[Jack-Devel] ALSA PCM substreams

When opening a PCM, would it make sense for JACK to open all the 
substreams on that PCM?

I was helping a user on IRC with the Audio 4 DJ, a 4-in/4-out USB sound 
card.  The card uses the `caiaq' driver in ALSA.  It didn't "just work" 
because the 4 channels are split up into stereo substreams.  So, when 
jack opens "hw:1,0", it opens "hw:1,0,0" (2 channels) and ignores 
"hw:1,0,1" (2 more channels).[1]

I spoke with the developers of the `caiaq' driver, and they said it 
would make sense for JACK to go ahead and open all the substreams. 
Substreams are intended to be a clock-synchronized concept.[2]

Looking at the ALSA driver within JACK, this seems feasible.  The 
solution also makes sense to me.  This type of situation *could* be in 
the `caiaq' driver[3] or even with card-specific asoundrc stuff... it 
makes the most sense to me that jack open all the substreams.

-gabe

[1] The issue was resolved by creating a virtual PCM in ~/.asoundrc
     That makes the two substreams look like a single substream.
[2] I can't find that principle written anywhere, but it can be
     verified using snd_pcm_info_get_sync() to see that the two
     substreams return the same ID (clock).
[3] Let substream 0 support up to 4 channels.  substream 1 returns
     EBUSY or EAGAIN if substrem 0 is using the channels.

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