[alsa-devel] multiple devices or one device w/ multiple subdevices?
Hi, I have a piece of custom hardware that provides 7 independent PCM outputs. This is not intended for multichannel playback, the content on each output can be completely different; playback will not be synchronized and will start separately. The only restriction is that all outputs rely on the same baserate and share the same bitclock, i.e. all the outputs work at 48kHz.
_From the mailling list archives, subdevices seem to be relevant when doing hw mixing or when lots of streams are handled. None of this applies to my case. Also it looks like PulseAudio knows nothing about subdevices in its detection loop, meaning the user would need to manually add some load-module commands in the PulseAudio configuration files. And last but not least, I didn't see very many lines of code (and no documentation) that showed how subdevices are registered/handled, etc.
My conclusion is that multiple devices would seem like a much better option. But I wanted to check with the community that I am on the right track. Thanks for your feedback! - P
Hi Pierre-Louis,
On Mon, 2009-08-24 at 12:26 -0500, pl bossart wrote:
Hi, I have a piece of custom hardware that provides 7 independent PCM outputs. This is not intended for multichannel playback, the content on each output can be completely different; playback will not be synchronized and will start separately. The only restriction is that all outputs rely on the same baserate and share the same bitclock, i.e. all the outputs work at 48kHz.
From the mailling list archives, subdevices seem to be relevant when
doing hw mixing or when lots of streams are handled. None of this applies to my case. Also it looks like PulseAudio knows nothing about subdevices in its detection loop, meaning the user would need to manually add some load-module commands in the PulseAudio configuration files. And last but not least, I didn't see very many lines of code (and no documentation) that showed how subdevices are registered/handled, etc.
My conclusion is that multiple devices would seem like a much better option. But I wanted to check with the community that I am on the right track. Thanks for your feedback!
- P
Fwiw, I worked on a similar piece of hardware a few years back (WM9713). It too had a shared 48kHz clock (AC97) and 3 independent outputs. I found creating a single card with 3 pcms worked very well in this case and kept the code simple.
I could independently play different audio data to each pcm with different start/stop times.
There is some example code in the ASoC WM9713 codec driver and in soc-core.c
HTH
Liam
pl bossart wrote:
Hi, I have a piece of custom hardware that provides 7 independent PCM outputs.
My conclusion is that multiple devices would seem like a much better option. But I wanted to check with the community that I am on the right track. Thanks for your feedback!
It used to be that there was a limit of 4 devices per card. But no (or large) limit on subdevices.
If this still applies you will need to use subdevices, and for simplicity you would create just one device with 7 subdevs.
-- Eliot
participants (3)
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Eliot Blennerhassett -
Liam Girdwood -
pl bossart