* Laurent Pinchart | 2011-08-01 13:13:15 [+0200]: >Hi Sebastian, Hi Laurent, >> What about using kmalloc() + dma_map_single() + dma_unmap_single() >> instead of arch dependend code in drivers? Those are nops x86 and >> perform the required syncs on other architectures. > >Do you mean creating and tearing down the mapping for each transform ? Isn't >that very costly on many non-x86 platforms ? You need consistent memory between hardware and cpu and this leaves you with two options: - use uncached memory - use cached memory and flush it If the architecture is cache coherent then you don't have to worry, the memory controller will snoop the cache line and invalidate the cache line or update the memory. If this is not the case then you have to do this on your own. Using uncached memory has the benefit that reads/writes are performed directly to main memory but since you bypass the cache, every access goes across the bus. So uncached memory is probably a good if you need to access the memory just once or twice. The dma ops usually flush the cache operations. So once the cache line for this memory address is clean then the CPU can fill its cache by fetching a bigger memory block instead of 4 bytes only (or 1 if the memcpy works on bytes). Is it possible to align the buffers somehow differrently and avoid the memcpy in the first place? Sebastian -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
