Re: [PATCH] WAVELAN - compile-time check for struct sizes

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Wed, Feb 06, 2008 at 01:04:48PM -0800, Andrew Morton wrote:
> On Wed, 6 Feb 2008 21:50:23 +0100
> Helge Deller <deller@gmx.de> wrote:
> 
> > > > +	/* compile-time check the sizes of structures */
> > > > +	BUILD_BUG_ON(sizeof(psa_t) != PSA_SIZE);
> > > > +	BUILD_BUG_ON(sizeof(mmw_t) != MMW_SIZE);
> > > 
> > > This assertion is now triggering with arm allmodconfig.
> > > 
> > > Rafael, please track this as a post-2.6.24 regression.
> > 
> > Hello Andrew,
> > 
> > with which arm platform did you found this assertion to trigger ?
> > I tried a few (e.g. ARM-poodle and CONFIG_ARCH_SA1100 w/ISA) but didn't saw it breaking.
> > Maybe you could send me you .config file ?
> > 
> 
> allmodconfig
> 
> > 
> > PS: I tried Linus' current git tree which now includes my patch above as well.
> 
> The assertion triggers with current mainline.  I'm using gcc-3.4.5, from
> http://userweb.kernel.org/~akpm/cross-compilers/

I assume that it's the second BUILD_BUG_ON() which is triggering?

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.