netdev - Re: [PATCH] Fix ARM BUILD_BUG_ON() errors with batman-adv

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Message-ID: <20131130230319.GB16735@n2100.arm.linux.org.uk>
Date:	Sat, 30 Nov 2013 23:03:20 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	David Miller <davem@...emloft.net>
Cc:	mareklindner@...mailbox.ch, sw@...onwunderlich.de,
	antonio@...hcoding.com, b.a.t.m.a.n@...ts.open-mesh.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH] Fix ARM BUILD_BUG_ON() errors with batman-adv

On Sat, Nov 30, 2013 at 04:05:47PM -0500, David Miller wrote:
> From: Russell King - ARM Linux <linux@....linux.org.uk>
> Date: Sat, 30 Nov 2013 19:15:53 +0000
> 
> > so there should be no undesired side effect from this packing.
> 
> There is a huge side effect from ever using the packed attribute, in
> that the compiler can assume absolutely nothing about the alignment of
> any object or sub-object of the type you apply this attribute to.
> 
> Even if it is "obvious" that some members will be aligned, the
> compiler cannot take advantage of this assumption because this
> attribute also means that an array of such elements might have
> arbitrary alignment.  So you when you get a pointer to one of these
> objects, the compiler has to assume the worst possible case.
> 
> This means using 4 byte loads to load a 32-bit quantity, always,
> unconditionally, no matter what.
> 
> That's why we should do whatever is necessary to align things properly
> by hand, and use packed only as the last possible and least desirable
> resort.
> 
> I'm not applying this, please try work to implement this more
> acceptably first.

Okay, someone else fixes this problem then, I've no idea how this can
be fixed.
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