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 2.6. September 2014 BLK_QUEUE_DMA_DRAIN(9)