At Mon, 09 Apr 2007 07:46:47 -0500, Jeff Rush wrote: > > A question re normal internal operation of ALSA. > > I'm using the poll() interface to wait for a audio playback device to ask for > more samples. The device is in the PREPARED state, not RUNNING state. At the > beginning of my program, _before_ I write any samples to it, I'm getting > repeated POLLOUT events, when I do not yet have data available to send. They > seem to come frequent enough to load down my CPU, rather than every N ticks, > so it seems. > > I thought a device was idle in the PREPARED state, and you had to either set > the start conditions and write data, or explicitly invoke snd_pcm_start(). In > my case, I wanted to use snd_pcm_start() once I had data to send, and also > used the start conditions, set at 50%, to cause the audio output to them > proceed to the speaker. > > The output snd_pcm_status_dump() right after exiting from the very first > poll() ever in the program shows the following, and that I have a POLLOUT > event pending. > > --- Status of Headphones Player --- > > state : PREPARED > > trigger_time: 0.000000 > > tstamp : 1176119509.658821000 > > delay : 0 > > avail : 2400 > > avail_max : 0 > > > > POLLOUT fired, calling ready_for_write() > > > I suppose I could force it back to an earlier state like SND_PCM_STATE_OPEN or > SND_PCM_STATE_SETUP, but that seems a bit harsh, since the snd_pcm_hw_params() > takes it to the SND_PCM_STATE_PREPARED itself. When the playback PCM is in the PREPARED state, it's actually ready for getting data and trigger. Thus poll() returns POLLOUT. Its logic is clear. I think you should change the program design. The data should have been prepared before poll. Or, if the flow is like: poll -> fetch data -> write data then you need some error control between fetch data and write data, for example, simply wait some time when no data is ready. Takashi _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
