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@@ -0,0 +1,75 @@ +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> + |