[alsa-devel] [topic/asoc][RFC 0/1] ASoC: OMAP: Add support for mono link configuration to McBSP DAI

[Mark Brown]

On Mon, Nov 24, 2008 at 03:45:33PM +0200, Jarkko Nikula wrote:
> My problem was this kind of switching correct to do in machine driver?

> 	cpu_dai->playback.channels_min = 1;
> 	cpu_dai->playback.channels_max = 1;
> 	cpu_dai->capture.channels_min = 1;
> 	cpu_dai->capture.channels_max = 1;

No, these should be constant.  Any dynamic constraints should be applied
with the constraints API.

> Defining channels_min = 1 in omap_mcbsp_dai initializer would cause that
> link is then switching between mono/stereo depending what user space is
> asking.

Yes.

> Like "aplay -c 1 /dev/urandom" causes that all machines having codec
> with "channels_min = 1" and "channels_max = 2" would cause incorrect
> mono link configuration where alsalib should do the mono->stereo
> conversion instead and vice versa for pure mono link.

So, this is a bit messy at the moment since we don't really have any
DAPM-style routing on the digital side yet.  What most of these codecs
should probably be doing is declaring a 2 channel minimuim - normally
that's what they require as a wire format even if they only actually use
the data from one of those channels.

This will be less efficient when these devices are used in a mono
configuration but is safer until we can map out the digital side better
in software; it's roughly what you're trying to do here on the CPU side.