Messages in this thread | | | Date | Tue, 30 Oct 2012 23:52:10 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 1/2] kfifo: round up the fifo size power of 2 |
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On Wed, 31 Oct 2012 07:30:33 +0100 Stefani Seibold <stefani@seibold.net> wrote:
> > 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).
Probably an insignificant change in performance.
It could be made much smaller by just never doing the modulus operation - instead do
if (++index == max) index = 0;
this does introduce one problem: it's no longer possible to distinguish the "full" and "empty" states by comparing the head and tail indices. But that is soluble.
> 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.
The power-of-2 thing is just a restriction in the current implementation - it's not a good idea to cement that into the interface. Of course, it could later be uncemented if the implementation's restriction was later relaxed.
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