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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>Chapter 11. Buffer and Memory Management</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="index.html" title="Writing an ALSA Driver" /><link rel="prev" href="ch10s03.html" title="IEC958 (S/PDIF)" /><link rel="next" href="ch11s02.html" title="External Hardware Buffers" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 11. Buffer and Memory Management</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch10s03.html">Prev</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="ch11s02.html">Next</a></td></tr></table><hr /></div><div class="chapter" title="Chapter 11. Buffer and Memory Management"><div class="titlepage"><div><div><h2 class="title"><a id="buffer-and-memory"></a>Chapter 11. Buffer and Memory Management</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="section"><a href="ch11.html#buffer-and-memory-buffer-types">Buffer Types</a></span></dt><dt><span class="section"><a href="ch11s02.html">External Hardware Buffers</a></span></dt><dt><span class="section"><a href="ch11s03.html">Non-Contiguous Buffers</a></span></dt><dt><span class="section"><a href="ch11s04.html">Vmalloc'ed Buffers</a></span></dt></dl></div><div class="section" title="Buffer Types"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="buffer-and-memory-buffer-types"></a>Buffer Types</h2></div></div></div><p>
ALSA provides several different buffer allocation functions
depending on the bus and the architecture. All these have a
consistent API. The allocation of physically-contiguous pages is
done via
<code class="function">snd_malloc_xxx_pages()</code> function, where xxx
is the bus type.
</p><p>
The allocation of pages with fallback is
<code class="function">snd_malloc_xxx_pages_fallback()</code>. This
function tries to allocate the specified pages but if the pages
are not available, it tries to reduce the page sizes until
enough space is found.
</p><p>
The release the pages, call
<code class="function">snd_free_xxx_pages()</code> function.
</p><p>
Usually, ALSA drivers try to allocate and reserve
a large contiguous physical space
at the time the module is loaded for the later use.
This is called <span class="quote">“<span class="quote">pre-allocation</span>”</span>.
As already written, you can call the following function at
pcm instance construction time (in the case of PCI bus).
</p><div class="informalexample"><pre class="programlisting">
snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
snd_dma_pci_data(pci), size, max);
</pre></div><p>
where <em class="parameter"><code>size</code></em> is the byte size to be
pre-allocated and the <em class="parameter"><code>max</code></em> is the maximum
size to be changed via the <code class="filename">prealloc</code> proc file.
The allocator will try to get an area as large as possible
within the given size.
</p><p>
The second argument (type) and the third argument (device pointer)
are dependent on the bus.
In the case of the ISA bus, pass <code class="function">snd_dma_isa_data()</code>
as the third argument with <code class="constant">SNDRV_DMA_TYPE_DEV</code> type.
For the continuous buffer unrelated to the bus can be pre-allocated
with <code class="constant">SNDRV_DMA_TYPE_CONTINUOUS</code> type and the
<code class="function">snd_dma_continuous_data(GFP_KERNEL)</code> device pointer,
where <code class="constant">GFP_KERNEL</code> is the kernel allocation flag to
use.
For the PCI scatter-gather buffers, use
<code class="constant">SNDRV_DMA_TYPE_DEV_SG</code> with
<code class="function">snd_dma_pci_data(pci)</code>
(see the
<a class="link" href="ch11s03.html" title="Non-Contiguous Buffers"><em class="citetitle">Non-Contiguous Buffers
</em></a> section).
</p><p>
Once the buffer is pre-allocated, you can use the
allocator in the <em class="structfield"><code>hw_params</code></em> callback:
</p><div class="informalexample"><pre class="programlisting">
snd_pcm_lib_malloc_pages(substream, size);
</pre></div><p>
Note that you have to pre-allocate to use this function.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch10s03.html">Prev</a> </td><td width="20%" align="center"> </td><td width="40%" align="right"> <a accesskey="n" href="ch11s02.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">IEC958 (S/PDIF) </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> External Hardware Buffers</td></tr></table></div></body></html>
