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

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Message-Id: <201106202239.31997.arnd@arndb.de>
Date:	Mon, 20 Jun 2011 22:39:31 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Nicolas Pitre <nico@...xnic.net>, gregkh@...e.de,
	linux-usb@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>,
	Rabin Vincent <rabin@....in>,
	Alan Stern <stern@...land.harvard.edu>,
	Alexander Holler <holler@...oftware.de>
Subject: Re: [PATCH] USB: ehci: use packed, aligned(4) instead of removing the packed attribute

On Monday 20 June 2011 22:28:49 Nicolas Pitre wrote:
> On Mon, 20 Jun 2011, Arnd Bergmann wrote:
> 
> > On Monday 20 June 2011 19:39:34 Alexander Holler wrote:
> > > That packed without an additional aligned() caused errors on ARM with 
> > > gcc 4.6 is another problem which got (currently) fixed by removing packed.
> > 
> > Packed caused errors because it is *wrong*. The code as it was used undefined
> > behavior in the language.
> 
> I wouldn't call this issue as such, but this is a Red herring.
> 
> Could you please provide a pointer to the structure definition so a 
> second opinion to the usefulness of __packed there could be provided?  

The structures in question are ehci_caps, ehci_regs and ehci_dbg_port.
The patch that remove the __packed attribute was 139540170 "USB: ehci:
remove structure packing from ehci_def".

The reason why I consider it a bug is that an access to a register
using readl/writel on the structure requires casting a pointer with
byte alignment to a pointer with word alignment, which is undefined
in C. Gcc just tries to be helpful and work around this by turning
the access into bytewise load/store instructions. In older gcc versions,
it would not do that if you happen to also case from non-volatile to
volatile pointer, but according to Uli that was not an intentional
feature of gcc but the ARM code just worked by pure coincidence.

> If it is not matching any of the fairly limited cases where having 
> __packed is relevant then we can just confirm that it should go.

It's already gone.

	Arnd
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