Add minimal systemd and udev support to alsactl so that mixer settings
are restored at boot, when sound cards are plugged in and saved on
shutdown.
This is similar to existing udev/init script solutions in various
distributions.
Note that alsactl is called both synchronously from the udev rules as
well as asynchronously at boot. This is intended, and to ensure two
things:
- At boot the asound.state file might not be readable, since it resides
on a different file system. That means exclusively restoring sound card
settings from udev rules will no suffice, since if the rule is
executed at early boot (for example within udev settle) then the file
will no be readable and cannot be restored.
- We need to ensure that applications monitoring sound cards coming and
going (such as PA) must not get these events before the mixer settings
have been restored. That means the mixer settings must be restored
synchronously withing the udev rules, before the events are passed on
to the apps.
That basically means we need to restore the settings once in udev, to
deal with sound cards becoming available during runtime. And once in
early boot to deal with coldplugged soundcards whose data files might
not have been available at time of plugging. Hence we call alsactl
twice: one from the udev rule, and once from he systemd unit file.
Signed-off-by: Lennart Poettering <mznyfn@0pointer.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
