From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Kok, Auke" Subject: Re: [PATCH] ixgbe: Introduce new 10GbE driver for Intel 82598 based PCI Express adapters... Date: Mon, 02 Jul 2007 16:57:12 -0700 Message-ID: <468990D8.6040604@intel.com> References: <4688F512.3030801@garzik.org> <20070702214238.GA7085@infradead.org> <46897611.9020207@intel.com> <200707030010.25431.mb@bu3sch.de> <46897934.5040101@garzik.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Michael Buesch , Christoph Hellwig , Stephen Hemminger , "Veeraiyan, Ayyappan" , netdev@vger.kernel.org, arjan@linux.intel.com, akpm@linux-foundation.org To: Jeff Garzik Return-path: Received: from mga01.intel.com ([192.55.52.88]:18873 "EHLO mga01.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757761AbXGBX5O (ORCPT ); Mon, 2 Jul 2007 19:57:14 -0400 In-Reply-To: <46897934.5040101@garzik.org> Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org Jeff Garzik wrote: > Michael Buesch wrote: >> On Tuesday 03 July 2007 00:02:57 Kok, Auke wrote: >>> well, FWIW when I started looking at adding these flags I looked in various >>> subsystems in the kernel and picked an implementation that suited. Guess what >>> pci.h has? ...: >>> >>> unsigned int msi_enabled:1; >>> unsigned int msix_enabled:1; >>> >>> this is literally where I copied the example from >>> >>> I suppose I can fix those, but I really don't understand what all the fuzz is >>> about here. We're only conserving memory and staying far away from the real >> I'm not sure if these bitfields actually _do_ conserve memory. >> Generated code gets bigger (need bitwise masks and stuff). >> Code also needs memory. It probably only conserves memory, if the >> structure is instanciated a lot. > > Actually, that's a good point. On several RISC architectures it > certainly generates bigger code. so, even on i386 it does (17 bytes and 6 instructions to test vs. 10:3 if using a "bool"): unsigned int:1: 8048365: 0f b6 45 f8 movzbl 0xfffffff8(%ebp),%eax 8048369: 0c 01 or $0x1,%al 804836b: 88 45 f8 mov %al,0xfffffff8(%ebp) 804836e: 0f b6 45 f8 movzbl 0xfffffff8(%ebp),%eax 8048372: 24 01 and $0x1,%al 8048374: 84 c0 test %al,%al bool: 8048365: c6 45 fb 01 movb $0x1,0xfffffffb(%ebp) 8048369: 0f b6 45 fb movzbl 0xfffffffb(%ebp),%eax 804836d: 84 c0 test %al,%al unsigned int: 8048365: c7 45 f8 01 00 00 00 movl $0x1,0xfffffff8(%ebp) 804836c: 8b 45 f8 mov 0xfffffff8(%ebp),%eax 804836f: 85 c0 test %eax,%eax using var & flag: 8048365: c7 45 f8 01 00 00 00 movl $0x1,0xfffffff8(%ebp) 804836c: 8b 45 f8 mov 0xfffffff8(%ebp),%eax 804836f: 25 00 04 00 00 and $0x400,%eax 8048374: 85 c0 test %eax,%eax note that using "var & flag" is slightly larger here... 1 extra instruction and 7 more bytes. interesting... but let's stay constructive here: ~/git/linux-2.6 $ grep -r 'unsigned int.*:1;' * | wc -l 748 Is anyone going to fix those? If we don't, someone will certainly again submit patches to add more of these bitfields, after all, some very prominent parts of the kernel still use them. Only recently for instance mac80211 merged like 30 of these.... and drivers/net, include etc.. certainly has a lot of these. Cheers, Auke