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




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