1 Note: The following comments are from the original FreeBSD 3.1 README
3 this file is: /sys/miscfs/devfs/README
9 expect it to be highly useless for a while,
10 as the only devices that register themselves are the floppy,
11 the pcaudio stuff, speaker, null,mem,zero,io,kmem.
15 There is a tree of nodes that describe the layout of the DEVFS as seen by
16 the drivers.. they add nodes to this tree. This is called the 'back' layer
17 for reasons that will become obvious in a second. Think of it as a
18 BLUEPRINT of the DEVFS tree. Each back node has associated with it
19 a "devnode" struct, that holds information about the device
20 (or directory) and a pointer to the vnode if one has been associated
21 with that node. The back node itself can be considered to be
22 a directory entry, and contains the default name of the device,
23 and a link to the directory that holds it. It is sometimes refered
24 to in the code as the dev_name. The devnode can be considered the inode.
26 When you mount the devfs somewhere (you can mount it multiple times in
27 multiple places), a front layer is created that contains a tree of 'front'
30 Think of this as a Transparency, layed over the top of the blueprint.
31 (or possibly a photocopy).
33 The front and back nodes are identical in type, but the back nodes
34 are reserved for kernel use only, and are protected from the user.
35 The back plane has a mount structure and all that stuff, but it is in
36 fact not really mounted. (and is thus not reachable via namei).
37 Internal kernel routines can open devices in this plane
38 even if the external devfs has not been mounted yet :)
39 (e.g. to find the root device)
41 To start with there is a 1:1 relationship between the front nodes
42 and the backing nodes, however once the front plane has been created
43 the nodes can be moved around within that plane (or deleted).
44 Think of this as the ability to revise a transparency...
45 the blueprint is untouched.
47 There is a "devnode" struct associated with each front note also.
48 Front nodes that refer to devices, use the same "devnode" struct that is used
49 by their associated backing node, so that multiple front nodes that
50 point to the same device will use the same "devnode" struct, and through
51 that, the same vnode, ops, modification times, flags, owner and group.
52 Front nodes representing directories and symlinks have their own
53 "devnode" structs, and may therefore differ. (have different vnodes)
54 i.e. if you have two devfs trees mounted, you can change the
55 directories in one without changing the other.
56 e.g. remove or rename nodes
58 Multiple mountings are like multiple transparencies,
59 each showing through to the original blueprint.
61 Information that is to be shared between these mounts is stored
62 in the 'backing' node for that object. Once you have erased 'front'
63 object, there is no memory of where the backing object was, and
64 except for the possibility of searching the entire backing tree
65 for the node with the correct major/minor/type, I don't see that
66 it is easily recovered.. Particularly as there will eventually be
67 (I hope) devices that go direct from the backing node to the driver
68 without going via the cdevsw table.. they may not even have
71 I see 'mount -u' as a possible solution to recovering a broken dev tree.
72 (though umount+mount would do the same)
74 Because non device nodes (directories and symlinks) have their own
75 "devnode" structs on each layer, these may have different
76 flags, owners, and contents on each layer.
77 e.g. if you have a chroot tree like erf.tfs.com has, you
78 may want different permissions or owners on the chroot mount of the DEVFS
79 than you want in the real one. You might also want to delete some sensitive
80 devices from the chroot tree.
82 Directories also have backing nodes but there is nothing to stop
83 the user from removing a front node from the directory front node.
84 (except permissions of course). This is because the front directory
85 nodes keep their own records as to which front nodes are members
86 of that directory and do not refer to their original backing node
89 The front nodes may be moved to other directories (including
90 directories) however this does not break the linkage between the
91 backing nodes and the front nodes. The backing node never moves. If
92 a driver decides to remove a device from the backing tree, the FS
93 code follows the links to all the front nodes linked to that backing
94 node, and deletes them, no matter where they've been moved to.
95 (active vnodes are redirected to point to the deadfs).
97 If a directory has been moved, and a new backing node is inserted
98 into its own back node, the new front node will appear in that front
99 directory, even though it's been moved, because the directory that
100 gets the front node is found via the links and not by name.
102 a mount -u might be considered to be a request to 'refresh' the
103 plane that controls to the mount being updated.. that would have the
104 effect of 're-propogating' through any backing nodes that find they
105 have no front nodes in that plane.
108 NOTES FOR RELEASE 1.2
109 1/ this is very preliminary
110 2/ the routines have greatly simplified since release 1.1
111 (I guess the break did me good :)
112 3/ many features are not present yet..
113 e.g. symlinks, a comprehensive registration interface (only a crude one)
114 ability to unlink and mv nodes.
115 4/ I'm pretty sure my use of vnodes is bad and it may be 'losing'
116 them, or alternatively, corrupting things.. I need a vnode specialist
projects / apple / xnu.git / blob