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:
- 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