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

From: David Miller
Date: Tue May 20 2008 - 18:43:43 EST

From: Scott Wood <scottwood@xxxxxxxxxxxxx>
Date: Tue, 20 May 2008 17:35:56 -0500

> Alan Cox wrote:
> >> It looks like we rely on -fno-strict-aliasing to prevent reordering
> >> ordinary memory accesses (such as to DMA descriptors) past the I/O
> >
> > DMA descriptors in main memory are dependant on cache behaviour anyway
> > and the dma_* operators should be the ones enforcing the needed behaviour.
> What about memory obtained from dma_alloc_coherent()? We still need a
> sync and a compiler barrier. The current I/O accessors have the former,
> but not the latter.

The __volatile__ in the asm construct disallows movement of the
inline asm relative to statements surrounding it.

The only reason barrier() in kernel.h needs a memory clobber is
because of a bug in ancient versions of gcc. In fact, I think
that memory clobber might even be removable.

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