[alsa-devel] Trigger problems while writing Alsa Shim
tiwai at suse.de
Thu Mar 13 11:57:48 CET 2008
At Fri, 07 Mar 2008 10:10:05 -0600,
bruce at azbarclay.com wrote:
> I am writing a shim from alsa to our own sound card drivers. I was
> able to get the init driver working and our sound card recognized.
> Things seem to be set up correctly. When I do aplay to my device (USB
> device works find), it calls my copy callback twice and then it calls
> the trigger and fails. The call that fails is in pcm_write
> (writei_func) with a -5. I know that my callback returns zero but
> there must be something else under the covers since that gets to be
> zero. Any help would be appreciated.
The error -5 is -EIO. Typically it indicates that the data isn't
handled properly by the PCM core.
ALSA PCM core needs proper (asynchronous) notification via
snd_pcm_period_elapsed(). Otherwise the internals won't be updated
and you'll have the error. Usually this is called from the irq
snd_pcm_period_elapsed() is supposed to be issued at the time when the
last period (fragment, chunk) has been processed -- meaning the sound
is really played back. It doesn't mean that the data is fed to the
Once after snd_pcm_period_elapsed() is called, PCM core calls the
pointer callback and updates the internals including the current
position. Then it wakes up the pending tasks, which will restart
writing to the buffer, etc.
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