linux-kernel - Re: [PATCH] USB: ehci: use packed, aligned(4) instead of removing the packed attribute

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Date:	Mon, 20 Jun 2011 15:20:44 -0400 (EDT)
From:	Nicolas Pitre <nico@...xnic.net>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Alan Stern <stern@...land.harvard.edu>, gregkh@...e.de,
	linux-usb@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>,
	Rabin Vincent <rabin@....in>,
	Alexander Holler <holler@...oftware.de>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] USB: ehci: use packed,
 aligned(4) instead of removing the packed attribute

On Mon, 20 Jun 2011, Russell King - ARM Linux wrote:

> On Mon, Jun 20, 2011 at 06:58:45PM +0200, Arnd Bergmann wrote:
> > A recent change in gcc changed the default behaviour when compiling the
> > ehci driver on ARM, but the behaviour was already nondeterministic
> > because the definition of the readl/writel macros on ARM relies on
> > unspecified behaviour (cast to pointer with larger aligment).
> 
> It's unspecified behaviour period.  If you pass a pointer to readl/writel
> which is not word aligned, what you get back is anyones guess.

The pointer _is_ aligned.  The problem is that it comes from the address 
of a structure member, which structure is marked __packed.

Older GCC would see the cast to unsigned int applied to that pointer 
within the ARM's readl()/writel() implementation and ignore that the 
source of the address is marked __packed, which __packed implies an 
alignment of 1.  REcent GCC versions would honnor the alignment of 1 and 
perform the unsigned int * access using byte sized loads/stores.


Nicolas
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