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    SubjectRe: [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.


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