linux-kernel - Re: [RFC] _

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Message-Id: <1240474923.3396.7.camel@localhost.localdomain>
Date:	Thu, 23 Apr 2009 09:22:03 +0100
From:	Steven Whitehouse <swhiteho@...hat.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [RFC] __ffs64()

Hi,

On Wed, 2009-04-22 at 16:59 -0400, Christoph Lameter wrote:
> On Wed, 22 Apr 2009, Steven Whitehouse wrote:
> 
> > I'd like to add a new bitop, __ffs64() which I need in order to fix a
> > bug in GFS2. The question is, where should it go?
> 
> I think the location is right.
> 
> > On 64 bit arches, __ffs64() would be a synonym for __ffs(), but on 32
> > bit arches it degenerates to a conditional plus a call to __ffs(). I'm
> > assuming that there would not be a lot of point in optimising this
> > operation on 32 bit arches even if such an instruction was available, so
> > that I should do something like the below patch.
> >
> > Does that seem reasonable, or should I give it a separate header file
> > under asm-generic/bitops/ like some of the similar operations? It looks
> > like I'd have to touch a lot of other files if I were to go that route,
> 
> One issue may be that some 32 bit architectures have a better way of doing
> 64 bit ffs.
> 
Yes, thats what I was worried about. I don't have a wide enough
knowledge of the different architectures to make a judgement about
whether this is likely or not.

I guess maybe the right thing to do is to leave it as I did it in the
patch and if an arch wants to create its own implementation, then it
could be moved at that stage.

Steve.


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