1BLK_QUEUE_DMA_DRAIN(9)           Block Devices          BLK_QUEUE_DMA_DRAIN(9)
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NAME

6       blk_queue_dma_drain - Set up a drain buffer for excess dma.
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SYNOPSIS

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

14       q
15           the request queue for the device
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17       dma_drain_needed
18           fn which returns non-zero if drain is necessary
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20       buf
21           physically contiguous buffer
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23       size
24           size of the buffer in bytes
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DESCRIPTION

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

36       This routine adjusts max_hw_segments to make room for appending the
37       drain buffer. If you call blk_queue_max_segments after calling this
38       routine, you must set the limit to one fewer than your device can
39       support otherwise there won´t be room for the drain buffer.
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42Kernel Hackers Manual 2.6.         June 2019            BLK_QUEUE_DMA_DRAIN(9)
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