- 1 Introduction
- 2 About modules auto-loading
- 3 About udev rules
- 4 Tips & Tricks
- 4.1 Auto mounting USB devices
- 4.1.1 Mount under Template:Filename; use partition label if present; support LUKS encryption
- 4.1.2 Mount under Template:Filename; use partition label if present; support user un-mounting
- 4.1.3 Mount under Template:Filename; create symbolic link under Template:Filename
- 4.1.4 Mount under Template:Filename; use partition label if present
- 4.1.5 Mount under Template:Filename only if the partition has a label
- 4.1.6 Mount SD cards
- 4.1.7 Accessing Firmware Programmers and USB Virtual Comm Devices
- 4.1 Auto mounting USB devices
- 5 Troubleshooting
- 5.1 Disabling modules auto-loading with the load_modules boot parameter
- 5.2 Blacklisting Modules
- 5.3 Known Problems with Hardware
- 5.4 Known Problems with Auto-Loading
- 5.5 Known Problems for Custom Kernel Users
- 6 Other Resources
"udev is the device manager for the Linux 2.6 kernel series. Primarily, it manages device nodes in Template:Filename. It is the successor of devfs and hotplug, which means that it handles the Template:Filename directory and all user space actions when adding/removing devices, including firmware load." Source: Wikipedia
udev loads kernel modules simultaneously, which can provide a speed increase during bootup. However, the downside is that it doesn't always load modules in the same order each time, which can cause problems with things like sound cards and network cards (if you have more than one of them). See below for more info on this.
About modules auto-loading
udev will not do any module loading for you unless Template:Codeline is enabled in Template:Filename. If you disable auto-loading you must manually load the modules you want/need by putting the list in the Template:Codeline array in Template:Filename, you can generate this list with the Template:Codeline command.
About udev rules
If you want to learn how to write udev rules see Writing udev rules.
To get a list of all the attributes of a device you can use to write rules:
# udevadm info -a -p $(udevadm info -q path -n [device name])
Replace [device name] with the device present in the system, such as '/dev/sda' or '/dev/ttyUSB0'.
To restart the udev system once you create or modify udev rules, run the following command. Hotpluggable devices, such as USB devices, will probably have to be reconnected for the new rules to take effect.
# udevadm control restart
Tips & Tricks
Auto mounting USB devices
Mount under Template:Filename; use partition label if present; support LUKS encryption
This udev rule set automatically mounts devices/partitions that are represented by /dev/sd* (USB drives, external hard drives and sometimes SD cards). If a partition label is available, it mounts the device to /media/<label> and otherwise to /media/usbhd-sd*, e.g. /media/usbhd-sdb1. If the plugged in device is a LUKS-encrypted partition, it will open a xterm window to ask for the passphrase (provided that xterm is installed). Also see this post and the follow-ups.
Mount under Template:Filename; use partition label if present; support user un-mounting
This is a variation on the above rule set. It uses pmount (which will need to be installed) instead of mount, allowing a non-root user to unmount udev-mounted devices. The required username must be hard-coded in the RUN command, so this rule set may not be suitable for multi-user systems. LUKS support has also been removed from the example, but can be easily reinstated as above. Template:File
The following rule set does not make use of partition labels; instead it mounts devices as usbhd-sdXY under the /mnt directory (ex: /mnt/usbhd-sdb1) and creates a symbolic link under /media. Template:File
Mount under Template:Filename; use partition label if present
The following rule set mounts under /media, using the partition label if present. If no label exists, the device is mounted as usbhd-sdXY (ex: /media/usbhd-sdb2): Template:File
Mount under Template:Filename only if the partition has a label
Mount SD cards
Accessing Firmware Programmers and USB Virtual Comm Devices
The following ruleset will allow normal users (within the "users" group) the ability to access the USBtinyISP USB programmer for AVR microcontrollers and a generic (SiLabs CP2102) USB to UART adapter. Adjust the permissions accordingly. Verified as of 2010-02-11.
Disabling modules auto-loading with the load_modules boot parameter
If you pass Template:Codeline on your kernel boot line, then udev will skip all the auto-loading business. This is to provide you with a big ripcord to pull if something goes wrong. If udev loads a problematic module that hangs your system or something equally awful, then you can bypass auto-loading with this parameter, then go in and blacklist the offensive module(s).
In rare cases, Udev can make mistakes and load the wrong modules. To prevent it from doing this, you can blacklist modules. Once blacklisted, udev will never load that module. Not at boot-time or later on when a hotplug event is received (ie, you plug in your USB flash drive).
Known Problems with Hardware
BusLogic devices can be broken and will cause a freeze during startup
This is a kernel bug and no fix has been provided yet.
PCMCIA Card readers are not treated as removable devices
To get access to them with hal's pmount backend add them to Template:Filename
Known Problems with Auto-Loading
CPU frequency modules
The current detection method for the various CPU frequency controllers is inadequate, so this has been omitted from the auto-loading process for the time being. To use CPU frequency scaling, load the proper module explicitly in your Template:Codeline array in Template:Filename.
Sound Problems or Some Modules Not Loaded Automatically
Some users have traced this problem to old entries in Template:Codeline. Try cleaning that file out and trying again.
Mixed Up Devices, Sound/Network Cards Changing Order Each Boot
Because udev loads all modules asynchronously, they are initialized in a different order. This can result in devices randomly switching names. For example, with two network cards, you may notice a switching of designations between Template:Codeline and Template:Codeline.
Arch Linux provides the advantage of specifying the module load order by listing the modules in the Template:Codeline array in Template:Filename. Modules in this array are loaded before udev begins auto-loading, so you have full control over the load order.
# Always load 8139too before e100 MODULES=(8139too e100)
Another method for network card ordering is to use the udev-sanctioned method of statically-naming each interface. Create the following file to bind the MAC address of each of your cards to a certain interface name: Template:File
A couple things to note:
- To get the MAC address of each card, use this command: Template:Codeline
- Make sure to use the lower-case hex values in your udev rules. It doesn't like upper-case.
- Some people have problems naming their interfaces after the old style: eth0, eth1, etc. Try something like "lan" or "wlan" if you experience this problem.
Don't forget to update your Template:Filename and other configuration files using the old ethX notation!
Known Problems for Custom Kernel Users
Udev doesn't start at all
Make sure you have a kernel version later than or equal to 2.6.15. Earlier kernels do not have the necessary uevent stuff that udev needs for auto-loading.
If you're using a 2.6.15 kernel, you'll need the uevent patch from ABS (which backports certain uevent functionality from 2.6.16). Just sync up your ABS tree with the Template:Codeline command, then you'll find the patch in Template:Codeline.