[alsa-devel] [RFC] Channel mapping API

[Takashi Iwai]

At Tue, 21 Aug 2012 14:06:09 +0200,
Takashi Iwai wrote:
> 
> 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.
> > >
> > >
> > > * 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.
> > 
> > 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.

I updated git trees to follow this now.


Takashi