linux-kernel - Re: bitmap API consistency

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Message-ID: <1513704756.7000.209.camel@linux.intel.com>
Date:   Tue, 19 Dec 2017 19:32:36 +0200
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 15:43 +0300, Andy Shevchenko wrote:
> 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.

7 months passed, can we eventually do something about it?

> >  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.

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