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To introduce the problem, USB devices are typically hot-pluggable external devices. Linux (the kernel) assigns a number for each device in the system at boot time, or later when the device is plugged into the system. This number (the minor device number) is used internally by several system (kernel) functions, but the user applications usually need a pseudo file, a device node, to open and access the device in a UNIX way. These files, residing usually in the /dev directory, contain the major and minor numbers that identify a device class and instance of each device. The problem with hot-pluggable devices is that changing the order used to plug the device, or the slot where it is attached you are pointing another /dev node to that device. And therefore you need to tell to your application that your music should be delivered to /dev/midi02 instead of /dev/midi01 as it was until yesterday. This problem is not limited to MIDI/Audio devices, but also to video-cameras, printers, flash memory disks, etc.

ALSA uses internal identifiers such as hw:1,0 or 64:1 to identify devices to the applications, instead of using the device nodes, but the problem remains: you want to keep a persistent identifier for each device to configure your applications, but it changes everyday depending on the plugging order and the socket used for your USB Audio and MIDI devices.

I think that a general solution for Linux, not only for ALSA, is udev using a 2.6 kernel and hotplug. But I didn't test it by myself yet, and don't know how well it fits with ALSA. You can find more about udev here:


Nevertheless, if you use a 2.4 kernel or can't use udev yet, ALSA's snd-usb-audio module has a feature that can solve your problems. As root, try this command:

# modinfo snd-usb-audio
filename:    /lib/modules/2.4.23/kernel/sound/usb/snd-usb-audio.o
description: "USB Audio"
author:      "Takashi Iwai <[email protected]>"
license:     "GPL"
parm:        index int array (min = 1, max = 8), description "Index value for the USB audio adapter."
parm:        id string array (min = 1, max = 8), description "ID string for the USB audio adapter."
parm:        enable int array (min = 1, max = 8), description "Enable USB audio adapter."
parm:        vid int array (min = 1, max = 8), description "Vendor ID for the USB audio device."
parm:        pid int array (min = 1, max = 8), description "Product ID for the USB audio device."
parm:        nrpacks int, description "Max. number of packets per URB."
parm:        async_unlink int, description "Use async unlink mode."

This module has several options, listed in lines starting with parm:. Index is a common option for every ALSA module, and it means the "card" order for your device. This is one of the key features we need. The other two are the vid and pid, meaning the Vendor and Product identifiers that every USB device shows as descriptor attributes. You can obtain the numbers using the following command:

# lsusb
Bus 001 Device 004: ID 0763:1110 Midiman
Bus 001 Device 003: ID 0582:0005 Roland Corp. Edirol UM-2 MIDI Adapter

The number 0763 is the Vendor ID for Midiman, and 0582 is the Vendor ID for Roland. The numbers following the colon are the Product Ids for the Midisport2x2 (1110) and the UM-2 (0005) respectively. Be warned that the Midisport has a different product id before loading the firmware. We need the number after the renumeration, when it has the firmware loaded and running.

The next step is to assign an index number to every device driven by ALSA, not only the USB ones. This can be done on /etc/modules.conf, or /etc/modprobe.conf, depending on your Linux vendor and kernel version.

Suppose that we have two USB devices (Midisport2x2 and UM-2) one PCI card and we need also the virmidi driver. The card order will be the following:

  1. PCI card
  2. USB Midisport2x2
  3. USB UM-2
  4. Virmidi

The relevant lines in your /etc/modules.conf will be the following:

alias snd-card-0 snd-via82xx
alias snd-card-1 snd-usb-audio
alias snd-card-2 snd-usb-audio
alias snd-card-3 snd-virmidi
options snd cards_limit=4
options snd-via82xx index=0
options snd-usb-audio index=1,2 vid=0x0763,0x0582 pid=0x1110,0x0005
options snd-virmidi index=3 midi_devs=2

snd-via82xx is the ALSA module containing the driver for the PCI card. Assign it the index=0 and it will be the first ALSA card in the system. That means: the PCM and raw MIDI devices will be named hw:0,N, and the sequencer client would have the number 64, but in this case, my PCI card is a VIA VT8233/A/8235 without MIDI, so the sequencer client number 64 will be inexistent/unused.

There are two cards using the module snd-usb-audio, and it has the assigned indexes 1 and 2. Notice the vid and pid parameters with the arguments for the Midisport2x2 and the UM-2. The three parameters are arrays, having up to eight comma separated elements. There should be the same number of elements for all parameters. Being the cards 1 and 2, the Midisport2x2 will be known as hw:1 for the raw MIDI interface, and the UM-2 as hw:2. The sequencer client numbers will be 72 for the Midisport2x2 and 80 for the UM-2. These numbers will be the same in despite of the plug order, or the USB socket used. Even if you only plug the UM-2 but not the Midisport, the sequencer client for it will be 80 and the raw MIDI interface will be hw:2.

NOTE: If you are using gentoo, you will need to run modules-update after modifying the files in /etc/modules.d.

Finally, we assign the persistent index 3 to the very useful virmidi (snd-virmidi) card. Use the following commands to verify your configuration:

# amidi -l
Device    Name
hw:1,0,0  Midisport 2x2 MIDI 1
hw:1,0,1  Midisport 2x2 MIDI 2
hw:2,0,0  UM-2 MIDI 1
hw:2,0,1  UM-2 MIDI 2
hw:3,0    Virtual Raw MIDI (16 subdevices)
hw:3,1    Virtual Raw MIDI (16 subdevices)

# aconnect -ol
client 72: 'Midisport 2x2' [type=kernel]
    0 'Midisport 2x2 MIDI 1'
    1 'Midisport 2x2 MIDI 2'
client 80: 'UM-2' [type=kernel]
    0 'UM-2 MIDI 1     '
    1 'UM-2 MIDI 2     '
client 88: 'Virtual Raw MIDI 3-0' [type=kernel]
    0 'VirMIDI 3-0     '
client 89: 'Virtual Raw MIDI 3-1' [type=kernel]
    0 'VirMIDI 3-1     '


Answer: with Udev

See also

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Category: Howto