linux-kernel - Re: [PATCH v9 1/2] lib/bitmap: add bitmap

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Message-ID: <ZTpYbCa0Qmry0HGH@smile.fi.intel.com>
Date:   Thu, 26 Oct 2023 15:15:40 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     catalin.marinas@....com, will@...nel.org, pcc@...gle.com,
        andreyknvl@...il.com, aleksander.lobakin@...el.com,
        linux@...musvillemoes.dk, yury.norov@...il.com,
        alexandru.elisei@....com, 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 v9 1/2] lib/bitmap: add bitmap_{read,write}()

On Wed, Oct 25, 2023 at 10:38:11AM +0200, Alexander Potapenko wrote:
> From: Syed Nayyar Waris <syednwaris@...il.com>
> 
> The two new functions allow reading/writing 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 number of changes and simplifications:
>  - 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());
>  - bitmap_get_value()/bitmap_set_value() are renamed to bitmap_read()
>    and bitmap_write();
>  - some redundant computations are omitted.

...

>   *  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_set_value8(map, value, start)        Set 8bit value to map at start
> + *  bitmap_read(map, start, nbits)              Read an nbits-sized value from
> + *                                              map at start
> + *  bitmap_write(map, value, start, nbits)      Write an nbits-sized value to
> + *                                              map at start

I still didn't get the grouping you implied with this...

>   * Note, bitmap_zero() and bitmap_fill() operate over the region of
>   * unsigned longs, that is, bits behind bitmap till the unsigned long

...

> +/**
> + * bitmap_read - read 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, nonzero, up to BITS_PER_LONG
> + *
> + * Returns: value of nbits located at the @start bit offset within the @map
> + * memory region.
> + *
> + * Note: callers on 32-bit systems must be careful to not attempt reading more
> + * than sizeof(unsigned long).

sizeof() here is misleading, We talk about bits, BITS_PER_LONG (which is 32),
here it's better to be explicit that reading more than 32 bits at a time on
32-bit platform will return 0. Actually what you need is to describe...

> + */
> +static inline unsigned long bitmap_read(const unsigned long *map,
> +					unsigned long start,
> +					unsigned long nbits)
> +{
> +	size_t index = BIT_WORD(start);
> +	unsigned long offset = start % BITS_PER_LONG;
> +	unsigned long space = BITS_PER_LONG - offset;
> +	unsigned long value_low, value_high;
> +
> +	if (unlikely(!nbits || nbits > BITS_PER_LONG))
> +		return 0;

...this return in the Return section.

> +
> +	if (space >= nbits)
> +		return (map[index] >> offset) & BITMAP_LAST_WORD_MASK(nbits);
> +
> +	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);
> +}

...

> +/**
> + * bitmap_write - write n-bit value within a memory region
> + * @map: address to the bitmap memory region
> + * @value: value to write, clamped to nbits
> + * @start: bit offset of the n-bit value
> + * @nbits: size of value in bits, nonzero, up to BITS_PER_LONG.
> + *
> + * bitmap_write() behaves as-if implemented as @nbits calls of __assign_bit(),
> + * i.e. bits beyond @nbits are ignored:
> + *
> + *   for (bit = 0; bit < nbits; bit++)
> + *           __assign_bit(start + bit, bitmap, val & BIT(bit));

> + * Note: callers on 32-bit systems must be careful to not attempt writing more
> + * than sizeof(unsigned long).

Ditto.

> + */

-- 
With Best Regards,
Andy Shevchenko