linux-kernel - Re: bitmap API consistency

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Date:   Wed, 24 May 2017 15:43:25 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: bitmap API consistency

On Wed, 2017-05-24 at 14:38 +0200, Rasmus Villemoes wrote:
> On 24 May 2017 at 14:11, Andy Shevchenko
> <andriy.shevchenko@...ux.intel.com> wrote:
> > Hi!
> > 
> > Surprisingly discovered today that bitmap API is not consistent in
> > some
> > cases (at least one I found recently).
> > 
> > bitmap_fill() sets area of bits in a bitmap.
> > bitmap_zero() clears them.
> > 
> > However, if _fill() does something sane, _zero() clears _all_ bits
> > up to
> > word size (long).
> > 
> > I think it should be fixed to be consistent with _fill() variant.
> 
> What do you want it to do?

Based on my vision and your answer below, thanks for it, I think we need
to
a) make _fill() to fill entire _aligned_ area
b) update comments in the header and documentation, if needed, to
specify that _fill() / _zero() operates on aligned to word size area,
while _set() and _clear() do exact amount of bits.

>  It always acts on whole words, so the last
> word must be set to something. One might as well say that _zero and
> _fill are consistent in that they both set the bits beyond nbits in
> the last word to 0.
> 
> If anything, I'd change bitmap_fill to do a memset(0xff) of the entire
> region. There used to be bugs where some of the bitmap_* functions
> didn't actually ignore the trailing bits, making it somewhat important
> that they were always 0, but I think they're fixed now.
> 
> Note that if one wants a guarantee that the trailing bits are not
> touched at all, the APIs to use are bitmap_{set, clear}(dst, 0,
> count). bitmap_{zero,fill} assumes that nbits is the total size of the
> bitmap (i.e. that the user will never care about bits beyond nbits).
> Maybe a few comments could be added somewhere.
> 
> Rasmus

-- 
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy