From: David Laight <David.Laight@ACULAB.COM>
To: "'James Hogan'" <james.hogan@imgtec.com>
Cc: "'Dan Carpenter'" <dan.carpenter@oracle.com>,
Chen Gang <gang.chen.5i5j@gmail.com>,
"devel@driverdev.osuosl.org" <devel@driverdev.osuosl.org>,
"andreas.dilger@intel.com" <andreas.dilger@intel.com>,
Antonio Quartulli <antonio@meshcoding.com>,
"Greg KH" <gregkh@linuxfoundation.org>,
"bergwolf@gmail.com" <bergwolf@gmail.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
David Miller <davem@davemloft.net>,
"oleg.drokin@intel.com" <oleg.drokin@intel.com>,
"jacques-charles.lafoucriere@cea.fr"
<jacques-charles.lafoucriere@cea.fr>,
"jinshan.xiong@intel.com" <jinshan.xiong@intel.com>,
netdev <netdev@vger.kernel.org>,
"linux-metag@vger.kernel.org" <linux-metag@vger.kernel.org>
Subject: RE: [PATCH] drivers: staging: lustre: lustre: include: add "__attribute__((packed))" for the related union
Date: Mon, 3 Feb 2014 10:35:02 +0000 [thread overview]
Message-ID: <063D6719AE5E284EB5DD2968C1650D6D0F6B777A@AcuExch.aculab.com> (raw)
In-Reply-To: <52EF6DCC.6040807@imgtec.com>
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
next prev parent reply other threads:[~2014-02-03 10:35 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-01-18 9:50 [PATCH] drivers: staging: lustre: lustre: include: add "__attribute__((packed))" for the related union Chen Gang
2014-01-18 10:05 ` Dan Carpenter
2014-01-18 10:26 ` Chen Gang
2014-01-18 14:24 ` Dan Carpenter
2014-01-19 10:07 ` Chen Gang
2014-01-20 11:56 ` James Hogan
2014-01-20 12:30 ` Dan Carpenter
2014-01-20 12:37 ` James Hogan
2014-01-20 12:56 ` Dan Carpenter
2014-01-20 13:01 ` Dan Carpenter
2014-01-20 13:38 ` James Hogan
2014-01-20 21:13 ` Dan Carpenter
2014-01-21 10:36 ` James Hogan
2014-01-25 11:55 ` Chen Gang
2014-02-01 13:57 ` Chen Gang
2014-02-03 8:58 ` Dan Carpenter
2014-02-03 10:03 ` Chen Gang
2014-02-03 11:35 ` Chen Gang
2014-02-03 10:05 ` David Laight
2014-02-03 10:22 ` James Hogan
2014-02-03 10:30 ` Chen Gang
2014-02-03 10:35 ` David Laight [this message]
2014-02-03 11:02 ` James Hogan
2014-02-03 11:54 ` David Laight
2014-02-03 10:25 ` Chen Gang
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