eudev/FAQ

Frequently Asked Questions about udev


Q: What's this udev thing, and what is it trying to do?
A: Read the OLS 2003 paper about udev, available in the docs/ directory,
   and at:
	<http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf>
   There is also a udev presentation given at OLS 2003 available at:
	<http://www.kroah.com/linux/talks/ols_2003_udev_talk/>

Q: How is udev related to devfs?
A: udev works entirely in userspace, using /sbin/hotplug calls that the
   kernel makes whenever a device is added or removed from the kernel.  All
   naming policy, and permission control is done in userspace.  devfs
   operated from within the kernel.

Q: Why was devfs marked OBSOLETE if udev is not finished yet?
A: To quote Al Viro (Linux VFS kernel maintainer):
	- it was determined that the same thing could be done in userspace
	- devfs had been shoved into the tree in hope that its quality will
	  catch up
	- devfs was found to have fixable and unfixable bugs
	- the former had stayed around for many months with maintainer
	  claiming that everything works fine
	- the latter had stayed, period.
	- the devfs maintainer/author disappeared and stoped maintaining
	  the code.

Q: But udev will not automatically load a driver if a /dev node is opened
   when it is not present like devfs will do.
A: If you really require this functionality, then use devfs.  It is still
   present in the kernel.

Q: But I really like the devfs naming scheme, will udev do that?
A: Yes, udev can create /dev nodes using the devfs naming policy.  A
   configuration file needs to be created to map the kernel default names
   to the devfs names.  Such a configuration file would be gladly added to
   the udev package if it is provided by anyone who can create such a
   mapping.

Q: What kinds of devices does udev create nodes for?
A: All devices that are shown in sysfs will work with udev.  If more
   support is added for devices to the kernel, udev will automatically
   start working for them.  All block devices are currently supported, and
   almost all major char devices are supported.  Kernel developers are
   working on adding support for all char devices at this time.  See the
   linux-kernel mailing list for patches and status of these patches.

Q: Will udev remove the limit on the number of anonymous devices?
A: udev is entirely in userspace.  If the kernel supports a greater number
   of anonymous devices, udev will support it.

Q: Will udev support symlinks?
A: Yes, patches are gladly accepted to add this functionality.

Q: How will udev support changes to device permissions?
A: On shutdown, udev will save the state of existing device permissions to
   its database, and then used the on the next boot time.

Q: How will udev handle the /dev filesystem?
A: /dev can be a ramfs, or a backing filesystem.  udev does not care what
   kind of filesystem it runs on.

Q: How will udev handle devices found before init runs?
A: udev will be placed in initramfs and run for every device that is found.
   Work to get this implemented is still underway.

Q: I have other questions about udev, where do I ask them?
A: The linux-hotplug-devel mailing list is the proper place for it.  The
   address for it is linux-hotplug-devel@lists.sourceforge.net
   Information on joining can be found at
	<https://lists.sourceforge.net/lists/listinfo/linux-hotplug-devel>
   Archives of the mailing list can be found at:
	<http://marc.theaimsgroup.com/?l=linux-hotplug-devel>