multiple devices or one device w/ multiple subdevices?

multiple devices or one device w/ multiple subdevices?

[pl bossart]

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

Re: multiple devices or one device w/ multiple subdevices?

[Liam Girdwood]

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 

Re: multiple devices or one device w/ multiple subdevices?

[Eliot Blennerhassett]

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