On Fri, Jan 09, 2009 at 11:19:36AM +0000, Ralf Baechle wrote: > 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. I really don't think that's a realistic option. You're asking for every call to the DMA API to ensure that the buffer and length are a multiple of the cache line size. So, what happens if, eg, SPI wants to send a 16 byte buffer, and your cache lines are 64 bytes? Does the SPI driver have to kmalloc a new chunk of memory 64 bytes long and copy the data into that before passing it into the DMA API? If you start enforcing that kind of thing, I think the cache coherent people will take violent exception and refuse to play such games - and quite rightly so. -- Russell King Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of: -- 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
