[alsa-devel] [RFC] Channel mapping API

Takashi Iwai tiwai at suse.de
Tue Aug 21 14:06:09 CEST 2012

At Tue, 21 Aug 2012 13:24:53 +0200,
David Henningsson wrote:
> On 08/21/2012 12:31 PM, Takashi Iwai wrote:
> > 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.
> >
> >
> >
> > 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.
> >
> >
> >
> > 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_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.
> Interesting. A few thoughts here:
> 1) You seem to have invented new constants for different channels 
> (SND_CHMAP_FL for "front left" etc). There is already a channel 
> enumeration in include/mixer.h, _snd_mixer_selem_channel_id. Is there a 
> reason we can't just reuse it for this purpose?

No big reason, but a minor reason would be that UNKNOWN = -1 is
a bit impractical and the mixer id definition lacks some new
positions.  And, this is for mixer, and another for PCM, so using
SND_MIXER_* doesn't sound right.

But adjusting to follow the same values won't be a big issue.
Let's see.

> If we can't, we should at least make sure that everywhere we have these 
> declarations, they always correspond to the same value and add comments 
> that they need to be kept in sync.
> 2) a chmap struct would probably be more elegant, something like:
> struct snd_chmap {
> 	int type; /* or snd_pcm_chmap_type? */
> 	int count;
> 	int channels[SND_MIXER_SCHN_LAST+1]; /* Or variable length? */
> }

It should be a variable length.  It could be with zero-size array,
	struct snd_chmap {
		int type;
		int count;
		int channels[0];

Though, the type would be used only for queries (see below), so it
should be named like struct snd_chmap_query.

> 3) Can you use snd_pcm_set_chmap before snd_pcm_prepare, and if so, it 
> seems a bit strange that snd_pcm_get_chmap does not return what you have 
> previously set.

Setting or getting the channel map doesn't work before prepare (or
more specifically hw_params) because the number of channels isn't
determined yet.  It's the only reason.

> 4) The question of memory allocation for _get and _query, who is 
> supposed to allocate and free the snd_chmap array?

The caller.  The helper function snd_pcm_free_chmaps() is provided,

> 5) I'm not sure I'm getting the _VAR and _PAIRED types, even though I 
> read the documentation. Is this for the query only? Or if not, what 
> would it mean to get/set a channelmap with _VAR type?

Oh sorry, I must correct the description: get_chmap() returns only the
number of channels followed by channel elements.  Ditto for
set_chmap().  The type is used only for query.



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