Date: Fri, 17 Nov 2006 19:33:01 +1100 (EST) From: Bruce Evans <bde@zeta.org.au> To: Yar Tikhiy <yar@comp.chem.msu.su> Cc: src-committers@freebsd.org, jkoshy@freebsd.org, cvs-all@freebsd.org, phk@phk.freebsd.dk, cvs-src@freebsd.org, "M. Warner Losh" <imp@bsdimp.com> Subject: Re: cvs commit: src/include ar.h Message-ID: <20061117192049.H10708@delplex.bde.org> In-Reply-To: <20061116090412.GB37133@comp.chem.msu.su> References: <3801.1163410519@critter.freebsd.dk> <20061113214928.P76443@delplex.bde.org> <20061113.101958.-861030824.imp@bsdimp.com> <20061116090412.GB37133@comp.chem.msu.su>
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On Thu, 16 Nov 2006, Yar Tikhiy wrote: > On Mon, Nov 13, 2006 at 10:19:58AM -0700, M. Warner Losh wrote: >> : >> : BTW, you are responsible for the __packed in <netinet/ip.h>. Please remove >> : it. The __CTASSERT() is enough to detect if heroic packing is ever needed. >> : The only danger is if something has grown to depend on __packed reducing >> : alignment as a side effect. E.g., suppose we had a byte string containing >> : a bytewise copy of a struct ip. If the copy might be misaligned, then it >> : should be copied to an actual struct ip before accessing it as a struct, >> : but code that accesses it directly using ((struct ip *)&bs[N]) would work >> : now due to the reduced alignment. Places that really need __packed should >> : probably use __aligned() to restore the natural alignment. >> >> DO NOT REMOVE IT. IT IS ABSOLUTELY REQUIRED FOR ARM TO WORK RIGHT. >> If you want to remove it, then you must make sure arm works right >> after it because I'll add it back. > > Many years ago I was taught that comments in code could help to > avoid such clashes in software development. Is this true no more? ;-) Apparently not. There has always been a relevant comment in <net/ethernet.h>: %%% /* * Mbuf adjust factor to force 32-bit alignment of IP header. * Drivers should do m_adj(m, ETHER_ALIGN) when setting up a * receive so the upper layers get the IP header properly aligned * past the 14-byte Ethernet header. */ #define ETHER_ALIGN 2 /* driver adjust for IP hdr alignment */ %%% Bruce
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