Messages in this thread | | | Date | Wed, 31 Oct 2012 14:49:40 +0800 | From | Yuanhan Liu <> | Subject | Re: [PATCH 1/2] kfifo: round up the fifo size power of 2 |
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On Wed, Oct 31, 2012 at 07:30:33AM +0100, Stefani Seibold wrote: > 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).
Right.
> > 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.
Yes, this would make this issue gone completely. Would you mind to let me do that?
--yliu
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