[alsa-devel] Playback Devices in PREPARED State Throw Off POLLOUT Events?
jeff at taupro.com
Mon Apr 9 14:46:47 CEST 2007
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
--- Status of Headphones Player ---
state : PREPARED
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.
Thanks for any tips,
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