async alsa applications and pcm plugin sound output

[Riku Voipio <riku.voipio@iki.fi>]

hi,

It possible to use async callbacks[1] using snd_async_add_pcm_handler()
to find out when it is possible to write to ALSA. It is also possible
to write a alsa-lib I/O Plugin for sound output[2]. Such 
plugins have been created atleast for alsa-bluetooth[3] and
dsp gateway[4]. Unfortunatly it seems IO plugins don't support
async playback. Using the pcm binary from alsa testsuite 
and dsp gateway I/O plugin:

-snip-
./pcm -Ddefault -m async
Playback device is default
Stream parameters are 44100Hz, S16_LE, 1 channels
Sine wave rate is 440.0000Hz
Using transfer method: async
Unable to register async handler
-snip-

It appears external IO plugins are not _allowed_ to
deal with async stuff:

-snip-
static int snd_pcm_ioplug_async(snd_pcm_t *pcm ATTRIBUTE_UNUSED,
                int sig ATTRIBUTE_UNUSED,
                pid_t pid ATTRIBUTE_UNUSED)
{
    return -ENOSYS;
}
-snip-

Does this mean that async callbacks should not be used if you
want to be sure sound works everywhere? Is there a chance that
plugin api will be changed to allow plugins implement async
callbacks?

Cheers,
Riku

[1] http://alsa2.opensrc.org/HowTo_Asynchronous_Playback
[2] http://alsa.cvs.sourceforge.net/alsa/alsa-lib/src/pcm/pcm_ioplug.c?revision=1.10&view=markup
[3] http://bluetooth-alsa.sourceforge.net/
[4] https://garage.maemo.org/projects/dsp-alsa/

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