Comment 2 for bug 1505584

control.1 {
  iface MIXER
  name 'Channel Mode'
  value '2ch'
  comment {
   access 'read write'
   type ENUMERATED
   count 1
   item.0 '2ch'
   item.1 '4ch'
   item.2 '6ch'
  }
 }

this channel mode control limit the availability of 5.1 since the two input jacks have to retask as output jacks

you need to discuss this issue in alsa-devel mailing list and pulseaudio-discuss mailing list

possible way is to add hook which change channel mode when using surround40, surround51

http://git.alsa-project.org/?p=alsa-lib.git;a=blob;f=src/conf/cards/HDA-Intel.conf;hb=HEAD

similar to

http://git.alsa-project.org/?p=alsa-lib.git;a=blob;f=src/conf/cards/ICH4.conf;hb=HEAD