1<html><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968"><title>blk_queue_make_request</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="index.html" title="The Linux Kernel API"><link rel="up" href="blkdev.html" title="Chapter&#160;14.&#160;Block Devices"><link rel="prev" href="API-blk-set-stacking-limits.html" title="blk_set_stacking_limits"><link rel="next" href="API-blk-queue-bounce-limit.html" title="blk_queue_bounce_limit"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center"><span class="phrase">blk_queue_make_request</span></th></tr><tr><td width="20%" align="left"><a accesskey="p" href="API-blk-set-stacking-limits.html">Prev</a>&#160;</td><th width="60%" align="center">Chapter&#160;14.&#160;Block Devices</th><td width="20%" align="right">&#160;<a accesskey="n" href="API-blk-queue-bounce-limit.html">Next</a></td></tr></table><hr></div><div class="refentry"><a name="API-blk-queue-make-request"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>blk_queue_make_request &#8212; 
2     define an alternate make_request function for a device
3 </p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><div class="funcsynopsis"><table border="0" class="funcprototype-table" summary="Function synopsis" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef">void <b class="fsfunc">blk_queue_make_request </b>(</code></td><td>struct request_queue * <var class="pdparam">q</var>, </td></tr><tr><td>&#160;</td><td>make_request_fn * <var class="pdparam">mfn</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer">&#160;</div></div></div><div class="refsect1"><a name="idp1128381580"></a><h2>Arguments</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="parameter"><code>q</code></em></span></dt><dd><p>
4     the request queue for the device to be affected
5    </p></dd><dt><span class="term"><em class="parameter"><code>mfn</code></em></span></dt><dd><p>
6     the alternate make_request function
7    </p></dd></dl></div></div><div class="refsect1"><a name="idp1128384228"></a><h2>Description</h2><p>
8   The normal way for <span class="structname">struct bios</span> to be passed to a device
9   driver is for them to be collected into requests on a request
10   queue, and then to allow the device driver to select requests
11   off that queue when it is ready.  This works well for many block
12   devices. However some block devices (typically virtual devices
13   such as md or lvm) do not benefit from the processing on the
14   request queue, and are served best by having the requests passed
15   directly to them.  This can be achieved by providing a function
16   to <code class="function">blk_queue_make_request</code>.
17</p></div><div class="refsect1"><a name="idp1128385924"></a><h2>Caveat</h2><p>
18   The driver that does this *must* be able to deal appropriately
19   with buffers in <span class="quote">&#8220;<span class="quote">highmemory</span>&#8221;</span>. This can be accomplished by either calling
20   <code class="function">__bio_kmap_atomic</code> to get a temporary kernel mapping, or by calling
21   <code class="function">blk_queue_bounce</code> to create a buffer in normal memory.
22</p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="API-blk-set-stacking-limits.html">Prev</a>&#160;</td><td width="20%" align="center"><a accesskey="u" href="blkdev.html">Up</a></td><td width="40%" align="right">&#160;<a accesskey="n" href="API-blk-queue-bounce-limit.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top"><span class="phrase">blk_set_stacking_limits</span>&#160;</td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top">&#160;<span class="phrase">blk_queue_bounce_limit</span></td></tr></table></div></body></html>
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