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

[Stefani Seibold <stefani@seibold.net>]

Am Mittwoch, den 31.10.2012, 13:59 +0800 schrieb Yuanhan Liu:
> 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@linux.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.
> > >
> > > ...
> > >  	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.
> 
> 
> 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?
> 

You can't remove the power-of-2-limitation, since this would result in a
performance decrease (bit wise and vs. modulo operation).

Andrew is right, this is an API miss design.  So it would be good to
rework the kfifo_init () and kfifo_alloc() to pass in log2 of the size,
not the size itself.

Stefani