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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>usb_buffer_map_sg</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="The Linux-USB Host Side API" /><link rel="up" href="ch05.html" title="Chapter 5. USB Core APIs" /><link rel="prev" href="re102.html" title="usb_buffer_unmap" /><link rel="next" href="re104.html" title="usb_buffer_dmasync_sg" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center"><span>usb_buffer_map_sg</span></th></tr><tr><td width="20%" align="left"><a accesskey="p" href="re102.html">Prev</a> </td><th width="60%" align="center">Chapter 5. USB Core APIs</th><td width="20%" align="right"> <a accesskey="n" href="re104.html">Next</a></td></tr></table><hr /></div><div class="refentry" title="usb_buffer_map_sg"><a id="API-usb-buffer-map-sg"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>usb_buffer_map_sg —
create scatterlist DMA mapping(s) for an endpoint
</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">int <b class="fsfunc">usb_buffer_map_sg </b>(</code></td><td>const struct usb_device * <var class="pdparam">dev</var>, </td></tr><tr><td> </td><td>int <var class="pdparam">is_in</var>, </td></tr><tr><td> </td><td>struct scatterlist * <var class="pdparam">sg</var>, </td></tr><tr><td> </td><td>int <var class="pdparam">nents</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1" title="Arguments"><a id="id3117520"></a><h2>Arguments</h2><div class="variablelist"><dl><dt><span class="term"><em class="parameter"><code>dev</code></em></span></dt><dd><p>
device to which the scatterlist will be mapped
</p></dd><dt><span class="term"><em class="parameter"><code>is_in</code></em></span></dt><dd><p>
mapping transfer direction
</p></dd><dt><span class="term"><em class="parameter"><code>sg</code></em></span></dt><dd><p>
the scatterlist to map
</p></dd><dt><span class="term"><em class="parameter"><code>nents</code></em></span></dt><dd><p>
the number of entries in the scatterlist
</p></dd></dl></div></div><div class="refsect1" title="Description"><a id="id3117598"></a><h2>Description</h2><p>
Return value is either < 0 (indicating no buffers could be mapped), or
the number of DMA mapping array entries in the scatterlist.
</p><p>
The caller is responsible for placing the resulting DMA addresses from
the scatterlist into URB transfer buffer pointers, and for setting the
URB_NO_TRANSFER_DMA_MAP transfer flag in each of those URBs.
</p><p>
Top I/O rates come from queuing URBs, instead of waiting for each one
to complete before starting the next I/O. This is particularly easy
to do with scatterlists. Just allocate and submit one URB for each DMA
mapping entry returned, stopping on the first error or when all succeed.
Better yet, use the usb_sg_*() calls, which do that (and more) for you.
</p><p>
This call would normally be used when translating scatterlist requests,
rather than <code class="function">usb_buffer_map</code>, since on some hardware (with IOMMUs) it
may be able to coalesce mappings for improved I/O efficiency.
</p><p>
Reverse the effect of this call with <code class="function">usb_buffer_unmap_sg</code>.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="re102.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch05.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="re104.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top"><span>usb_buffer_unmap</span> </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> <span>usb_buffer_dmasync_sg</span></td></tr></table></div></body></html>
