[alsa-devel] IEC switch issues

[pl bossart]

If I play my AC3 data on my HDAudio/SPDIF output using the hw:0,1
device, I can use alsamixer/amixer to mute/unmute.

[ume at plb PassThough]$ amixer cset numid=12 on
numid=12,iface=MIXER,name='IEC958 Playback Switch'
  ; type=BOOLEAN,access=rw------,values=1
  : values=on
[ume at plb PassThough]$ amixer cset numid=12 off
numid=12,iface=MIXER,name='IEC958 Playback Switch'
  ; type=BOOLEAN,access=rw------,values=1
  : values=off

Now if I use the iec958: plugin, I can't control the mute switch any longer:
[ume at plb PassThough]$ amixer cset numid=12 off
amixer: Control default element write error: Operation not permitted

[ume at plb PassThough]$ amixer cset numid=12 on
amixer: Control default element write error: Operation not permitted

What could possibly cause this issue? the IEC plugin relies on the
device 1, am I missing something here?
Thanks for your help
- Pierre