Messages in this thread | | | Date | Tue, 20 May 2008 17:43:58 -0500 | From | Scott Wood <> | Subject | Re: [PATCH] [POWERPC] Improve (in|out)_beXX() asm code |
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David Miller wrote: > From: Scott Wood <scottwood@freescale.com> > 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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