HOW-TO have an USB speaker/headset autoconfigured on Debian/Etch when plugged in...

I don't know why i have had to search this myself while neither Debian nor Linux in general
is my prefered system. People out there seems to like complicated things for a simple need.
Heh, who knows? Maybe they think it's fun to learn modprobe + ALSA scripts + udev in a row,
especially at 4:00 a.m.

Keywords: Debian Etch udev USB ALSA auto configure

1) First step: Go to root and run alsaconf, so that it will generate a default modprobe file.

folays@leonesse:~$ su
Password: 
leonesse:/home/folays# alsaconf

When it will ask you if you want to modify /etc/modprobe.d/sound, say Yes.

You should have now an /etc/modprobe.d/sound file, similary to this one:
<<<
alias snd-card-0 snd-intel8x0
options snd-intel8x0 index=0
>>>

Of course, you can have something different that snd-intel8x0 for your setup, it's hardware-
dependent.

The index= field hints the system about the priority of the device, the lower being favorite.
Change index=0 to index=2 for your native sound card.

I also had to add the following for my integrated ac97 modem which account for a sound card:
<<<
alias snd-card-1 snd-intel8x0m
options snd-intel8x0m index=3
>>>

You can pinpoint the name of the offending module by launching (as root) udevmonitor to watch
the last line like "UDEV [timestamp] add@/module/<module name>" which should be above another
line "UEVENT/UDEV [timestamp] add@/class/sound/{dspX,controlCX}":

<<<
leonesse:/home/folays# udevmonitor 
udevmonitor prints the received event from the kernel [UEVENT]
and the event which udev sends out after rule processing [UDEV]

[...]
UDEV  [1211772049.223883] add@/module/snd_usb_audio
[...]
UEVENT[1211772049.333136] add@/class/sound/dsp1
>>>

Now, plug in your USB headset and look at /var/log/messages for it's driver name, mine being
snd-usb-audio. Then, append it to /etc/modprobe.d/sound in aim of finally have the following:

leonesse:/etc/modprobe.d# cat sound
alias snd-card-0 snd-intel8x0
options snd-intel8x0 index=2
alias snd-card-1 snd-intel8x0m
options snd-intel8x0m index=3
alias snd-card-1 snd-usb-audio
options snd-usb-audio index=1

Note that snd-usb-audio has index=1, so that when plugged, it should take the precedence over
the integrated sound card.

We purposedly(?) omitted a device with index=0, so that we can manage ourselves all symbolics
links pointing to the real devices which primary files will be /dev/dspX, /dev/snd/controlCX.

Beware to not let junk files in the /etc/modprobe.d/ area misstakenly took into account.

2) Second step: Add into udev configuration files a program to be executed for those devices.

Change directory to /etc/udev/rules.d/, create a new local.rules file and have the following:

leonesse:/etc/udev# cat /etc/udev/rules.d/local.rules
KERNEL=="controlC[0-7]|dsp[0-7]", RUN+="/lib/udev/alsa-link $kernel $number"

3) Third step: Create the udev program script to be executed upon add/remove actions.

<<<
leonesse:/etc/udev# cat /lib/udev/alsa-link 
#!/bin/sh

{
    case "$1" in
	dsp*)
	    TARGET="${DEVNAME%/$1}/dsp"
	    ;;
	controlC*)
	    TARGET="${DEVNAME%/$1}/controlC0"
	    ;;
    esac
    rm $TARGET
    SOURCE="`ls ${DEVNAME%$2}* | head -n 1`"
    SOURCE="${SOURCE##*/}"
    case "$ACTION" in
	add|remove)
	ln -fs $SOURCE $TARGET
	;;
    esac
} &
exit 0
>>>

Basically, what it will do is create a symbolic file named /dev/dsp and /dev/snd/controlC0 each
respectively having the priority device as a target.

It could behave in some bad way if you did not carefully followed instructions.

4) Fourth step: Go for a ride!

Unplug your USB headset, /dev/dsp and /dev/snd/controlC0 should link to the native sound card:

folays@leonesse:~$ ls -l /dev/dsp* /dev/snd/controlC*
lrwxrwxrwx 1 root root        4 2008-05-26 05:20 /dev/dsp -> dsp2
crw-rw---- 1 root audio  14, 35 2008-05-26 05:11 /dev/dsp2
crw-rw---- 1 root audio  14, 51 2008-05-26 05:11 /dev/dsp3
lrwxrwxrwx 1 root root        9 2008-05-26 05:20 /dev/snd/controlC0 -> controlC2
crw-rw---- 1 root audio 116, 64 2008-05-26 05:11 /dev/snd/controlC2
crw-rw---- 1 root audio 116, 96 2008-05-26 05:11 /dev/snd/controlC3

Plug it back again, the symbolic links should now target the USB headset device files:

folays@leonesse:~$ ls -l /dev/dsp* /dev/snd/controlC*
lrwxrwxrwx 1 root root        4 2008-05-26 05:20 /dev/dsp -> dsp1
crw-rw---- 1 root audio  14, 19 2008-05-26 05:20 /dev/dsp1
crw-rw---- 1 root audio  14, 35 2008-05-26 05:11 /dev/dsp2
crw-rw---- 1 root audio  14, 51 2008-05-26 05:11 /dev/dsp3
lrwxrwxrwx 1 root root        9 2008-05-26 05:20 /dev/snd/controlC0 -> controlC1
crw-rw---- 1 root audio 116, 32 2008-05-26 05:20 /dev/snd/controlC1
crw-rw---- 1 root audio 116, 64 2008-05-26 05:11 /dev/snd/controlC2
crw-rw---- 1 root audio 116, 96 2008-05-26 05:11 /dev/snd/controlC3

Of couse, applications that do not purposely look for files changes/notifications will have to
be restarted for taking the new device into account.

=== Diagnostic ===

At any time, for debugging needs you can unload/reload your drivers modules with the following:

leonesse:/home/folays# modprobe -v <module name> (load the module)
leonesse:/home/folays# modprobe -v -r <module name> (unload it)

Where module name can be the following in my setup:
- snd-intel8x0, snd-intel8x0m, snd-usb-audio

At any time, you can list your PCI adapters:
leonesse:/home/folays# lspci | grep -i audio

You can also list your loaded modules:
leonesse:/home/folays# lsmod | grep -i snd

You would want to show what audio devices may be available in userland:
folays@leonesse:~$ aplay -l
[...]
(Basically this executable will solely look for /dev/dsp* files...)

udevmonitor can show you a lot of information on what's happening to dynamic device management:
leonesse:/home/folays# udevmonitor --env

And finally, alsamixer will allow you to change the mixer configuration:
folays@leonesse:~$ alsamixer

For the latter, the /dev/snd/controlC0 needs to be a valid file to works (and symbolic link are
okay), while most programs would probably only needs the /dev/dsp file.