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

[Scott Wood]

David Miller wrote:
> 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.

Current versions of GCC seem quite happy to move non-asm memory accesses 
around a volatile asm without a memory clobber; see the test Trent posted.

-Scott
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