FreeBSD/Linux Kernel Cross Reference
sys/miscfs/devfs/

	Name	Size	Last modified (GMT)	Description
	Parent directory		2008-12-15 12:26:16
	README	5436 bytes	1995-09-07 06:16:28
	devfs_proto.h	1249 bytes	1996-11-23 08:32:08
	devfs_tree.c	30438 bytes	1999-09-05 08:16:26
	devfs_vfsops.c	7788 bytes	1999-09-05 08:16:27
	devfs_vnops.c	46787 bytes	1999-09-05 08:16:29
	devfsdefs.h	5192 bytes	1999-09-05 08:16:30
	reproto.sh	913 bytes	1996-04-07 01:15:03

     this file is: /sys/miscfs/devfs/README
     
     to enable: add
     options DEVFS
     
     to your config file..
     expect it to be highly useless for a while,
     as the only devices that register themselves are the floppy,
     the pcaudio stuff, speaker, null,mem,zero,io,kmem.
    
    it works like this:
    
    There is a tree of nodes that describe the layout of the DEVFS as seen by
    the drivers.. they add nodes to this tree. This is called the 'back' layer
    for reasons that will become obvious in a second. Think of it as a
    BLUEPRINT of the DEVFS tree. Each back node has associated with it 
    a "devnode" struct, that holds information about the device
    (or directory) and a pointer to the vnode if one has been associated 
    with that node. The back node itself can be considered to be 
    a directory entry, and contains the default name of the device,
    and a link to the directory that holds it. It is sometimes refered
    to in the code as the dev_name. The devnode can be considered the inode.
    
    When you mount the devfs somewhere (you can mount it multiple times in
    multiple places), a front layer is created that contains a tree of 'front'
    nodes.
    
    Think of this as a Transparency, layed over the top of the blueprint.
    (or possibly a photocopy).
    
    The front and back nodes are identical in type, but the back nodes
    are reserved for kernel use only, and are protected from the user.
    The back plane has a mount structure and all that stuff, but it is in
    fact not really mounted. (and is thus not reachable via namei).
    Internal kernel routines can open devices in this plane
    even if the external devfs has not been mounted yet :)
    (e.g. to find the root device)
    
    To start with there is a 1:1 relationship between the front nodes
    and the backing nodes, however once the front plane has been created
    the nodes can be moved around within that plane (or deleted).
    Think of this as the ability to revise a transparency...
    the blueprint is untouched.
    
    There is a "devnode" struct associated with each front note also.
    Front nodes that refer to devices, use the same "devnode" struct that is used 
    by their associated backing node, so that multiple front nodes that
    point to the same device will use the same "devnode" struct, and through
    that, the same vnode, ops, modification times, flags, owner and group.
    Front nodes representing directories and symlinks have their own
    "devnode" structs, and may therefore differ. (have different vnodes)
    i.e. if you have two devfs trees mounted, you can change the 
    directories in one without changing the other. 
    e.g. remove or rename nodes
    
    Multiple mountings are like multiple transparencies,
    each showing through to the original blueprint.
    
    Information that is to be shared between these mounts is stored
    in the 'backing' node for that object.  Once you have erased 'front'
    object, there is no memory of where the backing object was, and
    except for the possibility of searching the entire backing tree
    for the node with the correct major/minor/type, I don't see that
    it is easily recovered.. Particularly as there will eventually be
    (I hope) devices that go direct from the backing node to the driver
    without going via the cdevsw table.. they may not even have
    major/minor numbers.
    
    I see 'mount -u' as a possible solution to recovering a broken dev tree.
    (though umount+mount would do the same)
    
    Because non device nodes (directories and symlinks) have their own
    "devnode" structs on each layer, these may have different
    flags, owners, and contents on each layer.
    e.g. if you have a chroot tree like erf.tfs.com has, you
    may want different permissions or owners on the chroot mount of the DEVFS
    than you want in the real one. You might also want to delete some sensitive
    devices from the chroot tree.
    
    Directories also have backing nodes but there is nothing to stop
    the user from removing a front node from the directory front node.
    (except permissions of course).  This is because the front directory
    nodes keep their own records as to which front nodes are members
    of that directory and do not refer to their original backing node
    for this information.
    
    The front nodes may be moved to other directories (including
    directories) however this does not break the linkage between the
    backing nodes and the front nodes. The backing node never moves. If
    a driver decides to remove a device from the backing tree, the FS
    code follows the links to all the front nodes linked to that backing
    node, and deletes them, no matter where they've been moved to.
    (active vnodes are redirected to point to the deadfs).
    
    If a directory has been moved, and a new backing node is inserted
    into it's own back node, the new front node will appear in that front
    directory, even though it's been moved, because the directory that
    gets the front node is found via the links and not by name.
    
   a mount -u might be considered to be a request to 'refresh' the
   plane that controls to the mount being updated.. that would have the
   effect of 're-propogating' through any backing nodes that find they
   have no front nodes in that plane.
   
   
   NOTES FOR RELEASE 1.2
   1/ this is very preliminary
   2/ the routines have greatly simplified since release 1.1
   (I guess the break did me good :)
   3/ many features are not present yet..
   e.g. symlinks, a comprehensive registration interface (only a crude one)
   ability to unlink and mv nodes.
   4/ I'm pretty sure my use of vnodes is bad and it may be 'losing'
   them, or alternatively, corrupting things.. I need a vnode specialist
   to look at this.