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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



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