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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>Resource Allocation</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="ch04.html" title="Chapter 4. PCI Resource Management" /><link rel="prev" href="ch04s02.html" title="Some Hafta's" /><link rel="next" href="ch04s04.html" title="Registration of Device Struct" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Resource Allocation</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch04s02.html">Prev</a> </td><th width="60%" align="center">Chapter 4. PCI Resource Management</th><td width="20%" align="right"> <a accesskey="n" href="ch04s04.html">Next</a></td></tr></table><hr /></div><div class="section" title="Resource Allocation"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="pci-resource-resource-allocation"></a>Resource Allocation</h2></div></div></div><p>
The allocation of I/O ports and irqs is done via standard kernel
functions. Unlike ALSA ver.0.5.x., there are no helpers for
that. And these resources must be released in the destructor
function (see below). Also, on ALSA 0.9.x, you don't need to
allocate (pseudo-)DMA for PCI like in ALSA 0.5.x.
</p><p>
Now assume that the PCI device has an I/O port with 8 bytes
and an interrupt. Then struct <span class="structname">mychip</span> will have the
following fields:
</p><div class="informalexample"><pre class="programlisting">
struct mychip {
struct snd_card *card;
unsigned long port;
int irq;
};
</pre></div><p>
</p><p>
For an I/O port (and also a memory region), you need to have
the resource pointer for the standard resource management. For
an irq, you have to keep only the irq number (integer). But you
need to initialize this number as -1 before actual allocation,
since irq 0 is valid. The port address and its resource pointer
can be initialized as null by
<code class="function">kzalloc()</code> automatically, so you
don't have to take care of resetting them.
</p><p>
The allocation of an I/O port is done like this:
</p><div class="informalexample"><pre class="programlisting">
err = pci_request_regions(pci, "My Chip");
if (err < 0) {
kfree(chip);
pci_disable_device(pci);
return err;
}
chip->port = pci_resource_start(pci, 0);
</pre></div><p>
</p><p>
It will reserve the I/O port region of 8 bytes of the given
PCI device. The returned value, chip->res_port, is allocated
via <code class="function">kmalloc()</code> by
<code class="function">request_region()</code>. The pointer must be
released via <code class="function">kfree()</code>, but there is a
problem with this. This issue will be explained later.
</p><p>
The allocation of an interrupt source is done like this:
</p><div class="informalexample"><pre class="programlisting">
if (request_irq(pci->irq, snd_mychip_interrupt,
IRQF_SHARED, "My Chip", chip)) {
printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
snd_mychip_free(chip);
return -EBUSY;
}
chip->irq = pci->irq;
</pre></div><p>
where <code class="function">snd_mychip_interrupt()</code> is the
interrupt handler defined <a class="link" href="ch05s07.html" title="Interrupt Handler"><em class="citetitle">later</em></a>.
Note that chip->irq should be defined
only when <code class="function">request_irq()</code> succeeded.
</p><p>
On the PCI bus, interrupts can be shared. Thus,
<code class="constant">IRQF_SHARED</code> is used as the interrupt flag of
<code class="function">request_irq()</code>.
</p><p>
The last argument of <code class="function">request_irq()</code> is the
data pointer passed to the interrupt handler. Usually, the
chip-specific record is used for that, but you can use what you
like, too.
</p><p>
I won't give details about the interrupt handler at this
point, but at least its appearance can be explained now. The
interrupt handler looks usually like the following:
</p><div class="informalexample"><pre class="programlisting">
static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
{
struct mychip *chip = dev_id;
....
return IRQ_HANDLED;
}
</pre></div><p>
</p><p>
Now let's write the corresponding destructor for the resources
above. The role of destructor is simple: disable the hardware
(if already activated) and release the resources. So far, we
have no hardware part, so the disabling code is not written here.
</p><p>
To release the resources, the <span class="quote">“<span class="quote">check-and-release</span>”</span>
method is a safer way. For the interrupt, do like this:
</p><div class="informalexample"><pre class="programlisting">
if (chip->irq >= 0)
free_irq(chip->irq, chip);
</pre></div><p>
Since the irq number can start from 0, you should initialize
chip->irq with a negative value (e.g. -1), so that you can
check the validity of the irq number as above.
</p><p>
When you requested I/O ports or memory regions via
<code class="function">pci_request_region()</code> or
<code class="function">pci_request_regions()</code> like in this example,
release the resource(s) using the corresponding function,
<code class="function">pci_release_region()</code> or
<code class="function">pci_release_regions()</code>.
</p><div class="informalexample"><pre class="programlisting">
pci_release_regions(chip->pci);
</pre></div><p>
</p><p>
When you requested manually via <code class="function">request_region()</code>
or <code class="function">request_mem_region</code>, you can release it via
<code class="function">release_resource()</code>. Suppose that you keep
the resource pointer returned from <code class="function">request_region()</code>
in chip->res_port, the release procedure looks like:
</p><div class="informalexample"><pre class="programlisting">
release_and_free_resource(chip->res_port);
</pre></div><p>
</p><p>
Don't forget to call <code class="function">pci_disable_device()</code>
before the end.
</p><p>
And finally, release the chip-specific record.
</p><div class="informalexample"><pre class="programlisting">
kfree(chip);
</pre></div><p>
</p><p>
Again, remember that you cannot
use the <em class="parameter"><code>__devexit</code></em> prefix for this destructor.
</p><p>
We didn't implement the hardware disabling part in the above.
If you need to do this, please note that the destructor may be
called even before the initialization of the chip is completed.
It would be better to have a flag to skip hardware disabling
if the hardware was not initialized yet.
</p><p>
When the chip-data is assigned to the card using
<code class="function">snd_device_new()</code> with
<code class="constant">SNDRV_DEV_LOWLELVEL</code> , its destructor is
called at the last. That is, it is assured that all other
components like PCMs and controls have already been released.
You don't have to stop PCMs, etc. explicitly, but just
call low-level hardware stopping.
</p><p>
The management of a memory-mapped region is almost as same as
the management of an I/O port. You'll need three fields like
the following:
</p><div class="informalexample"><pre class="programlisting">
struct mychip {
....
unsigned long iobase_phys;
void __iomem *iobase_virt;
};
</pre></div><p>
and the allocation would be like below:
</p><div class="informalexample"><pre class="programlisting">
if ((err = pci_request_regions(pci, "My Chip")) < 0) {
kfree(chip);
return err;
}
chip->iobase_phys = pci_resource_start(pci, 0);
chip->iobase_virt = ioremap_nocache(chip->iobase_phys,
pci_resource_len(pci, 0));
</pre></div><p>
and the corresponding destructor would be:
</p><div class="informalexample"><pre class="programlisting">
static int snd_mychip_free(struct mychip *chip)
{
....
if (chip->iobase_virt)
iounmap(chip->iobase_virt);
....
pci_release_regions(chip->pci);
....
}
</pre></div><p>
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