View source for Changing card IDs with udev

== Determining the device path ==

First, run the command
 udevadm monitor --kernel --subsystem-match=sound
Then plug your USB device in; the output will look like this:
 KERNEL[1343075515.559055] add      /devices/pci0000:00/0000:00:12.2/usb1/1-4/1-4.3/1-4.3:1.0/sound/card5 (sound)
 KERNEL[1343075515.559673] add      /devices/pci0000:00/0000:00:12.2/usb1/1-4/1-4.3/1-4.3:1.0/sound/card5/pcmC5D0p (sound)
 KERNEL[1343075515.559978] add      /devices/pci0000:00/0000:00:12.2/usb1/1-4/1-4.3/1-4.3:1.0/sound/card5/pcmC5D0c (sound)
 KERNEL[1343075515.561556] add      /devices/pci0000:00/0000:00:12.2/usb1/1-4/1-4.3/1-4.3:1.0/sound/card5/controlC5 (sound)
 KERNEL[1343075515.575976] change   /devices/pci0000:00/0000:00:12.2/usb1/1-4/1-4.3/1-4.3:1.0/sound/card5 (sound)
For PCI cards, unload and reload the driver module; the output will look like this:
 KERNEL[1343075492.525461] add      /devices/pci0000:00/0000:00:14.4/0000:06:05.0/sound/card2 (sound)
 KERNEL[1343075492.530679] add      /devices/pci0000:00/0000:00:14.4/0000:06:05.0/sound/card2/midiC2D0 (sound)
 KERNEL[1343075492.536421] add      /devices/pci0000:00/0000:00:14.4/0000:06:05.0/sound/card2/midi2 (sound)
 KERNEL[1343075492.541358] add      /devices/pci0000:00/0000:00:14.4/0000:06:05.0/sound/card2/dmmidi2 (sound)
 KERNEL[1343075492.546662] add      /devices/pci0000:00/0000:00:14.4/0000:06:05.0/sound/card2/pcmC2D1p (sound)
 KERNEL[1343075492.551704] add      /devices/pci0000:00/0000:00:14.4/0000:06:05.0/sound/card2/pcmC2D1c (sound)
 KERNEL[1343075492.557349] add      /devices/pci0000:00/0000:00:14.4/0000:06:05.0/sound/card2/pcmC2D0p (sound)
 KERNEL[1343075492.562739] add      /devices/pci0000:00/0000:00:14.4/0000:06:05.0/sound/card2/pcmC2D0c (sound)
 KERNEL[1343075492.568942] add      /devices/pci0000:00/0000:00:14.4/0000:06:05.0/sound/card2/controlC2 (sound)
 KERNEL[1343075492.574929] change   /devices/pci0000:00/0000:00:14.4/0000:06:05.0/sound/card2 (sound)

The device path we want is the string that is output for the last change event.<br>
In these example, the device path would be <tt>/devices/pci0000:00/0000:00:12.2/usb1/1-4/1-4.3/1-4.3:1.0/sound/card5</tt>
or <tt>/devices/pci0000:00/0000:00:14.4/0000:06:05.0/sound/card2</tt>.

== Creating udev rules ==

Create a file in <tt>/lib/udev/rules.d/</tt> with a name like <tt>85-my-usb-audio.rules</tt>
and contents like the following:

 SUBSYSTEM!="sound", GOTO="my_usb_audio_end"
 ACTION!="add", GOTO="my_usb_audio_end"
 
 DEVPATH=="/devices/pci0000:00/0000:00:12.2/usb1/1-4/1-4.3/1-4.3:1.0/sound/card?", ATTR{id}="MyDev_A"
 DEVPATH=="/devices/pci0000:00/0000:00:12.2/usb1/1-4/1-4.2/1-4.2:1.0/sound/card?", ATTR{id}="MyDev_B"
 
 LABEL="my_usb_audio_end"

The SUBSYSTEM and ACTION rules skip over the file if udev is called for any non-sound device,
or if some sound device is removed.

The DEVPATH=="..." check matches our cards.
Please note the "card?" at the end which will match for any random sound card number.

The ATTR{id}="..." assignment actually changes the card's ID.
Use whatever ID you want (but most special characters are not allowed).

== Using the devices ==

Use the ID string where you would otherwise use the card number, such as
 aplay -D default:MyDev_A something.wav
 mplayer -ao alsa:device=hw=MyDev_B somethingelse.avi