[alsa-devel] How to define card specific pcm devices?

Tanu Kaskinen tanuk at iki.fi
Sat Sep 8 10:04:39 CEST 2012

On Fri, 2012-09-07 at 11:35 +0200, Takashi Iwai wrote:
> At Fri, 07 Sep 2012 12:03:34 +0300,
> Tanu Kaskinen wrote:
> > A question about the syntax: should the PHASE_INVERSE and DRIVER_SPEC
> > flags be supported also here, or is it enough to have just a simple list
> > of channels?
> > 
> > I think the syntax will have to support the flags, because if
> > DRIVER_SPEC is set, then the channel position is effectively unknown to
> > PulseAudio. The channel map may potentially affect routing decisions,
> > which is the main reason why the channel map information is needed in
> > UCM in addition to the new channel map API. The channel map API can be
> > used only with an already opened pcm handle, which is not very useful
> > when doing routing decisions.
> Well, the kernel side API doesn't require that the PCM to be open.
> It's just a read of TLV from a control element.
> The problem is that the configuration is evaluated only at open in
> alsa-lib PCM abstraction.  That's the only reason chmap query takes
> snd_pcm_t handle.
> In other words, it's pretty easy to provide a query function limited
> only for hw device, such as
> snd_pcm_chmap_query_t **snd_pcm_query_chmaps_from_hw(int card, int device,
> 	int substream);
> Ideally it should be a query from a PCM name string like
> snd_pcm_chmap_query_t **snd_pcm_query_chmaps_from_name(const char *name);
> But it's hard for now without opening a PCM because we need to parse
> down the definition of the given PCM and the underlying plugin
> expansions require the opens of their slave PCMs.

I now realized that even if we had snd_pcm_query_chmaps_from_name(), the
channel map information would still be needed in the UCM configuration.
For example on N900, all audio playback and capture is done through hw:0
with two channels (the pcm can't be opened in mono mode). In different
situations the channel map is different: sometimes only one channel of
the pcm is useful. Getting a list of channel maps from
snd_pcm_query_chmaps_from_hw() doesn't tell PulseAudio which channel map
is the right one, so this information needs to come from the UCM

So, I was wrong when I said that the main reason for having the channel
map information in UCM is that the chmap API can only be used when the
pcm is open.


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