linux-kernel - Re: number of elements or bytes for kfifo

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Date:	Thu, 19 Aug 2010 08:52:58 +0200
From:	Stefani Seibold <stefani@...bold.net>
To:	Huang Ying <ying.huang@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: number of elements or bytes for kfifo_in etc

Am Donnerstag, den 19.08.2010, 09:15 +0800 schrieb Huang Ying:
> On Thu, 2010-08-19 at 08:32 +0800, Huang Ying wrote:
> > Hi, Stefani,
> > 
> > On Thu, 2010-08-19 at 00:35 +0800, Stefani Seibold wrote:
> > > Am Mittwoch, den 18.08.2010, 13:29 +0800 schrieb Huang Ying:
> > > > Hi, Stefani,
> > > > 
> > > > In new kfifo implementation in 2.6.36-rc1, description of the third
> > > > parameter is:
> > > > 
> > > > "@n: number of elements to be added"
> > > > 
> > > > But if my understanding is correct, the actual implementation is:
> > > > 
> > > > "@n: number of bytes to be added"
> > > > 
> > > 
> > > Number of bytes is wrong, this is only valid for byte stream fifo's
> > > where an element is a byte.
> > > 
> > > Have a look at samples/kfifo/inttype-example.c where an element is a int
> > > type.
> > 
> > Yes. You are right. I misunderstood your code. Sorry for bothering.
> 
> And I found __kfifo->esize is used in __kfifo_in/out etc to convert
> between elements and bytes. Maybe we can pull the conversion code to
> corresponding macro, because where esize is constant and may be
> optimized to bit shifting or nothing. What do you think about that?
> 

esize isn't constant, especially from the point of view inside
__kfifo_in and so on. The is ony good reason not to pass the size of the
element using sizeof(*fifo->type) as a parameter, it will generate more
code. Storing the size of the element in the kfifo structure is the
better decision.

But if you found a better way and can prove it under any circumstance
with a list of measurement results u will get an ack!

- Stefani


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