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

[Stefani Seibold <stefani@seibold.net>]

Am Dienstag, den 30.10.2012, 23:52 -0700 schrieb Andrew Morton:
> 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.
> 

And you will increase the code size, since kfifo_put and kfifo_get are
inline code. Also the speculative execution path of modern CPUs must
kick away the pipeline in case of are false branch prediction.
 
> > 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.

The power-of-2 thing is a design restriction, a balance between
performance and code size and usability.