On Mon, 20 Jun 2011, Alan Stern wrote:

On Mon, 20 Jun 2011, Nicolas Pitre wrote:

quoted

On Mon, 20 Jun 2011, Alan Stern wrote:

quoted

On Mon, 20 Jun 2011, Alexander Holler wrote:

quoted

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?

quoted

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.

quoted

 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