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-rw-r--r--Documentation/sound/kernel-api/writing-an-alsa-driver.rst41
1 files changed, 41 insertions, 0 deletions
diff --git a/Documentation/sound/kernel-api/writing-an-alsa-driver.rst b/Documentation/sound/kernel-api/writing-an-alsa-driver.rst
index 145bf6aad7cb..f169d58ca019 100644
--- a/Documentation/sound/kernel-api/writing-an-alsa-driver.rst
+++ b/Documentation/sound/kernel-api/writing-an-alsa-driver.rst
@@ -805,6 +805,7 @@ destructor and PCI entries. Example code is shown first, below.
return -EBUSY;
}
chip->irq = pci->irq;
+ card->sync_irq = chip->irq;
/* (2) initialization of the chip hardware */
.... /* (not implemented in this document) */
@@ -965,6 +966,15 @@ usually like the following:
return IRQ_HANDLED;
}
+After requesting the IRQ, you can passed it to ``card->sync_irq``
+field:
+::
+
+ card->irq = chip->irq;
+
+This allows PCM core automatically performing
+:c:func:`synchronize_irq()` at the necessary timing like ``hw_free``.
+See the later section `sync_stop callback`_ for details.
Now let's write the corresponding destructor for the resources above.
The role of destructor is simple: disable the hardware (if already
@@ -2059,6 +2069,37 @@ flag set, and you cannot call functions which may sleep. The
triggering the DMA. The other stuff should be initialized
``hw_params`` and ``prepare`` callbacks properly beforehand.
+sync_stop callback
+~~~~~~~~~~~~~~~~~~
+
+::
+
+ static int snd_xxx_sync_stop(struct snd_pcm_substream *substream);
+
+This callback is optional, and NULL can be passed. It's called after
+the PCM core stops the stream and changes the stream state
+``prepare``, ``hw_params`` or ``hw_free``.
+Since the IRQ handler might be still pending, we need to wait until
+the pending task finishes before moving to the next step; otherwise it
+might lead to a crash due to resource conflicts or access to the freed
+resources. A typical behavior is to call a synchronization function
+like :c:func:`synchronize_irq()` here.
+
+For majority of drivers that need only a call of
+:c:func:`synchronize_irq()`, there is a simpler setup, too.
+While keeping NULL to ``sync_stop`` PCM callback, the driver can set
+``card->sync_irq`` field to store the valid interrupt number after
+requesting an IRQ, instead. Then PCM core will look call
+:c:func:`synchronize_irq()` with the given IRQ appropriately.
+
+If the IRQ handler is released at the card destructor, you don't need
+to clear ``card->sync_irq``, as the card itself is being released.
+So, usually you'll need to add just a single line for assigning
+``card->sync_irq`` in the driver code unless the driver re-acquires
+the IRQ. When the driver frees and re-acquires the IRQ dynamically
+(e.g. for suspend/resume), it needs to clear and re-set
+``card->sync_irq`` again appropriately.
+
pointer callback
~~~~~~~~~~~~~~~~