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

From: Arnd Bergmann
Date: Mon Jun 20 2011 - 16:39:58 EST

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.

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