Linux Cross Reference
Linux/Documentation/filesystems/devfs/

	Name	Size	Last modified (GMT)	Description
	Parent directory		2000-11-28 01:47:38
	ChangeLog	48782 bytes	2000-07-06 04:36:49
	README	56044 bytes	2000-11-29 18:11:38
	ToDo	1128 bytes	2000-07-06 04:36:49
	boot-options	1702 bytes	2000-05-09 05:00:41
	rc.devfs	2511 bytes	2000-02-16 23:42:05

   Devfs (Device File System) FAQ
   
   
   Linux Devfs (Device File System) FAQ
   Richard Gooch
   3-JUL-2000
   
   -----------------------------------------------------------------------------
   
  NOTE: the master copy of this document is available online at:
  
  http://www.atnf.csiro.au/~rgooch/linux/docs/devfs.html
  and looks much better than the text version distributed with the
  kernel sources.
  
  There is also an optional daemon that may be used with devfs. You can
  find out more about it at:
  
  http://www.atnf.csiro.au/~rgooch/linux/
  
  NEWFLASH: The official 2.3.46 kernel has
  included the devfs patch. Future patches will be released which
  build on this. These patches are rolled into Linus' tree from time to
  time.
  
  A mailing list is available which you may subscribe to. Send
  email
  to majordomo@oss.sgi.com with the following line in the
  body of the message:
  subscribe devfs
  The list is archived at
  
  http://oss.sgi.com/projects/devfs/archive/.
  
  -----------------------------------------------------------------------------
  
  Contents
  
  
  What is it?
  
  Why do it?
  
  Who else does it?
  
  How it works
  
  Operational issues (essential reading)
  
  Instructions for the impatient
  Permissions persistence accross reboots
  Dealing with drivers without devfs support
  All the way with Devfs
  Other Issues
  Kernel Naming Scheme
  Devfsd Naming Scheme
  SCSI Host Probing Issues
  
  
  
  Device drivers currently ported
  
  Allocation of Device Numbers
  
  Questions and Answers
  
  Making things work
  Alternatives to devfs
  
  
  Other resources
  
  
  -----------------------------------------------------------------------------
  
  
  What is it?
  
  Devfs is an alternative to "real" character and block special devices
  on your root filesystem. Kernel device drivers can register devices by
  name rather than major and minor numbers. These devices will appear in
  devfs automatically, with whatever default ownership and
  protection the driver specified. A daemon (devfsd) can be used to
  override these defaults.
  
  NOTE that devfs is entirely optional. If you prefer the old
  disc-based device nodes, then simply leave CONFIG_DEVFS_FS=n (the
  default). In this case, nothing will change.  ALSO NOTE that if you do
  enable devfs, the defaults are such that full compatibility is
  maintained with the old devices names.
  
  There are two aspects to devfs: one is the underlying device
  namespace, which is a namespace just like any mounted filesystem. The
  other aspect is the filesystem code which provides a view of the
  device namespace. The reason I make a distinction is because devfs
  can be mounted many times, with each mount showing the same device
  namespace. Changes made are global to all mounted devfs filesystems.
  Also, because the devfs namespace exists without any devfs mounts, you
  can easily mount the root filesystem by referring to an entry in the
 devfs namespace.
 
 The cost of devfs is a small increase in kernel code size and memory
 usage. About 7 pages of code (some of that in __init sections) and 72
 bytes for each entry in the namespace. A modest system has only a
 couple of hundred device entries, so this costs a few more
 pages. Compare this with the suggestion to put /dev on a <a
 href="#why-faq-ramdisc">ramdisc.
 
 On a typical machine, the cost is under 0.2 percent. On a modest
 system with 64 MBytes of RAM, the cost is under 0.1 percent.  The
 accusations of "bloatware" levelled at devfs are not justified.
 
 -----------------------------------------------------------------------------
 
 
 Why do it?
 
 There are several problems that devfs addresses. Some of these
 problems are more serious than others (depending on your point of
 view), and some can be solved without devfs. However, the totality of
 these problems really calls out for devfs.
 
