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 4.8.                 January 2017                    BLK_QUEUE_DMA_DRAIN(9)