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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>Non-Contiguous Buffers</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="Writing an ALSA Driver" /><link rel="up" href="ch11.html" title="Chapter 11. Buffer and Memory Management" /><link rel="prev" href="ch11s02.html" title="External Hardware Buffers" /><link rel="next" href="ch11s04.html" title="Vmalloc'ed Buffers" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Non-Contiguous Buffers</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch11s02.html">Prev</a> </td><th width="60%" align="center">Chapter 11. Buffer and Memory Management</th><td width="20%" align="right"> <a accesskey="n" href="ch11s04.html">Next</a></td></tr></table><hr /></div><div class="section" title="Non-Contiguous Buffers"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="buffer-and-memory-non-contiguous"></a>Non-Contiguous Buffers</h2></div></div></div><p>
If your hardware supports the page table as in emu10k1 or the
buffer descriptors as in via82xx, you can use the scatter-gather
(SG) DMA. ALSA provides an interface for handling SG-buffers.
The API is provided in <code class="filename"><sound/pcm.h></code>.
</p><p>
For creating the SG-buffer handler, call
<code class="function">snd_pcm_lib_preallocate_pages()</code> or
<code class="function">snd_pcm_lib_preallocate_pages_for_all()</code>
with <code class="constant">SNDRV_DMA_TYPE_DEV_SG</code>
in the PCM constructor like other PCI pre-allocator.
You need to pass <code class="function">snd_dma_pci_data(pci)</code>,
where pci is the struct <span class="structname">pci_dev</span> pointer
of the chip as well.
The <span class="type">struct snd_sg_buf</span> instance is created as
substream->dma_private. You can cast
the pointer like:
</p><div class="informalexample"><pre class="programlisting">
struct snd_sg_buf *sgbuf = (struct snd_sg_buf *)substream->dma_private;
</pre></div><p>
</p><p>
Then call <code class="function">snd_pcm_lib_malloc_pages()</code>
in the <em class="structfield"><code>hw_params</code></em> callback
as well as in the case of normal PCI buffer.
The SG-buffer handler will allocate the non-contiguous kernel
pages of the given size and map them onto the virtually contiguous
memory. The virtual pointer is addressed in runtime->dma_area.
The physical address (runtime->dma_addr) is set to zero,
because the buffer is physically non-contigous.
The physical address table is set up in sgbuf->table.
You can get the physical address at a certain offset via
<code class="function">snd_pcm_sgbuf_get_addr()</code>.
</p><p>
When a SG-handler is used, you need to set
<code class="function">snd_pcm_sgbuf_ops_page</code> as
the <em class="structfield"><code>page</code></em> callback.
(See <a class="link" href="ch05s06.html#pcm-interface-operators-page-callback" title="page callback">
<em class="citetitle">page callback section</em></a>.)
</p><p>
To release the data, call
<code class="function">snd_pcm_lib_free_pages()</code> in the
<em class="structfield"><code>hw_free</code></em> callback as usual.
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch11s02.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch11.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ch11s04.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">External Hardware Buffers </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Vmalloc'ed Buffers</td></tr></table></div></body></html>
