Re: MMIO and gcc re-ordering issue

Benjamin Herrenschmidt
2008-05-27T22:20:03Z

On Tue, 2008-05-27 at 14:55 -0700, Linus Torvalds wrote:
>
> On Wed, 28 May 2008, Benjamin Herrenschmidt wrote:
> >
> > A problem with __raw_ though is that they -also- don't do byteswap,
>
> Well, that's why there is __readl() and __raw_readl(), no?

As I replied to somebody else, __readl() is news to me :-) we dont' have
those on powerpc.

> Neither does ordering, and __raw_readl() doesn't do byte-swap.

But I can add them :-)

> Of course, I'm not going to guarantee every architecture even has all
> those versions, nor am I going to guarantee they all work as advertised :)
>
> For x86, they have historially all been 100% identical. With the inline
> asm patch I posted, the "__" version (whether "raw" or not) lack the
> "memory" barrier, so they allow a *little* bit more re-ordering.
>
> (They won't be re-ordered wrt spinlocks etc, unless gcc starts reordering
> volatile asm's against each other, which would be a bug).
>
> In practice, I doubt it matters. Whatever small compiler re-ordering it
> might affect won't have any real performance impack one way or the other,
> I think.

I prefer explicit endian. Always. Thus I prefer introducing _be variants
(we already have those on powerpc and iomap has it's own _be versions
too) so we should probably generalize _be.

Ben.