[alsa-devel] Multiple channels mapping

Clemens Ladisch clemens at ladisch.de
Wed Feb 8 12:32:17 CET 2012

Takashi Iwai wrote:
> Clemens Ladisch wrote:
>> Takashi Iwai wrote:
>>> Currently the channel-mapping information is a missing piece in ALSA
>>> framework, and it's a very long-standing item on my TODO list.
>>> The implementation itself should be easy, but the only question is the
>>> API design.  If you have a good proposal, please speak up.
>> TLVs on media controller entities.
> Well, my concern is that it might be too far from the other PCM
> stuff.  How the implementation would look like?

There is one media controller device node per device (= ALSA card),
containing multiple entities (similar to how a ctl device contains
multiple controls).  /dev/media42 _is_ separate, but the entity that
represents a PCM device is handled by the PCM code, so it would be
possible to have a shortcut to get a PCM device's TLVs directly.

> More question is whether this information should be available before
> or after hw_params setup.

This is pretty much static information, which is available even when the
PCM device isn't opened.

> BTW, the channel-mapping info can be useful for the automatic plug
> layer, too.

And if there is a need for channel mapping information of ALSA plugins,
there needs to be a userspace library for the media controller API.


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