On Fri, 2010-06-11 at 02:38 +0100, Nick Piggin wrote: > On Thu, Jun 10, 2010 at 06:43:03PM -0600, Robert Hancock wrote: > > IMHO, it would be better for the platform code to ensure that MMIO > > access was strongly ordered with respect to each other and to RAM > > access. Drivers are just too likely to get this wrong, especially > > when x86, the most tested platform, doesn't have such issues. > > The plan is to make all platforms do this. writes should be > strongly ordered with memory. That serves to keep them inside > critical sections as well. Are there any public references to this discussion? Maybe a Documentation/ file (or update the memory-barriers.txt one would be useful). I guess correctness takes precedence here but on ARM, the only way to ensure relative ordering between non-cacheable writes and I/O writes is by flushing the write buffer (and an L2 write buffer if external cache is present). Hence the expensive mb(). The only reference of DMA buffers vs I/O I found in the DMA-API.txt file: Consistent memory is memory for which a write by either the device or the processor can immediately be read by the processor or device without having to worry about caching effects. (You may however need to make sure to flush the processor's write buffers before telling devices to read that memory.) But there is no API for "flushing the processor's write buffers". Does it mean that this should be taken care of in writel()? We would make the I/O accessors pretty expensive on some architectures. Thanks. -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html