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Udev and Pulse

To use a USB DAC (digital-analog audio converter) in Sailfish OS it is necessary to set up udev to respond to the device being plugged in and turn off the internal sound card.

You need to become root:

# devel-su // root

# echo 'ATTRS{id}=="mtsndcard", ENV{PULSE_IGNORE}="1"' > /etc/udev/rules.d/89-pulseaudio-usb.rules
# cd /etc/pulse/xpolicy.conf.d
# mv usbaudio.conf.disabled usbaudio.conf
# exit // back to user
# pactl load-module module-udev-detect

The last command should be done at system boot through pulse. It MAY work on your phone as is. Sadly on volla phones (gigaset GS5/rephone) it does not. This solution does:

# as root
devel-su

# modify the udev rules file
# /etc/udev/rules.d/89-pulseaudio-usb.rules

ATTRS{id}=="mtsndcard", ENV{PULSE_IGNORE}="1"
ACTION=="add", SUBSYSTEM=="sound", RUN+="/usr/local/bin/udev-create-flag"
ACTION=="change", SUBSYSTEM=="sound", RUN+="/usr/local/bin/udev-create-flag"


# create /usr/local/bin/udev-create-flag
# with contents

#!/bin/sh
# called by udev (runs as root)
# write a flag in the user's runtime dir so the user service notices it
USER="defaultuser"
FLAG="/run/user/$(id -u "$USER")/usb-dac-connected.flag"
mkdir -p "$(dirname "$FLAG")"
touch "$FLAG"

# make it executable
chmod 755 /usr/local/bin/udev-create-flag

# create /usr/local/bin/set-usb-dac-volume.sh
# with content this version is NOT setting volume, just making sure the dac is seen

#!/bin/sh
sleep 2
/usr/bin/pactl load-module module-udev-detect

# make it executable
 chmod 755 /usr/local/bin/set-usb-dac-volume.sh

# create /home/defaultuser/.config/systemd/user/usb-dac-volume.path
# with contents

[Unit]
Description=Watch for USB DAC flag
[Path]
PathExists=/run/user/%U/usb-dac-connected.flag
Unit=usb-dac-volume.service
[Install]
WantedBy=default.target


# enable
systemctl --user daemon-reload
systemctl --user enable --now usb-dac-volume.path



# create /home/defaultuser/.config/systemd/user/usb-dac-volume.service
# with contents

[Unit]
Description=Run USB DAC volume script when flag present
After=default.target
[Service]
Type=oneshot
Environment=XDG_RUNTIME_DIR=%t
ExecStart=/usr/local/bin/set-usb-dac-volume.sh
ExecStartPost=/bin/rm -f /run/user/%U/usb-dac-connected.flag
TimeoutStartSec=60
[Install]
WantedBy=default.target
#

enable

systemctl 
edit--user /home/defaultuser/.config/systemd/user/pulse_udev.service

[Unit]
Description=udev-pactl-start-usb-dac

[Service]
Type=oneshot
RemainAfterExit=no
ExecStart=/usr/bin/pactl load-module module-udev-detect

[Install]
WantedBy=multi-user.target

Then rundaemon-reload
systemctl --user enable pulse_udev.service--now usb-dac-volume.path

Article discussion: USB DAC playback on Sailfish OS Forum 

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