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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>Interrupt Handler</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="ch05s06.html" title="Operators" /><link rel="next" href="ch05s08.html" title="Atomicity" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Interrupt Handler</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch05s06.html">Prev</a> </td><th width="60%" align="center">Chapter 5. PCM Interface</th><td width="20%" align="right"> <a accesskey="n" href="ch05s08.html">Next</a></td></tr></table><hr /></div><div class="section" title="Interrupt Handler"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="pcm-interface-interrupt-handler"></a>Interrupt Handler</h2></div></div></div><div class="toc"><dl><dt><span class="section"><a href="ch05s07.html#pcm-interface-interrupt-handler-boundary">Interrupts at the period (fragment) boundary</a></span></dt><dt><span class="section"><a href="ch05s07.html#pcm-interface-interrupt-handler-timer">High frequency timer interrupts</a></span></dt><dt><span class="section"><a href="ch05s07.html#pcm-interface-interrupt-handler-both">On calling <code class="function">snd_pcm_period_elapsed()</code></a></span></dt></dl></div><p>
The rest of pcm stuff is the PCM interrupt handler. The
role of PCM interrupt handler in the sound driver is to update
the buffer position and to tell the PCM middle layer when the
buffer position goes across the prescribed period size. To
inform this, call the <code class="function">snd_pcm_period_elapsed()</code>
function.
</p><p>
There are several types of sound chips to generate the interrupts.
</p><div class="section" title="Interrupts at the period (fragment) boundary"><div class="titlepage"><div><div><h3 class="title"><a id="pcm-interface-interrupt-handler-boundary"></a>Interrupts at the period (fragment) boundary</h3></div></div></div><p>
This is the most frequently found type: the hardware
generates an interrupt at each period boundary.
In this case, you can call
<code class="function">snd_pcm_period_elapsed()</code> at each
interrupt.
</p><p>
<code class="function">snd_pcm_period_elapsed()</code> takes the
substream pointer as its argument. Thus, you need to keep the
substream pointer accessible from the chip instance. For
example, define substream field in the chip record to hold the
current running substream pointer, and set the pointer value
at open callback (and reset at close callback).
</p><p>
If you acquire a spinlock in the interrupt handler, and the
lock is used in other pcm callbacks, too, then you have to
release the lock before calling
<code class="function">snd_pcm_period_elapsed()</code>, because
<code class="function">snd_pcm_period_elapsed()</code> calls other pcm
callbacks inside.
</p><p>
Typical code would be like:
</p><div class="example"><a id="id2948235"></a><p class="title"><b>Example 5.3. Interrupt Handler Case #1</b></p><div class="example-contents"><pre class="programlisting">
static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
{
struct mychip *chip = dev_id;
spin_lock(&chip->lock);
....
if (pcm_irq_invoked(chip)) {
/* call updater, unlock before it */
spin_unlock(&chip->lock);
snd_pcm_period_elapsed(chip->substream);
spin_lock(&chip->lock);
/* acknowledge the interrupt if necessary */
}
....
spin_unlock(&chip->lock);
return IRQ_HANDLED;
}
</pre></div></div><p><br class="example-break" />
</p></div><div class="section" title="High frequency timer interrupts"><div class="titlepage"><div><div><h3 class="title"><a id="pcm-interface-interrupt-handler-timer"></a>High frequency timer interrupts</h3></div></div></div><p>
This happense when the hardware doesn't generate interrupts
at the period boundary but issues timer interrupts at a fixed
timer rate (e.g. es1968 or ymfpci drivers).
In this case, you need to check the current hardware
position and accumulate the processed sample length at each
interrupt. When the accumulated size exceeds the period
size, call
<code class="function">snd_pcm_period_elapsed()</code> and reset the
accumulator.
</p><p>
Typical code would be like the following.
</p><div class="example"><a id="id2948297"></a><p class="title"><b>Example 5.4. Interrupt Handler Case #2</b></p><div class="example-contents"><pre class="programlisting">
static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
{
struct mychip *chip = dev_id;
spin_lock(&chip->lock);
....
if (pcm_irq_invoked(chip)) {
unsigned int last_ptr, size;
/* get the current hardware pointer (in frames) */
last_ptr = get_hw_ptr(chip);
/* calculate the processed frames since the
* last update
*/
if (last_ptr < chip->last_ptr)
size = runtime->buffer_size + last_ptr
- chip->last_ptr;
else
size = last_ptr - chip->last_ptr;
/* remember the last updated point */
chip->last_ptr = last_ptr;
/* accumulate the size */
chip->size += size;
/* over the period boundary? */
if (chip->size >= runtime->period_size) {
/* reset the accumulator */
chip->size %= runtime->period_size;
/* call updater */
spin_unlock(&chip->lock);
snd_pcm_period_elapsed(substream);
spin_lock(&chip->lock);
}
/* acknowledge the interrupt if necessary */
}
....
spin_unlock(&chip->lock);
return IRQ_HANDLED;
}
</pre></div></div><p><br class="example-break" />
</p></div><div class="section" title="On calling snd_pcm_period_elapsed()"><div class="titlepage"><div><div><h3 class="title"><a id="pcm-interface-interrupt-handler-both"></a>On calling <code class="function">snd_pcm_period_elapsed()</code></h3></div></div></div><p>
In both cases, even if more than one period are elapsed, you
don't have to call
<code class="function">snd_pcm_period_elapsed()</code> many times. Call
only once. And the pcm layer will check the current hardware
pointer and update to the latest status.
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