Hi,
this is a progress report of my longstanding TODO, the channel map API implementation. I'm going to cover this at Plumbers audio uconf, so we can discuss details there, too.
The channel mapping API provides a method for user-space to query, get and set the channel map of a PCM stream. It's required for assigning channels properly for multi-channel streams.
* KERNEL IMPLEMENTATION
In my latest attempt, I implemented with control elements. A control element is created for each PCM substream with the corresponding device and substream index. Then it gives a TLV for querying maps, a read op for obtaining the current map, and optionally a write op for setting the map. The obvious merit by this way is that no extra kernel ABI is required.
A couple of new helper functions are provided for assigning standard channel maps. Currently, HD-audio and AC97 drivers has some implementation.
* ALSA-LIB IMPLEMENTATION
The additional alsa-lib API functions look like:
int **snd_pcm_query_chmaps(snd_pcm_t *pcm); int *snd_pcm_get_chmap(snd_pcm_t *pcm); int snd_pcm_set_chmap(snd_pcm_t *pcm, const int *map);
snd_pcm_query_chmaps() returns the list of channel maps. A channel map is represented by an integer array, beginning with the channel map type, followed by the number of channels, and the position of each channel, e.g. { SND_CHMAP_FIXED, 4, SND_CHMAP_FL, SND_CHMAP_FR, SND_CHMAP_RL, SND_CHMAP_RR }
snd_pcm_get_chmap() returns the currently assigned channel map for the given PCM stream. If the PCM is before prepared, it fills UNKNOWN.
When a driver allows user to change the channel map, user can call snd_pcm_set_chmap(). For example, HDMI allows you to choose whether it's 4.0 or 3.1 output.
* RESOURCES
The latest kernel patchset is found in sound-unstable tree topic/tlv-chmap branch. git://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound-unstable.git topic/tlv-chmap
The alsa-lib portion is found in a tree on github topic/chmap branch. git://github.com/tiwai/alsa-lib.git topic/chmap
thanks,
Takashi