linux-kernel - Re: [PATCH v6 1/5] lib/bitmap: add bitmap

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Message-ID: <ZSAeFfNOUZIrK3Yk@smile.fi.intel.com>
Date:   Fri, 6 Oct 2023 17:47:49 +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,
        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 v6 1/5] lib/bitmap: add bitmap_{read,write}()

On Fri, Oct 06, 2023 at 03:45:25PM +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.

...

> v6:
>  - As suggested by Yury Norov, do not require bitmap_read(..., 0) to
>    return 0.

Hmm... See below.

...

>   *  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

With the grouping as below I would add a blank line here. But was the intention
to group _arrXX() to these groups?

>   *  bitmap_get_value8(map, start)               Get 8bit value from map at start
> + *  bitmap_read(map, start, nbits)              Read an nbits-sized value from
> + *                                              map at start
>   *  bitmap_set_value8(map, value, start)        Set 8bit value to map at start
> + *  bitmap_write(map, value, start, nbits)      Write an nbits-sized value to
> + *                                              map at start

...

> +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))
> +		return 0;

Hmm... I didn't get was the comment to add or to remove these checks?

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

And don't you want to replace this GENMASK() as well?

> +	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);
> +}

-- 
With Best Regards,
Andy Shevchenko