linux-kernel - Re: [PATCH 1/2] kfifo: round up the fifo size power of 2

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Message-ID: <20121026123321.GG2778@yliu-dev.sh.intel.com>
Date:	Fri, 26 Oct 2012 20:33:21 +0800
From:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
To:	Stefani Seibold <stefani@...bold.net>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/2] kfifo: round up the fifo size power of 2

On Fri, Oct 26, 2012 at 11:30:27AM +0200, Stefani Seibold wrote:
> Am Freitag, den 26.10.2012, 15:56 +0800 schrieb Yuanhan Liu:
> > Say, if we want to allocate a filo with size of 6 bytes, it would be safer
> > to allocate 8 bytes instead of 4 bytes.
> > ----
> > I know it works with rounddown_pow_of_two as well, since size is maintained
> > in the kfifo internal part. But, I'm quite curious why Stefani chose
> > rounddown_pow_of_two. To reduce memory?
> > 
> 
> Yes, exactly, if a user do the wrong thing, than the user will get also
> a wrong result, and did not waste memory.

But, isn't it better to 'correct' it? ;-)

> 
> But anyway, if the majority like this patch it is okay for me.

Sorry, do you mean you are OK with this patch?

Thanks,
Yuanhan Liu
> > 
> > Cc: Stefani Seibold <stefani@...bold.net>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Signed-off-by: Yuanhan Liu <yuanhan.liu@...ux.intel.com>
> > ---
> >  kernel/kfifo.c |    6 +++---
> >  1 files changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/kfifo.c b/kernel/kfifo.c
> > index 59dcf5b..0f78378 100644
> > --- a/kernel/kfifo.c
> > +++ b/kernel/kfifo.c
> > @@ -39,11 +39,11 @@ int __kfifo_alloc(struct __kfifo *fifo, unsigned int size,
> >  		size_t esize, gfp_t gfp_mask)
> >  {
> >  	/*
> > -	 * round down to the next power of 2, since our 'let the indices
> > +	 * round up to the next power of 2, since our 'let the indices
> >  	 * wrap' technique works only in this case.
> >  	 */
> >  	if (!is_power_of_2(size))
> > -		size = rounddown_pow_of_two(size);
> > +		size = roundup_pow_of_two(size);
> >  
> >  	fifo->in = 0;
> >  	fifo->out = 0;
> > @@ -84,7 +84,7 @@ int __kfifo_init(struct __kfifo *fifo, void *buffer,
> >  	size /= esize;
> >  
> >  	if (!is_power_of_2(size))
> > -		size = rounddown_pow_of_two(size);
> > +		size = roundup_pow_of_two(size);
> >  
> >  	fifo->in = 0;
> >  	fifo->out = 0;
> 
--
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