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blk_queue_dma_drain(9) [centos man page]

BLK_QUEUE_DMA_DRAIN(9)						   Block Devices					    BLK_QUEUE_DMA_DRAIN(9)

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)

Check Out this Related Man Page

PHYSIO(9)						   BSD Kernel Developer's Manual						 PHYSIO(9)

physio -- initiate I/O on raw devices SYNOPSIS
int physio((*strategy)(struct buf *), struct buf *bp, dev_t dev, int flags, (*minphys)(struct buf *), struct uio *uio); DESCRIPTION
The physio() is a helper function typically called from character device read and write routines to start I/O on a user process buffer. It calls back on the provided strategy routine one or more times to complete the transfer described by uio. The maximum amount of data to transfer with each call to strategy is determined by the minphys routine. Since uio normally describes user space addresses, physio() needs to lock the appropriate data area into memory before each transaction with strategy (see uvm_vslock(9) and uvm_vsunlock(9)). The physio() function always awaits the completion of the entire requested transfer before returning, unless an error condition is detected earlier. In all cases, the buffer passed in bp is locked (marked as ``busy'') for the dura- tion of the entire transfer. A break-down of the arguments follows: strategy The device strategy routine to call for each chunk of data to initiate device I/O. bp The buffer to use with the strategy routine. The buffer flags will have B_BUSY, B_PHYS, and B_RAW set when passed to the strategy routine. If NULL, a buffer is allocated from a system pool. dev The device number identifying the device to interact with. flags Direction of transfer; the only valid settings are B_READ or B_WRITE. minphys A device specific routine called to determine the maximum transfer size that the device's strategy routine can handle. uio The description of the entire transfer as requested by the user process. Currently, the results of passing a uio structure with the 'uio_segflg' set to anything other than UIO_USERSPACE, are undefined. RETURN VALUES
If successful physio() returns 0. EFAULT is returned if the address range described by uio is not accessible by the requesting process. physio() will return any error resulting from calls to the device strategy routine, by examining the B_ERROR buffer flag and the 'b_error' field. Note that the actual transfer size may be less than requested by uio if the device signals an ``end of file'' condition. SEE ALSO
read(2), write(2) BSD
April 13, 2010 BSD
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