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

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Message-Id: <20121030235210.4dfc6ef7.akpm@linux-foundation.org>
Date:	Tue, 30 Oct 2012 23:52:10 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Stefani Seibold <stefani@...bold.net>
Cc:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] kfifo: round up the fifo size power of 2

On Wed, 31 Oct 2012 07:30:33 +0100 Stefani Seibold <stefani@...bold.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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