On Mon, 20 Jun 2011, Alan Stern wrote:
> On Mon, 20 Jun 2011, Nicolas Pitre wrote:
>
> > On Mon, 20 Jun 2011, Alan Stern wrote:
> >
> > > On Mon, 20 Jun 2011, Alexander Holler wrote:
> > >
> > > > I see it that way: packed is needed to be sure that at least for struct
> > > > ehci_regs there are no padding bytes inbetween the members.
> > >
> > > But is it _really_ needed?
> > >
> > > > It might
> > > > work without, but that depends on the compiler (-version, architecture,
> > > > whatever).
> > >
> > > Have there _ever_ been _any_ combinations of compiler, version,
> > > architecture, whatever, that had unwanted padding bytes in this
> > > structure?
> >
> > This can be determined by simple code inspection.
> >
> > If you must have struct members which are not aligned to their natural
> > size then you need __packed. Example:
> >
> > struct foo {
> > u8 a;
> > u16 b;
> > u32 c;
> > u64 d;
> > };
> >
> > Without __packed, there will be padding between a and b, and between c
> > and d.
>
> One byte of padding between a and b is enough. No more is needed, and
> the compiler would have to be pretty stupid to add anything else.
Obviously, my mistake. I meant to make c a u16 too but failed to
correct the example before posting.
> > If the order of the members in this struct were reversed, then
> > everything would be naturally aligned and no padding between members
> > would be inserted.
> >
> > The size of structures is normally rounded up with padding to the size
> > of the largest basic element it contains. Example:
> >
> > struct foo {
> > u64 a;
> > u8 b;
> > };
> >
> > Here sizeof(struct foo) would return 16, even if the actual content
> > occupies 9 bytes only. That's because the largest basic element is u64
> > i.e. 8 bytes. Normally this trailing padding is not an issue, unless
> > you have an array of such a struct or if it is a member of another
> > struct. If you want to get rid of that padding, you need to use
> > __packed again (which of course would make all subsequent instances of
> > that structure in your array completely misaligned too).
> >
> > Two odd exceptions with the old ABI on ARM:
> >
> > - The alignment of a 64-bit value is always 4 bytes not 8.
> >
> > - The size of all structures are always rounded up to a 4-byte boundary,
> > irrespective of their content.
> >
> > If you fall into none of the above issues, then you don't need any
> > __packed, period.
>
> We don't fall into any of these cases, and therefore as you say, we
> don't need packed. Arnd and I have both explained this. So why do you
> keep arguing that we do need it?
Please show me where I keep arguing that you need it?
Nicolas
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