Start Saraswati service after USB sound devices have been registered

[amotl]

Hi there,

when invoking sudo saraswati setup --systemd, Saraswati installs itself as a systemd unit. For that, it uses the saraswati.service systemd unit definition file.

@MKO1640 reported a problem with that at https://community.hiveeyes.org/t/installation-von-saraswati-auf-einem-industrie-pc-mit-upload-auf-synology-nas/4048/39:

There is also a small problem with the USB microphones. They are not always recognized by the GStreamer after a reboot. They have to be unplugged once and plugged in again.

With lsusb, the microphones are also always recognized. But it also recognizes 2 internal Linux USB hubs.
With my test computer I could not reproduce the problem so far.

He already offers a probable suggestion for the root cause:

I guess that the service starts before the USB is initialized and therefore it doesn't work. So it might be enough to restart ALSA and GStreamer before Saraswati.

With kind regards,
Andreas.

/cc @ClemensGruber

[amotl]

When investigating how to improve the situation, I found those resources on the internet.

How could I say "after USB is ready" for a systemd unit?

• https://superuser.com/questions/1322777/in-systemd-service-file-how-do-i-say-after-usb-is-ready

[amotl]

357b148 is an attempt to address this. By using path-based activation mechanics, it will make Saraswati only start if /dev/bus/usb becomes non-empty.

I am not sure yet if that works best in all occasions. A built-in soundcard is probably not made available through USB, so the Saraswati unit configuration should be improved further to account for this scenario where there might a built-in soundcard but no USB devices at all.

[MKO1640]

I will soon check and report this in my test environment. There is also an onboard sound card that I am not currently using.
In any case, it is now running perfectly via usb.