On Thu, Jan 08, 2009 at 09:36:03AM +0100, Haavard Skinnemoen wrote: > In the general case, however, I think MIPS has a bug: I've seen drivers > DMA to/from tiny buffers stored inside another struct. This is legal > because the driver can guarantee that the other fields in the struct > aren't accessed in the mean time, but any fields sharing a cacheline > with the buffer must be written back before the lines are invalidated. Depending on the implementation details, the use of such a struct might be relying on implementation-specific behaviour. This is what Documentation/DMA-API.txt has to say: [...] int dma_get_cache_alignment(void) Returns the processor cache alignment. This is the absolute minimum alignment *and* width that you must observe when either mapping memory or doing partial flushes. Notes: This API may return a number *larger* than the actual cache line, but it will guarantee that one or more cache lines fit exactly into the width returned by this call. It will also always be a power of two for easy alignment. [...] Since dma_get_cache_alignment() is a function not a constant its result can't be used in the definition of a struct unless possibly excessive padding is used. The debate has shown that we problably need BUG_ON() assertions in the DMA API implementations to catch this sort of dangerous use. Ralf -- 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
