Man page 9 : blk_queue_dma_drain

blk_queue_dma_drain - Set up a drain buffer for excess dma.

int blk_queue_dma_drain(struct request_queue *  q , dma_drain_needed_fn *  dma_drain_needed , void *  buf , unsigned int  size );

q
    the request queue for the device

dma_drain_needed
    fn which returns non-zero if drain is necessary

buf
    physically contiguous buffer

size
    size of the buffer in bytes

Some devices have excess DMA problems and canAqt simply discard (or zero fill) the unwanted piece of the transfer. They have to have a real area of memory to transfer it into. The use case for this is ATAPI devices in DMA mode. If the packet command causes a transfer bigger than the transfer size some HBAs will lock up if there arenAqt DMA elements to contain the excess transfer. What this API does is adjust the queue so that the buf is always appended silently to the scatterlist.

This routine adjusts max_hw_segments to make room for appending the drain buffer. If you call blk_queue_max_segments after calling this routine, you must set the limit to one fewer than your device can support otherwise there wonAqt be room for the drain buffer.