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

From: Scott Wood
Date: Tue May 20 2008 - 18:45:41 EST

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.

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