Re: [PATCH] [POWERPC] Improve (in|out)_beXX() asm code

[Trent Piepho]

On Wed, 21 May 2008, Benjamin Herrenschmidt wrote:
> > Depends on what you define as "necessary".  It's seem clear that I/O accessors
> > _no not_ need to be strictly ordered with respect to normal memory accesses,
> > by what's defined in memory-barriers.txt.  So if by "necessary" you mean what
> > the Linux standard for I/O accessors requires (and what other archs provide),
> > then yes, they have the necessary ordering guarantees.
> >
> > But, if you want them to be strictly ordered w.r.t to normal memory, that's
> > not the case.
>
> They should be.

Someone should update memory-barriers.txt, because it doesn't say that, and
all I/O accessors for all the arches, because none of them are.

> > Here's a quick hack I stuck in a driver to test.  compile with -save-temps and
> > check the resulting asm.  gcc will do the optimization I described above.
> >
> > static void __iomem *baz = (void*)0x1234;
> > static struct bar {
> >      u32 bar[256];
> > } bar;
> >
> > void foo(void) {
> >      bar.bar[0] = 44;
> >      out_be32(baz+100, 200);
> >      bar.bar[0] = 45;
> >      out_be32(baz+101, 201);
> > }
>
> Have you removed -fno-strict-aliasing ? Just don't do that.

No, it's compiled with a normal kernel build, which includes
-fno-strict-aliasing.
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