mirror of
https://github.com/systemd/systemd.git
synced 2024-11-06 08:26:52 +03:00
109 lines
5.2 KiB
Plaintext
109 lines
5.2 KiB
Plaintext
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>
|
|
|