From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754283Ab2JaGw0 (ORCPT ); Wed, 31 Oct 2012 02:52:26 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:52689 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754098Ab2JaGwY (ORCPT ); Wed, 31 Oct 2012 02:52:24 -0400 Date: Tue, 30 Oct 2012 23:52:10 -0700 From: Andrew Morton To: Stefani Seibold Cc: Yuanhan Liu , linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] kfifo: round up the fifo size power of 2 Message-Id: <20121030235210.4dfc6ef7.akpm@linux-foundation.org> In-Reply-To: <1351665033.23165.6.camel@wall-e> References: <1351238218-22648-1-git-send-email-yuanhan.liu@linux.intel.com> <20121029135935.bb8b0b2a.akpm@linux-foundation.org> <20121031055916.GC29509@yliu-dev.sh.intel.com> <1351665033.23165.6.camel@wall-e> X-Mailer: Sylpheed 2.7.1 (GTK+ 2.18.9; x86_64-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 31 Oct 2012 07:30:33 +0100 Stefani Seibold 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.