[alsa-devel] arecord: does hardware or software throw away unused

[Rob Nertney]

Hi Clemens,

That is where my confusion was. When you said hw_params are only what the
HW supports (makes sense :) ). Making minchan=maxchan, I can now record
less than max channels.

Thanks for the help!

On Wed, Sep 21, 2016 at 12:01 AM, Clemens Ladisch <clemens at ladisch.de>
wrote:

> Rob Nertney wrote:
> > I'm building the hardware, so I can technically do whatever I want. I
> > didn't want to add more to the hardware if ALSA can handle this.
> >
> > The hardware is spec-ed to *always* have 16 channels worth of data being
> > written into DDR. I think this comes full-circle to my original question
> of
> > whether arecord can filter out channels.
>
> When the ALSA library automatically converts sample formats, it can also
> adjust the number of channels.  (This does not happen if some program
> decides to use the "hw" device to bypass all conversions.)
>
> > It sounds like arecord can handle multiple channels, but its under the
> > assumption that the hardware itself changes (in the pcm_open function) to
> > only copy in the requested channels worth of data; i.e. arecord assumes
> > that the data in dma_area is exactly as requested by arecord.
>
> The ALSA kernel framework assumes that the snd_pcm_hw constraints correctly
> describe the capabilities of your hardware.  Any conversions happen in
> userspace.
>
> Your driver never sees what happens in userspace; the values to be set by
> the .hw_params callback are guaranteed to fit into your driver's
> constraints.
>
>
> Regards,
> Clemens
>