linux-kernel - Re: Yet more ARM breakage in linux-next

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Message-ID: <20081204003143.GA21350@flint.arm.linux.org.uk>
Date:	Thu, 4 Dec 2008 00:31:44 +0000
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	Rusty Russell <rusty@...tcorp.com.au>, nico@....org
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: Yet more ARM breakage in linux-next

On Thu, Dec 04, 2008 at 09:52:44AM +1030, Rusty Russell wrote:
> On Thursday 04 December 2008 07:11:09 Andrew Morton wrote:
> > On Wed, 3 Dec 2008 19:29:05 +0000
> >
> > Russell King <rmk+lkml@....linux.org.uk> wrote:
> > > This seems to be causing lots of ARM breakage:
> > >
> > > lib/find_next_bit.c:183: error: implicit declaration of function '__fls'
> > >
> > > Whoever's responsible,
> >
> > git-blame?
> 
> It's me.  Turns out sparc, avr32 and arm all don't define __fls in their
> asm/bitops.h, and I'm the first one to use it in generic code.
> 
> But as I prepared this patch, I note that the armv5 __fls/fls is wrong:

__fls is wrong.

> 
>   /* Implement fls() in C so that 64-bit args are suitably truncated */
>   static inline int fls(int x)
>   {
>       return __fls(x);
>   }
> 
> __fls(x) returns a bit number (0-31).  fls() returns 0 or bitnumber+1.

The 'clz' instruction returns 32 for a zero input, or (31 - most significant
set bit) - which seems to work for fls() but not __fls().

Sending to Nicolas.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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