Re: drivers/block/ub.c

[David S. Miller]

On Mon, 28 Jun 2004 10:15:17 -0400
Scott Wood <scott@xxxxxxxxxxx> wrote:

> On Sun, Jun 27, 2004 at 02:26:28PM -0700, David S. Miller wrote:
> > On Sun, 27 Jun 2004 12:42:21 +0200
> > Oliver Neukum <oliver@xxxxxxxxxx> wrote:
> > 
> > > OK, then it shouldn't be used in this case. However, shouldn't we have
> > > an attribute like __nopadding__ which does exactly that?
> > 
> > It would have the same effect.  CPU structure layout rules don't pack
> > (or using other words, add padding) exactly in cases where it is
> > needed to obtain the necessary alignment.
> 
> No, it wouldn't, as you could drop the assumption that the base of
> the struct can be misaligned.  Thus, the compiler only needs to
> generate unaligned loads and stores for fields which are unaligned
> within the struct, which in this case would be none of them.
> 
> While it's rather unlikely that a struct like this one would ever
> need packing, it would help those structs that do need it by reducing
> the number of fields subjected to unaligned loads and stores.

That's true.  But if one were to propose such a feature to the gcc
guys, I know the first question they would ask.  "If no padding of
the structure is needed, why are you specifying this new
__nopadding__ attribute?"

I think it's bad to just "smack this attribute onto any structure that
_MIGHT_ need it on some platform"  I never do that in my drivers,
and they work on all platforms.  For example, if you have a simple
DMA descriptor structure such as:

	struct txd {
		u32 dma_addr;
		u32 length;
	};

It is just total and utter madness to put a packed or the proposed
__nopadding__ attribute on that structure.  Yet this seems to be
what was suggested now and at the beginning of this thread.
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