On Sun, 19 Jun 2011, Arnd Bergmann wrote:
> On Sunday 19 June 2011 21:00:01 Alan Stern wrote:
> > On Sun, 19 Jun 2011, Nicolas Pitre wrote:
> > > On Thu, 16 Jun 2011, Arnd Bergmann wrote:
> > > > On Thursday 16 June 2011 22:10:53 Alexander Holler wrote:
> > > > At least I would be happier without the patch. I'm trying to convince
> > > > people to not use these attributes unless required because too much
> > > > harm is done when they are used without understanding the full
> > > > consequences. I also recommend using __packed as localized as possible,
> > > > i.e. set it for the members that need it, not the entire struct.
> > > >
> > > > I agree that your patch is harmless, it's just the opposite of
> > > > a cleanup in my opinion.
> > >
> > > The question is: does the structure really has to be packed?
> >
> > What do you mean? The structure really does need to be allocated
> > without padding between the fields; is that the same thing? So do a
> > bunch of other structures that currently have no annotations at all.
>
> I guess the issue is that some ABIs actually require a minimum alignment,
> like the old ARM ABI that you can still use to build the kernel.
>
> If a structure is not a multiple of four bytes in size, that ABI
> will add padding at the end, e.g. in
>
> struct s {
> char c[2];
> };
>
> struct t {
> struct s t1;
> unsigned short t2[3];
> };
>
> On most architectures, struct s will be two bytes in size and one byte
> aligned, while struct t is eight bytes and two byte aligned.
>
> On ARM oABI, struct s ends up with four byte size and alignment while
> struct t is twelve bytes long. All this is ok for regular structures,
> but not when they are used to describe memory layout of hardware
> registers on on-wire packets.
Agreed. Is that the case with EHCI though? In your example, you'd have
to mark that structure as packed,aligned(2).
> > > If it does, then the follow-up question is: is a packing on word
> > > boundaries sufficient?
> >
> > > If the answer is yes in both cases, then having packed,aligned(4) is not
> > > a frivolity but rather a correctness issue.
> >
> > Why so? Current systems work just fine without it.
>
> I think Nicolas got it backwards here, adding both packed and
> aligned(4) would make a structure like the one above consistently
> incorrect when used to describe a tightly packed hardware structure.
I didn't look at the details, but your example above requires
aligned(2). That tells the compiler that it may use instructions to
access the data (or hardware) that are up to 2-byte wide (on ARM that
means STRh/LDRH) for the data of that width instead of the byte per byte
loads.
>
> In this case, we would have to do
>
> struct s {
> char c[2];
> } __packed;
>
> struct t {
> struct s t1;
> unsigned short t2[3] __aligned(2);
> } __packed;
>
> To tell the compiler that t2 is indeed aligned, while struct t
> is packed to include no padding around t.
... and give the aligned(2) attribute to struct t so those shorts are
accessed with a 16-bit wide load/store not byte loads/stores.
Nicolas
--
To unsubscribe from this list: send the line "unsubscribe linux-usb" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
