Wednesday Why: Audio Setting Persistence

Wednesday, August 22. 2007

Posted by Chris Tyler in Wednesday Why at 00:01 | Comments (6) | Trackbacks (0)

Wednesday Why: Audio Setting Persistence

Fedora employs the Advanced Linux Sound Architecture (ALSA) system, a feature of the 2.6 kernel series. In addition to the kernel drivers, ALSA incorporates a library and a set of utility programs.

You've probably noticed that ALSA settings for various input and output controls are preserved across reboots. The audio device state is saved in the file /etc/alsa/asound.state, a text file which contains a description of each device control and the current values.

This file is created and read by the program /sbin/salsa, which is part of the alsa-utils package. At shutdown, the script /etc/rc.d/init.d/halt calls /sbin/salsa with the -s (save) argument:

# Save mixer settings, here for lack of a better place.
grep -q "$alsa$" /proc/devices
if [ $? = 0 -a -x /sbin/salsa ]; then
action $"Saving mixer settings" /sbin/salsa -s
fi

When the udev subsystem discovers audio devices -- usually at system startup, but possibly at later time if an audio device such as a USB soundcard is not present at boot and hot-plugged later -- these two rules in /etc/udev/rules.d/90-alsa-rules are invoked:

SUBSYSTEM=="sound", KERNEL=="controlC*" RUN+="/sbin/salsa"
SUBSYSTEM=="sound", KERNEL=="pcm*" RUN+="/sbin/salsa"

The RUN part of each rule executes /sbin/salsa to restore the audio settings for all devices.

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/All/ devices? Does that mean that if a machine is booted, the user adjusts the levels on the main soundcard, then plugs in a headset that the levels for the main card will be reset to the saved at shutdown values?

#1 Ewan Mac Mahon on 2007-08-22 05:54 (Reply)

I haven't tested that, but it looks like it to me -- the udev rules restore the settings for all devices identified in the saved settings file, so it will probably do just what you suggest.

#1.1 Chris Tyler on 2007-08-22 10:16 (Reply)

(I think this is implied in the original comment, but just to be clear: plugging analog headphones into a sound card won't cause the settings to revert, but plugging in a USB headset, or a USB sound dongle, would).

#1.1.1 Chris Tyler on 2007-08-22 12:54 (Reply)

Is the RUN rule to restore audio settings correct? My machine (with alsa-utils-1.0.14-1.fc7.x86_64) doesn't restore volume levels after a reboot. Simply running /sbin/salsa appears to have no effect. I have to explicitly specify the -l option to load settings. Should the RUN rule be:

SUBSYSTEM=="sound", KERNEL=="controlC*" RUN+="/sbin/salsa -l"
SUBSYSTEM=="sound", KERNEL=="pcm*" RUN+="/sbin/salsa -l"

Or is there something wrong with my ALSA setup?

#2 Naveed Hasan on 2007-08-27 13:43 (Reply)

Yes ? I saw from some time that mu PCM level isn't saved during shutdown process. After boot this PCM level is again at 50% even if i every time, when login to gnome set it to level 90%. So it's bug form me.

#3 Ireneusz Piasecki on 2007-08-28 08:22 (Reply)

not sure what happened, but in my case it does not work for my lenovo T60. The PCM setting always reset after reboot.....

#4 Edmond Hui (Homepage) on 2007-08-31 17:34 (Reply)

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