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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 wait, I really want udev to automatically load drivers when they
are not present but the device node is opened. It's the only reason I
like using devfs. Please make udev do this.
A: No. udev is for managing /dev, not loading kernel drivers.
Q: Oh come on, pretty please. It can't be that hard to do.
A: Such a functionality isn't needed on a properly configured system. All
devices present on the system should generate hotplug events, loading
the appropriate driver, and udev will notice and create the
appropriate device node. If you don't want to keep all drivers for your
hardware in memory, then use something else to manage your modules
(scripts, modules.conf, etc.) This is not a task for udev.
Q: But I love that feature of devfs, please?
A: The devfs approach caused a lot of spurious modprobe attempts as
programs probed to see if devices were present or not. Every probe
attempt created a process to run modprobe, almost all of which were
spurious.
Q: 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. See the initial udev.rules.devfs file in the udev
release. It is the start of such a configuration file. If there are
any things missing, please let the udev authors know.
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, It now does. Multiple symlinks per device node too.
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: Can I use udev to automount a USB device when I connect it?
A: Technically, yes, but udev is not intended for this. Projects that do
automount hotplugged storage devices are:
* Usb-mount http://users.actrix.co.nz/michael/usbmount.html
* devlabel http://linux.dell.com/projects.shtml#devlabel
Alternatively, it is easy to add the following to fstab:
/udev/pendrive /pendrive vfat user,noauto 0 0
This means that users can access the device with:
$ mount /pendrive
And don't have to be root but will get full permissions on /pendrive.
This works even without udev if /udev/pendrive is replaced by /dev/sda1
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>
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