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

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Date:   Fri, 6 Oct 2023 09:53:51 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Alexander Potapenko <glider@...gle.com>, catalin.marinas@....com,
        will@...nel.org, pcc@...gle.com, andreyknvl@...il.com,
        aleksander.lobakin@...el.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 v6 1/5] lib/bitmap: add bitmap_{read,write}()

On Fri, Oct 06, 2023 at 05:47:49PM +0300, Andy Shevchenko wrote:
> 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.

 [...]
 
> > +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?

The sentence relates to the test, and the comment that confused you
should to to the 2nd patch.

I.e., bitmap_read(..., 0) is not required to return 0, and it's purely
an implementation details.

> 
> > +	if (space >= nbits)
> > +		return (map[index] >> offset) & GENMASK(nbits - 1, 0);
> 
> And don't you want to replace this GENMASK() as well?

+1

> > +	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
>