Keeping the codec running at all times

[Pierre-Louis Bossart]

>> 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.
> 
> The driver may limit the available rates (thus it may be possible to set 
> the one accepted rate via the module parameter or so which may be used 
> for the codec initialization before an application uses the PCM device).

It's a valid request, some platforms want to avoid any glitches due to 
clocks and require that they remain active, even if it means writing-off 
power optimizations.

If your codec exposes a clock object then you could have e.g. a board or 
machine driver configure the clock 
(clk_get/clk_set_rate/clk_prepare_enable) and leave it on regardless of 
the streaming usages. You would still need to make sure that the clock 
rates are compatible with the hw_params when streaming does happen.
that's what e.g. was done for Intel to make sure the MCLK, BCLK and 
FSYNC could be enabled even when the DSP was idle.