[alsa-devel] [RFC] Channel mapping API (take 2)

Takashi Iwai tiwai at suse.de
Tue Sep 4 17:47:36 CEST 2012


my proposal for channel mapping API seems accepted fairly well through
discussions at Plumbers, so I continued to work on it and uploaded the
updated version.

The kernel part is found in sound-unstable tree topic/tlv-chmap branch

and the alsa-lib part is found in github tree topic/chmap branch

The updated parts are:

- SNDRV_CHMAP_NA was newly added, indicating the channel is not
  available or silent.

- The channel position value is masked in lower 16bit.
  The upper bits are used for more channel attributes as bit flags.


  The phase inverted channel has this bit:
  #define SNDRV_CHMAP_PHASE_INVERSE	(0x01 << 16)

  And the driver-specific non-standard channel position has this:
  #define SNDRV_CHMAP_DRIVER_SPEC	(0x02 << 16)

- The alsa-lib API functions use snd_pcm_chmap_query_t and
  snd_pcm_chmap_t instead of ambiguous integer arrays.

/** the channel map header */
typedef struct snd_pcm_chmap {
	unsigned int channels;
	unsigned int pos[0];
} snd_pcm_chmap_t;

/** the header of array items returned from snd_pcm_query_chmaps() */
typedef struct snd_pcm_chmap_query {
	enum snd_pcm_chmap_type type;
	snd_pcm_chmap_t map;
} snd_pcm_chmap_query_t;

snd_pcm_chmap_query_t **snd_pcm_query_chmaps(snd_pcm_t *pcm);
void snd_pcm_free_chmaps(snd_pcm_chmap_query_t **maps);
snd_pcm_chmap_t *snd_pcm_get_chmap(snd_pcm_t *pcm);
int snd_pcm_set_chmap(snd_pcm_t *pcm, const snd_pcm_chmap_t *map);

The rest aren't so much changed.  Only slight bug fixes.

A big remaining question is whether the current kernel-side
implementation is OK.  We may add some PCM ops pointer instead of the
current style using the direct control tlv/read/write override.
But the kernel-side API can be changed even later, so I don't worry
about it so much for now, as long as we can keep the ABI definition.



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