About Devices

Multiple Sound Cards

If there are multiple sound cards in a system, the "default" sound card becomes random. The method to establish sound card order depends on whether the drivers are modules or not. If the sound card drivers are compiled into the kernel, control is via kernel command line parameters in /boot/grub/grub.cfg. For example, if a system has both an FM801 card and a SoundBlaster PCI card, the following can be appended to the command line:

snd-fm801.index=0 snd-ens1371.index=1

If the sound card drivers are built as modules, the order can be established in the /etc/modprobe.conf file with:

options snd-fm801 index=0
options snd-ens1371 index=1

USB Device Issues

USB devices usually have two kinds of device nodes associated with them.

The first kind is created by device-specific drivers (e.g., usb_storage/sd_mod or usblp) in the kernel. For example, a USB mass storage device would be /dev/sdb, and a USB printer would be /dev/usb/lp0. These device nodes exist only when the device-specific driver is loaded.

The second kind of device nodes (/dev/bus/usb/BBB/DDD, where BBB is the bus number and DDD is the device number) are created even if the device doesn't have a kernel driver. By using these "raw" USB device nodes, an application can exchange arbitrary USB packets with the device, i.e., bypass the possibly-existing kernel driver.

Access to raw USB device nodes is needed when a userspace program is acting as a device driver. However, for the program to open the device successfully, the permissions have to be set correctly. By default, due to security concerns, all raw USB devices are owned by user root and group usb, and have 0664 permissions (the read access is needed, e.g., for lsusb to work and for programs to access USB hubs). Packages (such as SANE and libgphoto2) containing userspace USB device drivers also ship udev rules that change the permissions of the controlled raw USB devices. That is, rules installed by SANE change permissions for known scanners, but not printers. If a package maintainer forgot to write a rule for your device, report a bug to both BLFS (if the package is there) and upstream, and you will need to write your own rule.

There is one situation when such fine-grained access control with pre-generated udev rules doesn't work. Namely, PC emulators such as KVM, QEMU and VirtualBox use raw USB device nodes to present arbitrary USB devices to the guest operating system (note: patches are needed in order to get this to work without the obsolete /proc/bus/usb mount point described below). Obviously, maintainers of these packages cannot know which USB devices are going to be connected to the guest operating system. You can either write separate udev rules for all needed USB devices yourself, or use the default catch-all "usb" group, members of which can send arbitrary commands to all USB devices.

Before Linux-2.6.15, raw USB device access was performed not with /dev/bus/usb/BBB/DDD device nodes, but with /proc/bus/usb/BBB/DDD pseudofiles. Some applications (e.g., VMware Workstation) still use only this deprecated technique and can't use the new device nodes. For them to work, use the "usb" group, but remember that members will have unrestricted access to all USB devices. To create the fstab entry for the obsolete usbfs filesystem:

usbfs  /proc/bus/usb  usbfs  devgid=14,devmode=0660  0  0

Udev Device Attributes

Fine-tuning of device attributes such as group name and permissions is possible by creating extra udev rules, matching on something like this. The vendor and product can be found by searching the /sys/devices directory entries or using udevadm info after the device has been attached. See the documentation in the current udev directory of /usr/share/doc for details.

SUBSYSTEM=="usb_device", SYSFS{idVendor}=="05d8", SYSFS{idProduct}=="4002", \
  GROUP:="scanner", MODE:="0660"

Devices for Servers

In some cases, it makes sense to disable udev completely and create static devices. Servers are one example of this situation. Does a server need the capability of handling dynamic devices? Only the system administrator can answer that question, but in many cases the answer will be no.

If dynamic devices are not desired, then static devices must be created on the system. In the default configuration, the /etc/rc.d/rcS.d/S10udev boot script mounts a tmpfs partition over the /dev directory. This problem can be overcome by mounting the root partition temporarily:

mount --bind / /mnt
cp -a /dev/* /mnt/dev
rm /etc/rc.d/rcS.d/{S10udev,S50udev_retry}
umount /mnt

At this point, the system will use static devices upon the next reboot. Create any desired additional devices using mknod.

If you want to restore the dynamic devices, recreate the /etc/rc.d/rcS.d/{S10udev,S50udev_retry} symbolic links and reboot again. Static devices do not need to be removed (console and null are always needed) because they are covered by the tmpfs partition. Disk usage for devices is negligible (about 20–30 bytes per entry.)