BLK_QUEUE_DMA_DRAIN - Online Linux Manual PageSection : 9
Updated : June 2018
Source : Kernel Hackers Manual 2​.6​.
Note : Block Devices

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NAMEblk_queue_dma_drain − Set up a drain buffer for excess dma​.

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

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ARGUMENTSq 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

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DESCRIPTIONSome 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​.

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NOTEThis 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​.

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