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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>Constructor</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="ch05.html" title="Chapter 5. PCM Interface" /><link rel="prev" href="ch05s02.html" title="Full Code Example" /><link rel="next" href="ch05s04.html" title="... And the Destructor?" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Constructor</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch05s02.html">Prev</a> </td><th width="60%" align="center">Chapter 5. PCM Interface</th><td width="20%" align="right"> <a accesskey="n" href="ch05s04.html">Next</a></td></tr></table><hr /></div><div class="section" title="Constructor"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="pcm-interface-constructor"></a>Constructor</h2></div></div></div><p>
A pcm instance is allocated by the <code class="function">snd_pcm_new()</code>
function. It would be better to create a constructor for pcm,
namely,
</p><div class="informalexample"><pre class="programlisting">
static int __devinit snd_mychip_new_pcm(struct mychip *chip)
{
struct snd_pcm *pcm;
int err;
err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
if (err < 0)
return err;
pcm->private_data = chip;
strcpy(pcm->name, "My Chip");
chip->pcm = pcm;
....
return 0;
}
</pre></div><p>
</p><p>
The <code class="function">snd_pcm_new()</code> function takes four
arguments. The first argument is the card pointer to which this
pcm is assigned, and the second is the ID string.
</p><p>
The third argument (<em class="parameter"><code>index</code></em>, 0 in the
above) is the index of this new pcm. It begins from zero. If
you create more than one pcm instances, specify the
different numbers in this argument. For example,
<em class="parameter"><code>index</code></em> = 1 for the second PCM device.
</p><p>
The fourth and fifth arguments are the number of substreams
for playback and capture, respectively. Here 1 is used for
both arguments. When no playback or capture substreams are available,
pass 0 to the corresponding argument.
</p><p>
If a chip supports multiple playbacks or captures, you can
specify more numbers, but they must be handled properly in
open/close, etc. callbacks. When you need to know which
substream you are referring to, then it can be obtained from
struct <span class="structname">snd_pcm_substream</span> data passed to each callback
as follows:
</p><div class="informalexample"><pre class="programlisting">
struct snd_pcm_substream *substream;
int index = substream->number;
</pre></div><p>
</p><p>
After the pcm is created, you need to set operators for each
pcm stream.
</p><div class="informalexample"><pre class="programlisting">
snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
&snd_mychip_playback_ops);
snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
&snd_mychip_capture_ops);
</pre></div><p>
</p><p>
The operators are defined typically like this:
</p><div class="informalexample"><pre class="programlisting">
static struct snd_pcm_ops snd_mychip_playback_ops = {
.open = snd_mychip_pcm_open,
.close = snd_mychip_pcm_close,
.ioctl = snd_pcm_lib_ioctl,
.hw_params = snd_mychip_pcm_hw_params,
.hw_free = snd_mychip_pcm_hw_free,
.prepare = snd_mychip_pcm_prepare,
.trigger = snd_mychip_pcm_trigger,
.pointer = snd_mychip_pcm_pointer,
};
</pre></div><p>
All the callbacks are described in the
<a class="link" href="ch05s06.html" title="Operators"><em class="citetitle">
Operators</em></a> subsection.
</p><p>
After setting the operators, you probably will want to
pre-allocate the buffer. For the pre-allocation, simply call
the following:
</p><div class="informalexample"><pre class="programlisting">
snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
snd_dma_pci_data(chip->pci),
64*1024, 64*1024);
</pre></div><p>
It will allocate a buffer up to 64kB as default.
Buffer management details will be described in the later section <a class="link" href="ch11.html" title="Chapter 11. Buffer and Memory Management"><em class="citetitle">Buffer and Memory
Management</em></a>.
</p><p>
Additionally, you can set some extra information for this pcm
in pcm->info_flags.
The available values are defined as
<code class="constant">SNDRV_PCM_INFO_XXX</code> in
<code class="filename"><sound/asound.h></code>, which is used for
the hardware definition (described later). When your soundchip
supports only half-duplex, specify like this:
</p><div class="informalexample"><pre class="programlisting">
pcm->info_flags = SNDRV_PCM_INFO_HALF_DUPLEX;
</pre></div><p>
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