At Fri, 07 Mar 2008 10:10:05 -0600, bruce@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 handler.

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 hardware.

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.

Takashi