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95 lines
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95 lines
4.5 KiB
Plaintext
Frequently Asked Questions about udev
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Q: What's this udev thing, and what is it trying to do?
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A: Read the OLS 2003 paper about udev, available in the docs/ directory,
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and at:
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<http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf>
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There is also a udev presentation given at OLS 2003 available at:
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<http://www.kroah.com/linux/talks/ols_2003_udev_talk/>
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Q: How is udev related to devfs?
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A: udev works entirely in userspace, using /sbin/hotplug calls that the
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kernel makes whenever a device is added or removed from the kernel. All
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naming policy, and permission control is done in userspace. devfs
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operated from within the kernel.
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Q: Why was devfs marked OBSOLETE if udev is not finished yet?
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A: To quote Al Viro (Linux VFS kernel maintainer):
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- it was determined that the same thing could be done in userspace
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- devfs had been shoved into the tree in hope that its quality will
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catch up
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- devfs was found to have fixable and unfixable bugs
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- the former had stayed around for many months with maintainer
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claiming that everything works fine
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- the latter had stayed, period.
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- the devfs maintainer/author disappeared and stoped maintaining
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the code.
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Q: But udev will not automatically load a driver if a /dev node is opened
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when it is not present like devfs will do.
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A: If you really require this functionality, then use devfs. It is still
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present in the kernel.
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Q: But wait, I really want udev to automatically load drivers when they
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are not present but the device node is opened. It's the only reason I
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like using devfs. Please make udev do this.
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A: No. udev is for managing /dev, not loading kernel drivers.
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Q: Oh come on, pretty please. It can't be that hard to do.
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A: Such a functionality isn't needed on a properly configured system. All
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devices present on the system should generate hotplug events, loading
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the appropriate driver, and udev will notice and create the
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appropriate device node. If you don't want to keep all drivers for your
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hardware in memory, then use something else to manage your modules
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(scripts, modules.conf, etc.) This is not a task for udev.
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Q: But I love that feature of devfs, please?
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A: The devfs approach caused a lot of spurious modprobe attempts as
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programs probed to see if devices were present or not. Every probe
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attempt created a process to run modprobe, almost all of which were
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spurious.
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Q: I really like the devfs naming scheme, will udev do that?
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A: Yes, udev can create /dev nodes using the devfs naming policy. A
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configuration file needs to be created to map the kernel default names
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to the devfs names. See the initial udev.rules.devfs file in the udev
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release. It is the start of such a configuration file. If there are
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any things missing, please let the udev authors know.
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Q: What kinds of devices does udev create nodes for?
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A: All devices that are shown in sysfs will work with udev. If more
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support is added for devices to the kernel, udev will automatically
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start working for them. All block devices are currently supported, and
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almost all major char devices are supported. Kernel developers are
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working on adding support for all char devices at this time. See the
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linux-kernel mailing list for patches and status of these patches.
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Q: Will udev remove the limit on the number of anonymous devices?
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A: udev is entirely in userspace. If the kernel supports a greater number
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of anonymous devices, udev will support it.
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Q: Will udev support symlinks?
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A: Yes, It now does. Multiple symlinks per device node too.
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Q: How will udev support changes to device permissions?
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A: On shutdown, udev will save the state of existing device permissions to
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its database, and then used the on the next boot time.
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Q: How will udev handle the /dev filesystem?
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A: /dev can be a ramfs, or a backing filesystem. udev does not care what
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kind of filesystem it runs on.
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Q: How will udev handle devices found before init runs?
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A: udev will be placed in initramfs and run for every device that is found.
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Work to get this implemented is still underway.
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Q: I have other questions about udev, where do I ask them?
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A: The linux-hotplug-devel mailing list is the proper place for it. The
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address for it is linux-hotplug-devel@lists.sourceforge.net
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Information on joining can be found at
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<https://lists.sourceforge.net/lists/listinfo/linux-hotplug-devel>
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Archives of the mailing list can be found at:
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<http://marc.theaimsgroup.com/?l=linux-hotplug-devel>
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