns value of nbits located at the @start bit offset within the @map
> + * memory region.
> + */
> +static inline unsigned long bitmap_get_value(const unsigned long *map,
> +					     unsigned long start,
> +					     unsigned long nbits)
> +{
> +	const size_t index = BIT_WORD(start);
> +	const unsigned long offset = start % BITS_PER_LONG;
> +	const unsigned long space = BITS_PER_LONG - offset;
> +	unsigned long value_low, value_high;
> +
> +	if (space >= nbits)
> +		return (map[index] >> offset) & GENMASK(nbits - 1, 0);
> +	value_low = map[index] & BITMAP_FIRST_WORD_MASK(start);
> +	value_high = map[index + 1] & BITMAP_LAST_WORD_MASK(start + nbits);
> +	return (value_low >> offset) | (value_high << space);
> +}

When nbits == 0, copy-like functions shouldn't touch any memory. See how
other bitmap and find_bit functions hold it.

Thanks,
Yury