'\" t
.\"     Title: blk_queue_dma_drain
.\"    Author: [FIXME: author] [see http://docbook.sf.net/el/author]
.\" Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/>
.\"      Date: January 2017
.\"    Manual: Block Devices
.\"    Source: Kernel Hackers Manual 4.8.15
.\"  Language: English
.\"
.TH "BLK_QUEUE_DMA_DRAIN" "9" "January 2017" "Kernel Hackers Manual 4\&.8\&." "Block Devices"
.\" -----------------------------------------------------------------
.\" * Define some portability stuff
.\" -----------------------------------------------------------------
.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.\" http://bugs.debian.org/507673
.\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html
.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.ie \n(.g .ds Aq \(aq
.el       .ds Aq '
.\" -----------------------------------------------------------------
.\" * set default formatting
.\" -----------------------------------------------------------------
.\" disable hyphenation
.nh
.\" disable justification (adjust text to left margin only)
.ad l
.\" -----------------------------------------------------------------
.\" * MAIN CONTENT STARTS HERE *
.\" -----------------------------------------------------------------
.SH "NAME"
blk_queue_dma_drain \- Set up a drain buffer for excess dma\&.
.SH "SYNOPSIS"
.HP \w'int\ blk_queue_dma_drain('u
.BI "int blk_queue_dma_drain(struct\ request_queue\ *\ " "q" ", dma_drain_needed_fn\ *\ " "dma_drain_needed" ", void\ *\ " "buf" ", unsigned\ int\ " "size" ");"
.SH "ARGUMENTS"
.PP
\fIq\fR
.RS 4
the request queue for the device
.RE
.PP
\fIdma_drain_needed\fR
.RS 4
fn which returns non\-zero if drain is necessary
.RE
.PP
\fIbuf\fR
.RS 4
physically contiguous buffer
.RE
.PP
\fIsize\fR
.RS 4
size of the buffer in bytes
.RE
.SH "DESCRIPTION"
.PP
Some devices have excess DMA problems and can\*(Aqt 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\*(Aqt 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\&.
.SH "NOTE"
.PP
This routine adjusts max_hw_segments to make room for appending the drain buffer\&. If you call
\fBblk_queue_max_segments\fR
after calling this routine, you must set the limit to one fewer than your device can support otherwise there won\*(Aqt be room for the drain buffer\&.
.SH "COPYRIGHT"
.br