Re: [alsa-devel] period_time in slave definition

1 Mar 2011


      Jaroslav Kysela napsal(a):
...
On Tue, 1 Mar 2011, Pavel Hofman wrote:
...
Jaroslav Kysela napsal(a):
...
It's correct, pcm:plug does not support buffer modifiers. Look to
"Automatic conversion plugin".
...
I checked the source code and the keywords seem to be ok
pcm.c:
static const char *const names[SND_PCM_HW_PARAM_LAST_INTERVAL + 1] = {
    [SND_PCM_HW_PARAM_FORMAT] = "format",
    [SND_PCM_HW_PARAM_CHANNELS] = "channels",
    [SND_PCM_HW_PARAM_RATE] = "rate",
    [SND_PCM_HW_PARAM_PERIOD_TIME] = "period_time",
    [SND_PCM_HW_PARAM_PERIOD_SIZE] = "period_size",
    [SND_PCM_HW_PARAM_BUFFER_TIME] = "buffer_time",
    [SND_PCM_HW_PARAM_BUFFER_SIZE] = "buffer_size",
    [SND_PCM_HW_PARAM_PERIODS] = "periods"
};
It's just a table to translate all possibilites between index and name.
The plug plugins allow only this:
err = snd_pcm_slave_conf(root, slave, &sconf, 3,
             SND_PCM_HW_PARAM_FORMAT, SCONF_UNCHANGED, &sformat,
             SND_PCM_HW_PARAM_CHANNELS, SCONF_UNCHANGED, &schannels,
             SND_PCM_HW_PARAM_RATE, SCONF_UNCHANGED, &srate);

                Jaroslav


Well, still I do not understand why we are talking about the plug
plugin, when I am trying to redefine the slave via pcm_slave. The
documentation at
http://www.alsa-project.org/alsa-doc/alsa-lib/pcm_plugins.html says the
pcm_slave can define period_time and buffer_time, while a slightly
modified example from the wiki page fails:
pcm_slave.slave_rate44100Hz {
       pcm "hw:0,0"
       rate 44100
--->    period_time 1024
}
pcm.rate44100Hz {
       type plug
       slave slave_rate44100Hz
}
How does the slave definition relate to the plug plugin? Sorry for these
basics but I am really trying to understand that, so far no success.
The above description is an universal description for all slaves.
Unfortunately, not all plugins accept the whole range of parameters.
You have to look to the snd_pcm_slave_conf() caller which parameters are
accepted.
Thanks a lot for your clarification. Would it make sense to add these
parameters to the plug plugin? I guess it would not be as simple as in
dshare.c:
params.period_size = psize;
    params.buffer_size = bsize;
when opening the slave.
Or should I use dshare when I need to specify period and buffer
parameters outside of the application?
Thanks a lot,
Pavel.