[alsa-devel] Channel mapping

Jaroslav Kysela perex at perex.cz
Wed Nov 21 16:27:47 CET 2007

On Wed, 21 Nov 2007, Clemens Ladisch wrote:

> Takashi Iwai wrote:
> > Yes, querying channel mapping is another missing piece with popular
> > demand.
> > 
> > The implementation would be easy, I guess.  But we have to define the
> > way to inform this from kernel to user space: whether create a new
> > ioctl or extend the existing ones (if possible)...
> It's just metadata that describes a PCM device, so I think we should use
> TLV for this.
> The existing struct snd_ctl_tlv uses a single integer to identify
> control elements.  We could restrict control numid's to 31 bits and
> use the upper bit to signal that this value includes device type and
> device number in the lower bits, if we want to reuse the same TLV
> ioctls.

We can also encode PCM device / subdevice numbers to data structure. But
I think that best way is to extend channel_info PCM ioctl (create new 
version and emulate old one - it should be quite easy to implement).


Jaroslav Kysela <perex at perex.cz>
Linux Kernel Sound Maintainer
ALSA Project

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