linux-kernel - Re: [RFC] _

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Date:	Thu, 23 Apr 2009 12:07:10 +0300
From:	Benny Halevy <bhalevy@...asas.com>
To:	Steven Whitehouse <swhiteho@...hat.com>
CC:	Christoph Lameter <cl@...ux.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC] __ffs64()

On Apr. 23, 2009, 11:22 +0300, Steven Whitehouse <swhiteho@...hat.com> wrote:
> 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.

Agreed.

Benny

> 
> Steve.
> 
> 
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