[alsa-devel] Errors iterating over PCM stream hints and opening based on NAME/IOID

[Takashi Iwai]

On Sat, 07 May 2016 05:19:45 +0200,
Trent Reed wrote:
> 
> Hey All,
> 
> (Tried to send an email earlier, it didn't seem to send?)
> 
> I'm trying to understand ALSA, having a little trouble with some
> errors that I can't ration over.
> Gist Repro: https://gist.github.com/TReed0803/353ce27d37bc1ebe965854ea88c8b97b
> 
> Whenever I run the above simple code (iterate over PCM stream hints,
> open for Playback/Capture depending on IOID; output also provided at
> link above) I notice that I sometimes hit errors. I'm having trouble
> understanding these errors, because as I understand I'm using the ALSA
> library correctly. For example, dmix is telling me that it "Only
> supports playback streams", well then, why does the hint IOID return
> that it supports capture as well? (same for dsnoop).

Oh, these are real bugs.  They shouldn't have been listed as
bidirectional, indeed.  The fix patches are below.

> More confusing
> are the "ALSA lib pcm_route.c:867:(find_matching_chmap) Found no
> matching channel map" errors.

This is a bit more complicated.  I suppose you're testing with
HD-audio, right?  HD-audio has a restriction that it accepts only even
numbered streams (2, 4, 6, 8).  The error comes from the definition of
surround21, i.e. the odd numbers.  But, you can still use this
definition by wrapping with plug, so that the missing one channel is
filled automatically while using the given channel allocation.


thanks,

Takashi

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