linux-kernel - Re: [PATCH v11 1/4] bitops: Introduce the for_each_set

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Date:   Tue, 6 Oct 2020 14:27:45 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Syed Nayyar Waris <syednwaris@...il.com>
Cc:     linus.walleij@...aro.org, akpm@...ux-foundation.org,
        vilhelm.gray@...il.com, arnd@...db.de, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 1/4] bitops: Introduce the for_each_set_clump macro

On Tue, Oct 06, 2020 at 02:52:16PM +0530, Syed Nayyar Waris wrote:
> This macro iterates for each group of bits (clump) with set bits,
> within a bitmap memory region. For each iteration, "start" is set to
> the bit offset of the found clump, while the respective clump value is
> stored to the location pointed by "clump". Additionally, the
> bitmap_get_value() and bitmap_set_value() functions are introduced to
> respectively get and set a value of n-bits in a bitmap memory region.
> The n-bits can have any size less than or equal to BITS_PER_LONG.
> Moreover, during setting value of n-bit in bitmap, if a situation arise
> that the width of next n-bit is exceeding the word boundary, then it
> will divide itself such that some portion of it is stored in that word,
> while the remaining portion is stored in the next higher word. Similar
> situation occurs while retrieving the value from bitmap.

...

> @@ -75,7 +75,11 @@
>   *  bitmap_from_arr32(dst, buf, nbits)          Copy nbits from u32[] buf to dst
>   *  bitmap_to_arr32(buf, src, nbits)            Copy nbits from buf to u32[] 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

Formatting here is done with solely spaces, no TABs.

...

> +/**
> + * 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 (must be between 1 and BITS_PER_LONG inclusive).


> + *	nbits less than 1 or more than BITS_PER_LONG causes undefined behaviour.

Please, detach this from field description and move to a main description.

> + *
> + * Returns value of nbits located at the @start bit offset within the @map
> + * memory region.
> + */

...

> +		return (map[index] >> offset) & GENMASK(nbits - 1, 0);

Have you considered to use rather BIT{_ULL}(nbits) - 1?
It maybe better for code generation.

...

> +/**
> + * bitmap_set_value - set n-bit value within a memory region
> + * @map: address to the bitmap memory region
> + * @value: value of nbits
> + * @start: bit offset of the n-bit value
> + * @nbits: size of value in bits (must be between 1 and BITS_PER_LONG inclusive).

> + *	nbits less than 1 or more than BITS_PER_LONG causes undefined behaviour.

Please, detach this from field description and move to a main description.

> + */

...

> +	value &= GENMASK(nbits - 1, 0);

Ditto.

> +		map[index] &= ~(GENMASK(nbits + offset - 1, offset));

Last time I checked such GENMASK) use, it gave a lot of code when
GENMASK(nbits - 1, 0) << offset works much better, but see also above.

-- 
With Best Regards,
Andy Shevchenko