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<html xmlns="http://www.w3.org/1999/xhtml"><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 V1.75.2" /><link rel="home" href="index.html" title="The Linux Kernel API" /><link rel="up" href="ch14.html" title="Chapter 14. Block Devices" /><link rel="prev" href="re543.html" title="blk_set_default_limits" /><link rel="next" href="re545.html" title="blk_queue_bounce_limit" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center"><span>blk_queue_make_request</span></th></tr><tr><td width="20%" align="left"><a accesskey="p" href="re543.html">Prev</a> </td><th width="60%" align="center">Chapter 14. Block Devices</th><td width="20%" align="right"> <a accesskey="n" href="re545.html">Next</a></td></tr></table><hr /></div><div class="refentry" title="blk_queue_make_request"><a id="API-blk-queue-make-request"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>blk_queue_make_request —
define an alternate make_request function for a device
</p></div><div class="refsynopsisdiv" title="Synopsis"><h2>Synopsis</h2><div class="funcsynopsis"><table border="0" summary="Function synopsis" cellspacing="0" cellpadding="0" class="funcprototype-table"><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> </td><td>make_request_fn * <var class="pdparam">mfn</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1" title="Arguments"><a id="id3034757"></a><h2>Arguments</h2><div class="variablelist"><dl><dt><span class="term"><em class="parameter"><code>q</code></em></span></dt><dd><p>
the request queue for the device to be affected
</p></dd><dt><span class="term"><em class="parameter"><code>mfn</code></em></span></dt><dd><p>
the alternate make_request function
</p></dd></dl></div></div><div class="refsect1" title="Description"><a id="id3034801"></a><h2>Description</h2><p>
The normal way for <span class="structname">struct bios</span> to be passed to a device
driver is for them to be collected into requests on a request
queue, and then to allow the device driver to select requests
off that queue when it is ready. This works well for many block
devices. However some block devices (typically virtual devices
such as md or lvm) do not benefit from the processing on the
request queue, and are served best by having the requests passed
directly to them. This can be achieved by providing a function
to <code class="function">blk_queue_make_request</code>.
</p></div><div class="refsect1" title="Caveat"><a id="id3034829"></a><h2>Caveat</h2><p>
The driver that does this *must* be able to deal appropriately
with buffers in <span class="quote">“<span class="quote">highmemory</span>”</span>. This can be accomplished by either calling
<code class="function">__bio_kmap_atomic</code> to get a temporary kernel mapping, or by calling
<code class="function">blk_queue_bounce</code> to create a buffer in normal memory.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="re543.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch14.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="re545.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top"><span>blk_set_default_limits</span> </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> <span>blk_queue_bounce_limit</span></td></tr></table></div></body></html>
