From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stephen Hemminger Subject: Re: wireless vs. alignment requirements Date: Tue, 27 Nov 2007 10:39:40 -0800 Message-ID: <20071127103940.314c7844@freepuppy.rosehill> References: <1195893216.4149.186.camel@johannes.berg> <20071124133200.GA27531@gondor.apana.org.au> <1195912176.4149.194.camel@johannes.berg> <20071124141319.GA27819@gondor.apana.org.au> <4748855C.5090103@linux-foundation.org> <474C50D7.5010901@zytor.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: Herbert Xu , Johannes Berg , dsd-aBrp7R+bbdUdnm+yROfE0A@public.gmane.org, davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org, netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-wireless-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: "H. Peter Anvin" Return-path: In-Reply-To: <474C50D7.5010901-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org> Sender: linux-wireless-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: netdev.vger.kernel.org On Tue, 27 Nov 2007 09:16:07 -0800 "H. Peter Anvin" wrote: > Stephen Hemminger wrote: > > Herbert Xu wrote: > >> On Sat, Nov 24, 2007 at 02:49:36PM +0100, Johannes Berg wrote: > >> > >>> Right. I just didn't think that would be a valid value for an > >>> architecture to set. > >>> > >> > >> OK. Let me clarify this a bit more. We require at least one > >> of the following rules to be met: > >> > >> * the IPv4/IPv6 header is aligned by 8 bytes on reception; > >> * or the platform provides unaligned exception handlers. > >> > >> So if your platform violates both rules then it won't work with > >> the IP stack, simple as that. Fortunately I don't think such a > >> platform exists currently on Linux. > >> > >> Cheers, > >> > > > > Then what about hardware that can't dma ethernet to non-aligned address. > > Sky2 hardware breaks if DMA is not 8 byte aligned. IMHO the IP stack > > should handle any alignment, and do the appropriate memove if the CPU requires > > alignment. > > I wrote a patch for the IP stack to realign packets if necessary at one > point. I should dredge it up again and submit it for collective flamage. > > -hpa > Is there any standard kernel config define for "this platform can't do unaligned accesses"? -- Stephen Hemminger