General notes

• The default names for devices come from the kernel.

• Our naming policy is to follow the standard devices.txt suggested names, applying them where the kernel driver uses a mnemonic name.

• We do not supply special names, no, you cannot have /dev/pony.

• However if ponies are a class of devices that share a common subsystem or driver, and are commonly addressed in this way, you could have /dev/pony0.../dev/ponyN

• The numbers on the end of the enumerated devices are assigned by the kernel on a first-come, first-served basis.
• This means every time you boot, the numbers swap around.
• This is not a bug.

• We counter the above by creating symlinks with device-unique names (serial numbers, UUIDs, etc.) e.g. /dev/disk/by-uuid and /dev/input/by-id

• udev does not mount your disks, HAL, gnome-volume-manager and pmount do that.
• udev doesn't pick which module gets loaded, modprobe does that.

Common symptoms

• /dev/null with wrong permissions: udev failed to start in the real filesystem, make sure that the udevd from the initramfs was killed, and that there is only one udev startup script.

• 3-minute hang on boot: one of the devices rules is failing, check /var/log/udev as noted below.

  • often this is ifup, hdparm, etc.

Useful things

• sudo udevmonitor -e ... this should be run for any problems when a device is plugged in, or removed. It will output UEVENT blocks for the event from the kernel, and a following UDEV block; these are timestamped so you know how long it takes to process. The UDEV block contains more information, which udev adds through various helper programs.

  • note that in Ubuntu 8.10, the udevmonitor command has been replaced with the "udevadm monitor" command. This changes the above command to sudo udevadm monitor --e

• /var/log/udev, this is the output from above for the boot process. Especially note whether a UDEV block takes a long amount of time to appear, or doesn't appear at all.

• To see the actions udevd is taking, you can run it in verbose debug mode:
  • sudo pkill udevd
  • sudo udevd --debug-trace --verbose --suppress-syslog
• To get more debug info from udev,
  • edit /usr/share/initramfs-tools/scripts/init-top/udev, and change the line starting udev by adding --debug and sending stdout and stderr into a file under/dev/.initramfs. For instance:

    • /sbin/udevd --daemon --debug --resolve-names=never > /dev/.initramfs/udev.out 2>&1

  • then run sudo update-initramfs -k all -u
  • After reboot, /dev/.initramfs/udev.out will have the udev debug info.

Known bugs

Description of known issues, how to recognise them and stock responses/actions.

Open

Closed

Non-bugs

TBD.

CategoryBugSquad CategoryDebugging