linux-kernel - Re: [RFC] [PATCH] Fix b43 driver build for arm

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Date:	Mon, 18 Feb 2008 23:00:58 +0000
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	Michael Buesch <mb@...sch.de>
Cc:	Gordon Farquharson <gordonfarquharson@...il.com>,
	linux-kernel@...r.kernel.org, linville@...driver.com,
	stefano.brivio@...imi.it
Subject: Re: [RFC] [PATCH] Fix b43 driver build for arm

On Mon, Feb 18, 2008 at 10:53:54PM +0000, Russell King wrote:
> On Mon, Feb 18, 2008 at 11:43:12PM +0100, Michael Buesch wrote:
> > On Monday 18 February 2008 23:34:10 Russell King wrote:
> > > On Mon, Feb 18, 2008 at 11:24:44PM +0100, Michael Buesch wrote:
> > > > On Monday 18 February 2008 23:13:24 Russell King wrote:
> > > > > On Mon, Feb 18, 2008 at 11:08:56PM +0100, Michael Buesch wrote:
> > > > > > On Monday 18 February 2008 23:03:10 Gordon Farquharson wrote:
> > > > > > > The b43 driver in 2.6.25-rc[12] fails to build for arm on an x86_64
> > > > > > > box using a cross-compiler:
> > > > > > > 
> > > > > > > FATAL: drivers/net/wireless/b43/b43: sizeof(struct ssb_device_id)=6 is
> > > > > > > not a modulo of the size of section __mod_ssb_device_table=64.
> > > > > > > Fix definition of struct ssb_device_id in mod_devicetable.h
> > > > > > 
> > > > > > Why does ARM have this special requirement and what is it about?
> > > > > 
> > > > > Because ARM is a 32 bit architecture.
> > > > 
> > > > Ehm, I didn't see this warning on any other architecture nor did we
> > > > have _any_ problem with it on any other architecture.
> > > > This code runs fine on x86_32, x86_64, powerpc and mips, at least.
> > > 
> > > Well, don't expect this driver to work until you fix your broken
> > > assumptions about alignment requirements.
> > 
> > Mr King, I'm not an idiot!
> 
> I get extremely pissed off everytime I have to try to explain random
> alignment issues to people.  "It doesn't work like i386 so it must be
> broken" is a rediculous position to take.
> 
> > Can you _please_ explain what makes ARM so special here?
> 
> No because I don't really know.
> 
> > Why can't we have an array of this structure on ARM?
> > 
> > struct ssb_device_id {
> >        __u16   vendor;
> 
> 2 bytes
> 
> >        __u16   coreid;
> 
> 2 bytes
> 
> >        __u8    revision;
> 
> 1 byte
> 
> > };
> 
> and therefore sizeof this structure will be 5 bytes, but because of the
> ABI rules (which are _explicitly_ allowed by the C standard), it'll
> become 8 bytes due to padding afterwards.

Another guess might be that, if using AEABI, this structure might
be 6 bytes in size, but the linker will align structures to 4 bytes.

As I say, this is all guess work.  I don't know for sure.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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