In certain system-on-chip systems, with separate ADCs and DACs for instance, the ADC could generate clocks for the DAC, where it not for the fact that ALSA shuts down whatever device is not being used in order to conserve power. Is it possible to instruct ALSA not to do this, i.e. once a codec has been configured to operate at a given sample rate, it will continue to do so, even after all streams have stopped.
I suppose one way to do this would be to change the codec set_bias_level() callback so that the BIAS_OFF and BIAS_STANDBY cases don't do anything but leave the codec running. But it doesn't sound like a clean way of doing this.
Of course, one complication is that at system startup, before any capture or playback operations have been attempted, ALSA doesn't know which sample rate should be configured, as there is no concept of a 'default sample rate'; the sample rate is always set when a stream is opened.
/Ricard