On Mon, 20 Apr 2009, Nicola Mfb wrote:
Hi! I'm sorry if this is an old issue but I did not found help in archives/alsaweb/google and I'm new to alsa, so this report is what I guessed reading sources and compiling test apps for about 2 days, and the following assumptions may be completely wrong.
Asterisk has an alsa channel that works fine when using plughw:0,0 both for read and for writing, but if use plug:dmix/plug:dsnoop or default with a dmix/dsnoop wrapper in /etc/asound.conf it does not emit any sound. It uses select on poll fd descriptor to know if the pcm is ready to be sent output samples. The select will never return as it seems the dmix plugin is not able to signal when is ready. I noted that using snd_pcm_wait works well but cannot be used in asterisk as it needs to select on two fd. I guess that snd_pcm_wait with dmix check the shared memory and not the fd descriptor. So should poll/select be avoided completely in applications (as they does not know if the default pcm is a dmix) and asterisk alsa channel be rewritten or is there a workaround?
Asterisk does not use snd_pcm_poll_descriptors_revents to translate events returned from poll to correct poll event. Also, only file descriptor number from snd_pcm_poll_descriptors() - asterisk should handle correctly returned poll events as well.
See chan_alsa.c - readdev / writedev variables and sound_thread().
Jaroslav
----- Jaroslav Kysela perex@perex.cz Linux Kernel Sound Maintainer ALSA Project, Red Hat, Inc.