Return-path: Received: from mail3-relais-sop.national.inria.fr ([192.134.164.104]:46843 "EHLO mail3-relais-sop.national.inria.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753916AbaAFJfd (ORCPT ); Mon, 6 Jan 2014 04:35:33 -0500 Date: Mon, 6 Jan 2014 10:35:14 +0100 (CET) From: Julia Lawall To: Geert Uytterhoeven cc: Emmanuel Grumbach , Johannes Berg , Henrique de Moraes Holschuh , Julia Lawall , "kernel-janitors@vger.kernel.org" , Emmanuel Grumbach , Intel Linux Wireless , "John W. Linville" , linux-wireless , "netdev@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH 4/11] use ether_addr_equal_64bits In-Reply-To: Message-ID: (sfid-20140106_103557_020901_E8CF8712) References: <1388427307-8691-1-git-send-email-Julia.Lawall@lip6.fr> <1388427307-8691-5-git-send-email-Julia.Lawall@lip6.fr> <1388429761.4410.1.camel@jlt4.sipsolutions.net> <1388438724.4573.2.camel@jlt4.sipsolutions.net> <20131230215701.GA4938@khazad-dum.debian.net> <1388445188.18164.0.camel@jlt4.sipsolutions.net> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-wireless-owner@vger.kernel.org List-ID: On Mon, 6 Jan 2014, Geert Uytterhoeven wrote: > On Tue, Dec 31, 2013 at 7:26 AM, Emmanuel Grumbach wrote: > > On Tue, Dec 31, 2013 at 1:13 AM, Johannes Berg > > wrote: > >> > >> On Mon, 2013-12-30 at 19:57 -0200, Henrique de Moraes Holschuh wrote: > >> > On Mon, 30 Dec 2013, Johannes Berg wrote: > >> > > On Mon, 2013-12-30 at 20:58 +0100, Julia Lawall wrote: > >> > > > > Is there any way we could catch (sparse, or some other script?) that > >> > > > > struct reorganising won't break the condition needed ("within a > >> > > > > structure that contains at least two more bytes")? > >> > > > > >> > > > What kind of reorganizing could happen? Do you mean that the programmer > >> > > > might do at some time in the future, or something the compiler might do? > >> > > > >> > > I'm just thinking of a programmer, e.g. changing a struct like this: > >> > > > >> > > struct foo { > >> > > u8 addr[ETH_ALEN]; > >> > > - u16 dummy; > >> > > }; > >> > > > >> > > for example. > >> > > >> > That is easily resolved by: > >> > > >> > struct foo { > >> > u8 addr[ETH_ALEN]; > >> > u16 required_padding; /* do not remove upon pain of death */ > >> > }; > > Adding the u16 also changes the alignment of the whole struct. So it may > cost one additional byte _in front of_ the struct. > > > While you're at it, why not just making a new 64-bit aligned type for > Ethernet addresses, so it'll also work for > !CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS? > > > >> That'd be a stupid waste of struct space. If anything, there should be > >> *only* a comment saying that at least two bytes are needed - I'd still > >> prefer an automated check. > > > > Frankly I am not sure I like the patch. This flow is not a fast path > > I also don't like it. To me this sounds like wasting space for nothing. > BTW, would it be that more expensive to always do a 32+16 bit > comparison? No space is wasted by the patch. The patch is only generated in the case where there is currently enough space. julia > > at all. While I don't really care for the waste in iwlwifi (because > > there isn't), I don't see the real point is make the code more > > sensitive to changes to earn basically nothing. > > Thanks to this discussion, my eye fell on: > > static inline unsigned compare_ether_addr(const u8 *addr1, const u8 *addr2) > { > const u16 *a = (const u16 *) addr1; > const u16 *b = (const u16 *) addr2; > > BUILD_BUG_ON(ETH_ALEN != 6); > return ((a[0] ^ b[0]) | (a[1] ^ b[1]) | (a[2] ^ b[2])) != 0; > } > > What if addr1 or addr2 are odd, and this is running on an architecture that > doesn't support unaligned accesses at all?? Have we been lucky forever? > > Gr{oetje,eeting}s, > > Geert > > -- > Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org > > In personal conversations with technical people, I call myself a hacker. But > when I'm talking to journalists I just say "programmer" or something like that. > -- Linus Torvalds > -- > To unsubscribe from this list: send the line "unsubscribe kernel-janitors" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html >