Date: Mon, 18 Feb 2008 22:53:54 +0000
From: Russell King <rmk+lkml@arm.linux.org.uk>
To: Michael Buesch <mb@bu3sch.de>
Cc: Gordon Farquharson <gordonfarquharson@gmail.com>,
       linux-kernel@vger.kernel.org, linville@tuxdriver.com,
       stefano.brivio@polimi.it
Subject: Re: [RFC] [PATCH] Fix b43 driver build for arm
Message-ID: <20080218225354.GE23181@flint.arm.linux.org.uk>
References: <97a0a9ac0802181403ja79c32v864b093414b2755@mail.gmail.com> <200802182324.45267.mb@bu3sch.de> <20080218223410.GD23181@flint.arm.linux.org.uk> <200802182343.12749.mb@bu3sch.de>
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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.

At a _guess_ and its only a guess, the linker will enforce this rule
between compilation units, otherwise the implications are disgusting
(would probably result in all loads having to be individual byte loads
and instructions to combine the result - since ARM has strict alignment
requirements.)

What I can say is that the ABI will not be changed because someone in the
kernel decides they don't like it.  So the options are: either fix it so
it works, or accept that the code is broken and will never work on ARM.

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