Date: Thu, 7 Feb 2008 10:49:12 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: "John W. Linville" <linville@tuxdriver.com>
Cc: Russell King - ARM Linux <linux@arm.linux.org.uk>, deller@gmx.de,
	jt@hpl.hp.com, rjw@sisk.pl, linux-wireless@vger.kernel.org,
	linux-arm-kernel@lists.arm.linux.org.uk
Subject: Re: [PATCH] WAVELAN - compile-time check for struct sizes
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On Thu, 7 Feb 2008 10:51:52 -0500 "John W. Linville" <linville@tuxdriver.com> wrote:

> On Wed, Feb 06, 2008 at 01:59:50PM -0800, Andrew Morton wrote:
> > On Wed, 6 Feb 2008 21:47:47 +0000
> > Russell King - ARM Linux <linux@arm.linux.org.uk> wrote:
> 
> > > I assume that it's the second BUILD_BUG_ON() which is triggering?
> > 
> > yup.
> > 
> > > Given that:
> > > 
> > > #define MMW_SIZE        37
> > > 
> > > is not a multiple of sizeof(unsigned long) this is hardly surprising.
> > > 
> > > If structures are used to define a layout of something and must not
> > > contain compiler padding, it must be packed.  Given these structures
> > > contain just unsigned char, there's no concerns about >8bit loads
> > > becoming less efficient.
> 
> Does a patch like this suffice?  I haven't checked whether such a
> patch implies that the BUILD_BUG_ON()'s become unnecessary...

With your patch applied and arm allmodconfig, this

        BUILD_BUG_ON(sizeof(ha_t) != HA_SIZE);

triggers

Without your patch applied, these two

        BUILD_BUG_ON(sizeof(mmw_t) != MMW_SIZE);
        BUILD_BUG_ON(sizeof(ha_t) != HA_SIZE);

are triggering.