Re: [PATCH anybus v3 4/6] bus: support HMS Anybus-S bus

[Geert Uytterhoeven]

On Fri, Nov 9, 2018 at 10:03 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Fri, Nov 9, 2018 at 5:25 PM Sven Van Asbroeck <thesven73@xxxxxxxxx> wrote:
> > On Thu, Nov 8, 2018 at 9:07 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > > > +struct anybus_mbox_hdr {
> > > > +       u16 id;
> > > > +       u16 info;
> > > > +       u16 cmd_num;
> > > > +       u16 data_size;
> > > > +       u16 frame_count;
> > > > +       u16 frame_num;
> > > > +       u16 offset_high;
> > > > +       u16 offset_low;
> > > > +       u16 extended[8];
> > > > +} __packed;
> > >
> > > I don't think you want this to be __packed, it won't change the layout
> > > but instead force byte accesses on architectures that don't have
> > > fast unaligned load/store instructions.
> > >
> > > Instead of the __packed, it's almost always better to ensure that
> > > the structure itself is aligned to a 16-bit address.
> > >
> >
> > A general question about __packed.
> >
> > My current understanding is this:
> > (please tell me if it's incorrect or incomplete)
> >
> > + without __packed, the compiler is free to pad the struct in whatever
> > way it feels is best.
> > + with __packed, the fields have to be laid out EXACTLY as specified.
>
> It's not up to the compiler but the ELF ABI. The rules are largely consisten
> among the architectures we support, but there are a couple of notable
> exceptions:
>
> - ARM OABI requires 32-bit alignment for structures
> - x86-32 aligns 64-bit members to 32-bit rather than 64-bit
> - m68k has some oddities, I think they can pack certain
>   members (don't remember the details)

M68k aligns 16-bit and larger members to 16-bit.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds