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

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Date:	Wed, 31 Oct 2012 13:59:16 +0800
From:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, Stefani Seibold <stefani@...bold.net>
Subject: Re: [PATCH 1/2] kfifo: round up the fifo size power of 2

On Mon, Oct 29, 2012 at 01:59:35PM -0700, Andrew Morton wrote:
> On Fri, 26 Oct 2012 15:56:57 +0800
> Yuanhan Liu <yuanhan.liu@...ux.intel.com> wrote:
> 
> > 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.
> >
> > ...
> >
> > --- 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;
> 
> hm, well, if the user asked for a 100-element fifo then it is a bit
> strange and unexpected to give them a 128-element one.

Hi Andrew,

Yes, and I guess the same to give them a 64-element one.

> 
> If there's absolutely no prospect that the kfifo code will ever support
> 100-byte fifos then I guess we should rework the API so that the caller
> has to pass in log2 of the size, not the size itself.  That way there
> will be no surprises and no mistakes.
> 
> That being said, the power-of-2 limitation isn't at all intrinsic to a
> fifo, so we shouldn't do this.  Ideally, we'd change the kfifo
> implementation so it does what the caller asked it to do!

I'm fine with removing the power-of-2 limitation. Stefani, what's your
comment on that?

	--yliu
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