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If buffer is to be accessed by cpu after dma is over, but between dma mapping and dma unmapping, we should use dma_sync_single_for_cpu to sync the buffer between cpu with device. And dma_sync_single_for_device is used to let device gain the buffer again. v2: Felix pointed out dma_sync_single_for_device is needed to return buffer to device if an unsuccessful status bit check is found. Signed-off-by: Ming Lei <tom.leiming@gmail.com> Acked-by: Felix Fietkau <nbd@openwrt.org> Signed-off-by: John W. Linville <linville@tuxdriver.com>