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Date: Mon, 3 Feb 2014 10:35:02 +0000 From: David Laight <David.Laight@...LAB.COM> To: 'James Hogan' <james.hogan@...tec.com> CC: 'Dan Carpenter' <dan.carpenter@...cle.com>, Chen Gang <gang.chen.5i5j@...il.com>, "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>, "andreas.dilger@...el.com" <andreas.dilger@...el.com>, Antonio Quartulli <antonio@...hcoding.com>, "Greg KH" <gregkh@...uxfoundation.org>, "bergwolf@...il.com" <bergwolf@...il.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, David Miller <davem@...emloft.net>, "oleg.drokin@...el.com" <oleg.drokin@...el.com>, "jacques-charles.lafoucriere@....fr" <jacques-charles.lafoucriere@....fr>, "jinshan.xiong@...el.com" <jinshan.xiong@...el.com>, netdev <netdev@...r.kernel.org>, "linux-metag@...r.kernel.org" <linux-metag@...r.kernel.org> Subject: RE: [PATCH] drivers: staging: lustre: lustre: include: add "__attribute__((packed))" for the related union From: James Hogan > On 03/02/14 10:05, David Laight wrote: > > From: Dan Carpenter > >> On Sat, Feb 01, 2014 at 09:57:39PM +0800, Chen Gang wrote: > >>> It seems, our kernel still stick to treate 'pack' region have effect > >>> with both 'align' and 'sizeof'. > >> > >> It's not about packed regions. It's about unions. It's saying the > >> sizeof() a union is a multiple of 4 unless it's packed. > >> > >> union foo { > >> short x; > >> short y; > >> }; > >> > >> The author intended the sizeof(union foo) to be 2 but on metag arch then > >> it is 4. > > > > The same is probably be true of: struct foo { _u16 bar; }; > > Yes indeed. > > > Architectures that define such alignment rules are a right PITA. > > You either need to get the size to 2 without using 'packed', or > > just not define such structures. > > It is worth seeing if adding aligned(2) will change the size - I'm > > not sure. > > __aligned(2) alone doesn't seem to have any effect on sizeof() or > __alignof__() unless it is accompanied by __packed. x86_64 is similar in > that respect (it just packs sanely in the first place). > > Combining __packed with __aligned(2) does the trick though (__packed > alone sets __aligned(1) which is obviously going to be suboptimal). Compile some code for a cpu that doesn't support misaligned transfers (probably one of sparc, arm, ppc) and see if the compiler generates a single 16bit request or two 8 bits ones. You don't want the compiler generating multiple byte-sized memory transfers. David -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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