<?xml version="1.0" encoding="ANSI_X3.4-1968" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968" /><title>Callbacks</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="ch07.html" title="Chapter 7. API for AC97 Codec" /><link rel="prev" href="ch07s03.html" title="Constructor" /><link rel="next" href="ch07s05.html" title="Updating Registers in The Driver" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Callbacks</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch07s03.html">Prev</a> </td><th width="60%" align="center">Chapter 7. API for AC97 Codec</th><td width="20%" align="right"> <a accesskey="n" href="ch07s05.html">Next</a></td></tr></table><hr /></div><div class="section" title="Callbacks"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="api-ac97-callbacks"></a>Callbacks</h2></div></div></div><p>
The standard callbacks are <em class="structfield"><code>read</code></em> and
<em class="structfield"><code>write</code></em>. Obviously they
correspond to the functions for read and write accesses to the
hardware low-level codes.
</p><p>
The <em class="structfield"><code>read</code></em> callback returns the
register value specified in the argument.
</p><div class="informalexample"><pre class="programlisting">
static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
unsigned short reg)
{
struct mychip *chip = ac97->private_data;
....
return the_register_value;
}
</pre></div><p>
Here, the chip can be cast from ac97->private_data.
</p><p>
Meanwhile, the <em class="structfield"><code>write</code></em> callback is
used to set the register value.
</p><div class="informalexample"><pre class="programlisting">
static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
unsigned short reg, unsigned short val)
</pre></div><p>
</p><p>
These callbacks are non-atomic like the control API callbacks.
</p><p>
There are also other callbacks:
<em class="structfield"><code>reset</code></em>,
<em class="structfield"><code>wait</code></em> and
<em class="structfield"><code>init</code></em>.
</p><p>
The <em class="structfield"><code>reset</code></em> callback is used to reset
the codec. If the chip requires a special kind of reset, you can
define this callback.
</p><p>
The <em class="structfield"><code>wait</code></em> callback is used to
add some waiting time in the standard initialization of the codec. If the
chip requires the extra waiting time, define this callback.
</p><p>
The <em class="structfield"><code>init</code></em> callback is used for
additional initialization of the codec.
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch07s03.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch07.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ch07s05.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Constructor </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Updating Registers in The Driver</td></tr></table></div></body></html>
