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 configuration.
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.