linux-kernel - Re: [RESEND PATCH v2 3/9] asm-generic: Move some macros from linux/bitops.h to a new bits.h file

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Message-ID: <20180711094326.GB13963@arm.com>
Date:   Wed, 11 Jul 2018 10:43:26 +0100
From:   Will Deacon <will.deacon@....com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...nel.org, linux-arm-kernel@...ts.infradead.org,
        yamada.masahiro@...ionext.com
Subject: Re: [RESEND PATCH v2 3/9] asm-generic: Move some macros from
 linux/bitops.h to a new bits.h file

Hi again, Andrew,

On Mon, Jul 09, 2018 at 03:10:06PM -0700, Andrew Morton wrote:
> On Mon, 9 Jul 2018 12:32:51 +0100 Will Deacon <will.deacon@....com> wrote:
> > On Fri, Jul 06, 2018 at 05:30:49PM -0700, Andrew Morton wrote:
> > > On Tue, 19 Jun 2018 13:53:08 +0100 Will Deacon <will.deacon@....com> wrote:
> > > 
> > > > In preparation for implementing the asm-generic atomic bitops in terms
> > > > of atomic_long_*, we need to prevent asm/atomic.h implementations from
> > > > pulling in linux/bitops.h. A common reason for this include is for the
> > > > BITS_PER_BYTE definition, so move this and some other BIT() and masking
> > > > macros into a new header file, linux/bits.h
> > > > 
> > > > --- a/include/linux/bitops.h
> > > > +++ b/include/linux/bitops.h
> > > > @@ -2,29 +2,9 @@
> > > >  #ifndef _LINUX_BITOPS_H
> > > >  #define _LINUX_BITOPS_H
> > > >  #include <asm/types.h>
> > > > +#include <linux/bits.h>
> > > >  
> > > > -#ifdef	__KERNEL__
> > > > -#define BIT(nr)			(1UL << (nr))
> > > > -#define BIT_ULL(nr)		(1ULL << (nr))
> > > > -#define BIT_MASK(nr)		(1UL << ((nr) % BITS_PER_LONG))
> > > > -#define BIT_WORD(nr)		((nr) / BITS_PER_LONG)
> > > > -#define BIT_ULL_MASK(nr)	(1ULL << ((nr) % BITS_PER_LONG_LONG))
> > > > -#define BIT_ULL_WORD(nr)	((nr) / BITS_PER_LONG_LONG)
> > > > -#define BITS_PER_BYTE		8
> > > >  #define BITS_TO_LONGS(nr)	DIV_ROUND_UP(nr, BITS_PER_BYTE * sizeof(long))
> > > > -#endif
> > > 
> > > Why does it leave BITS_TO_LONGS() in place?
> > > 
> > > That becomes unfortunate with Chris's patch, so I'm moving
> > > BITS_TO_LONGS() into bits.h.
> > 
> > The reason I avoided that was because it would pull in the dreaded
> > kernel.h for DIV_ROUND_UP, and then we're back to circular include hell :(
> > 
> 
> Well we should comment that so every reader doesn't wonder what I
> wondered.
> 
> Refactoring works well.  I suppose DIV_ROUND_UP and friends await a new
> <arithmacros.h>.

Yes; pulling apart kernel.h would certainly help solve some of the fragility
here.

> Also, all these macros no longer `#ifdef __KERNEL__' protection, which
> wasn't mentioned in the changelog.  Deliberate?

Yes, these aren't under uapi/ so I dropped the guards now that they're not
needed (my understanding is that they're only there because of the way the
uapi split was originally scripted). I can also confirm that the generated
user headers match exactly with and without my changes.

Will