centos man page for blk_queue_dma_drain

BLK_QUEUE_DMA_DRAIN(9)						   Block Devices					    BLK_QUEUE_DMA_DRAIN(9)

NAME
       blk_queue_dma_drain - Set up a drain buffer for excess dma.

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

ARGUMENTS
       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

DESCRIPTION
       Some devices have excess DMA problems and can't 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 aren't 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.

NOTE
       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 won't be room for the drain buffer.

COPYRIGHT
Kernel Hackers Manual 3.10					     June 2014						    BLK_QUEUE_DMA_DRAIN(9)