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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

create a udev rule to deal with the usb dac and activate the pulse usbaudio config.

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 to normal user and execute

pactl load-module module-udev-detect

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as root:

devel-su

modify the udev rules file

vi /etc/udev/rules.d/89-pulseaudio-usb.rules

with the following

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 a flag file

vi /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 and create the script to trigger the dac (or set volume, for instance)

chmod 755 /usr/local/bin/udev-create-flag
vi /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

or for also setting volume (apple dongle for instance) add

# !!! update sink name to yours - it contains unique serial number !!! `pactlpacmd list list-sinks|grep Name`alsa_output.usb`
# someone could improve this to autotdetec script
sleep 2
/usr/bin/pactl load-module module-udev-detect
pactl set-sink-volume alsa_output.usb-Apple__Inc._USB-C_to_3.5mm_Headphone_Jack_Adapter_DWH504202DRL1MQAF-00.analog-stereo 100%

make it executable

 chmod 755 /usr/local/bin/set-usb-dac-volume.sh

Now create the systemd triggers (these are prompted by the udev create flag script)

you may need to make the directories first:

 

mkdir /home/defaultuser/.config/systemd/user
vi /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
vi /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

and enable

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

You should now have automatic switching between internal and usb dacs.

Article discussion: USB DAC playback on Sailfish OS Forum 

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