How to make the sound work

From Multiseat

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Introduction

Sound devices move around on boot. If the alsa driver supports it you can use index=X to create the device on a specific device but that doesn't help with multiseat sound.

Fortunately ALSA provides a way to set the alsa default sound card - simply set the ALSA_CARD env var for the bash session.

Setting up ALSA

Here's a hack for setting ALSA_CARD for each mdm seat.

First

cat /proc/asound/cards

You should see something like this:

0 [Audio          ]: USB-Audio - USB Audio
                     C-Media INC. USB Audio at usb-0000:01:06.1-1, full speed
1 [Audio_1        ]: USB-Audio - USB Audio
                     C-Media INC. USB Audio at usb-0000:01:06.2-3.1, full speed
2 [NVidia         ]: HDA-Intel - HDA NVidia
                     HDA NVidia at 0xfe020000 irq 20
3 [CX8801         ]: CX88x - Conexant CX8801
                     Conexant CX8801 at 0xfa000000

This system has 4 sound cards. Pick out an identifier that uniquely identifies the sound card you want to use for each seat. If you use USB sound, don't use the card name; these can move around with each reboot. I use the physical address of the usb sound card.

Then add a section to /etc/mdm/mdm.conf, identifying your sound card to use with each seat:

# Sound

SOUND[1]="NVidia"
SOUND[2]="usb-0000:01:06.1-1"
SOUND[3]="usb-0000:01:06.2-3.1"

The hard part is done. Now we need to create a script that creates a short config file connecting our sound cards with our displays.

#!/bin/sh

config_file="/etc/mdm/mdm.conf"

[[ -f $config_file ]] || exit 1

. $config_file

ncards=${#SOUND[@]}
index=1

echo "#automatically created sound file DO NOT EDIT" > /etc/asound_card.conf

OIFS=$IFS
IFS=';'

for card in `awk '{i1=$0; getline ; i2=$0 ; x= i1 i2 ; print x ";"}' /proc/asound/cards` ; do
        IFS=$OIFS
        index=1
        while [ "$index" -le "$ncards" ] ; do
                if echo $card | grep ${SOUND[$index]} -qs ; then
                        echo SOUND$index=`echo $card | awk '{print $1}'`>> /etc/asound_card.conf
                        fi
                let "index = $index + 1"
                done
        IFS=';'
        done

Put this into /usr/local/bin/asound_config, make sure to

chmod +x /usr/local/bin/asound_config

and set it up so it runs on every boot, and before mdm. Now you're almost done.

Now you need to tell bash to add the alsa information to every shell. Edit /etc/profile and add the following lines to the front of the file:

if [ -n $DISPLAY ] ; then
        [ -f /etc/asound_card.conf ] && . /etc/asound_card.conf
        export ALSA_CARD=`eval echo '$SOUND'$(echo $DISPLAY | cut -f2 -d: | cut -f1 -d.)`
fi