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1 | Frequently Asked Questions about udev |
2 | ||
3 | ||
4 | Q: What's this udev thing, and what is it trying to do? | |
5 | A: Read the OLS 2003 paper about udev, available in the docs/ directory, | |
6 | and at: | |
7 | <http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf> | |
8 | There is also a udev presentation given at OLS 2003 available at: | |
9 | <http://www.kroah.com/linux/talks/ols_2003_udev_talk/> | |
10 | ||
11 | Q: How is udev related to devfs? | |
12 | A: udev works entirely in userspace, using /sbin/hotplug calls that the | |
13 | kernel makes whenever a device is added or removed from the kernel. All | |
14 | naming policy, and permission control is done in userspace. devfs | |
15 | operated from within the kernel. | |
16 | ||
17 | Q: Why was devfs marked OBSOLETE if udev is not finished yet? | |
18 | A: To quote Al Viro (Linux VFS kernel maintainer): | |
19 | - it was determined that the same thing could be done in userspace | |
20 | - devfs had been shoved into the tree in hope that its quality will | |
21 | catch up | |
22 | - devfs was found to have fixable and unfixable bugs | |
23 | - the former had stayed around for many months with maintainer | |
24 | claiming that everything works fine | |
25 | - the latter had stayed, period. | |
26 | - the devfs maintainer/author disappeared and stoped maintaining | |
27 | the code. | |
28 | ||
29 | Q: But udev will not automatically load a driver if a /dev node is opened | |
30 | when it is not present like devfs will do. | |
31 | A: If you really require this functionality, then use devfs. It is still | |
32 | present in the kernel. | |
33 | ||
34 | Q: But I really like the devfs naming scheme, will udev do that? | |
35 | A: Yes, udev can create /dev nodes using the devfs naming policy. A | |
36 | configuration file needs to be created to map the kernel default names | |
37 | to the devfs names. Such a configuration file would be gladly added to | |
38 | the udev package if it is provided by anyone who can create such a | |
39 | mapping. | |
40 | ||
41 | Q: What kinds of devices does udev create nodes for? | |
42 | A: All devices that are shown in sysfs will work with udev. If more | |
43 | support is added for devices to the kernel, udev will automatically | |
44 | start working for them. All block devices are currently supported, and | |
45 | almost all major char devices are supported. Kernel developers are | |
46 | working on adding support for all char devices at this time. See the | |
47 | linux-kernel mailing list for patches and status of these patches. | |
48 | ||
49 | Q: Will udev remove the limit on the number of anonymous devices? | |
50 | A: udev is entirely in userspace. If the kernel supports a greater number | |
51 | of anonymous devices, udev will support it. | |
52 | ||
53 | Q: Will udev support symlinks? | |
54 | A: Yes, patches are gladly accepted to add this functionality. | |
55 | ||
56 | Q: How will udev support changes to device permissions? | |
57 | A: On shutdown, udev will save the state of existing device permissions to | |
58 | its database, and then used the on the next boot time. | |
59 | ||
60 | Q: How will udev handle the /dev filesystem? | |
61 | A: /dev can be a ramfs, or a backing filesystem. udev does not care what | |
62 | kind of filesystem it runs on. | |
63 | ||
64 | Q: How will udev handle devices found before init runs? | |
65 | A: udev will be placed in initramfs and run for every device that is found. | |
66 | Work to get this implemented is still underway. | |
67 | ||
68 | Q: I have other questions about udev, where do I ask them? | |
69 | A: The linux-hotplug-devel mailing list is the proper place for it. The | |
70 | address for it is linux-hotplug-devel@lists.sourceforge.net | |
71 | Information on joining can be found at | |
72 | <https://lists.sourceforge.net/lists/listinfo/linux-hotplug-devel> | |
73 | Archives of the mailing list can be found at: | |
74 | <http://marc.theaimsgroup.com/?l=linux-hotplug-devel> | |
75 |