于 Sat, 4 Jul 2009 19:07:00 +0100 Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> 写道: > On Sat, Jul 04, 2009 at 11:41:46PM +0800, Ming Lei wrote: > > dma_map_*() can invalidate cache lines, so dma_unmap_*() may do > > nothing. But if someone accesses the dma buffer during dma > > transfer, you may get the inconsistent result. > > You missed an important statement: accessing the dma buffer during a > dma transfer is not permitted without an intervening call to the > dma_sync_* functions. > > DMA buffers have the idea of "ownership" - they initially start off > being owned by the CPU. Upon dma_map_*(), ownership transfers to the > DMA agent, and remains there until either a dma_unmap_*() or > dma_sync_single_for_cpu(). In the case of dma_sync_single_for_cpu(), > it can only be given back to the DMA agent using > dma_sync_single_for_device(). > IMHO, only if dma transfer is over dma_sync_*_cpu() can be called to access the dma buffer safely. Currently, dma_sync_*_cpu() does nothing like dma_unmap_* in ARM if dma bounce isdisabled. > The important point from the above is that _only_ the current owner > may access the buffer. So, if you've mapped a buffer, accessing the > buffer subsequently without a call to dma_sync_single_for_cpu() is > illegal. -- Lei Ming -- 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