 The choice is a patchwork of inefficient user space solutions, which
 are complex and likely to be fragile, or to use a simple and efficient
 devfs which is robust.
 
 There have been many counter-proposals to devfs, all seeking to
 provide some of the benefits without actually implementing devfs. So
 far there has been an absence of code and no proposed alternative has
 been able to provide all the features that devfs does. Further,
 alternative proposals require far more complexity in user-space (and
 still deliver less functionality than devfs). Some people have the
 mantra of reducing "kernel bloat", but don't consider the effects on
 user-space.
 
 A good solution limits the total complexity of kernel-space and
 user-space.
 
 
 Major&minor allocation
 
 The existing scheme requires the allocation of major and minor device
 numbers for each and every device. This means that a central
 co-ordinating authority is required to issue these device numbers
 (unless you're developing a "private" device driver), in order to
 preserve uniqueness. Devfs shifts the burden to a namespace. This may
 not seem like a huge benefit, but actually it is. Since driver authors
 will naturally choose a device name which reflects the functionality
 of the device, there is far less potential for namespace conflict.
 Solving this requires a kernel change.
 
 /dev management
 
 Because you currently access devices through device nodes, these must
 be created by the system administrator. For standard devices you can
 usually find a MAKEDEV programme which creates all these (hundreds!)
 of nodes. This means that changes in the kernel must be reflected by
 changes in the MAKEDEV programme, or else the system administrator
 creates device nodes by hand.
 The basic problem is that there are two separate databases of
 major and minor numbers. One is in the kernel and one is in /dev (or
 in a MAKEDEV programme, if you want to look at it that way). This is
 duplication of information, which is not good practice.
 Solving this requires a kernel change.
 
 /dev growth
 
 A typical /dev has over 1200 nodes! Most of these devices simply don't
 exist because the hardware is not available. A huge /dev increases the
 time to access devices (I'm just referring to the dentry lookup times
 and the time taken to read inodes off disc: the next subsection shows
 some more horrors).
 
 An example of how big /dev can grow is if we consider SCSI devices:
 
 host           6  bits  (say up to 64 hosts on a really big machine)
 channel        4  bits  (say up to 16 SCSI buses per host)
 id             4  bits
 lun            3  bits
 partition      6  bits
 TOTAL          23 bits
 
 
 This requires 8 Mega (1024*1024) inodes if we want to store all
 possible device nodes. Even if we scrap everything but id,partition
 and assume a single host adapter with a single SCSI bus and only one
 logical unit per SCSI target (id), that's still 10 bits or 1024
 inodes. Each VFS inode takes around 256 bytes (kernel 2.1.78), so
 that's 256 kBytes of inode storage on disc (assuming real inodes take
 a similar amount of space as VFS inodes). This is actually not so bad,
 because disc is cheap these days. Embedded systems would care about
 256 kBytes of /dev inodes, but you could argue that embedded systems
 would have hand-tuned /dev directories. I've had to do just that on my
 embedded systems, but I would rather just leave it to devfs.
 Another issue is the time taken to lookup an inode when first
 referenced. Not only does this take time in scanning through a list in
 memory, but also the seek times to read the inodes off disc.
 This could be solved in user-space using a clever programme which
 scanned the kernel logs and deleted /dev entries which are not
 available and created them when they were available. This programme
 would need to be run every time a new module was loaded, which would
 slow things down a lot.
 
 There is an existing programme called scsidev which will automatically
 create device nodes for SCSI devices. It can do this by scanning files
 in /proc/scsi. Unfortunately, to extend this idea to other device
 nodes would require significant modifications to existing drivers (so
 they too would provide information in /proc). This is a non-trivial
 change (I should know: devfs has had to do something similar). Once
 you go to this much effort, you may as well use devfs itself (which
 also provides this information).  Furthermore, such a system would
 likely be implemented in an ad-hoc fashion, as different drivers will
 provide their information in different ways.
 
 Devfs is much cleaner, because it (natually) has a uniform mechanism
 to provide this information: the device nodes themselves!
 
 
 Node to driver file_operations translation
 
 There is an important difference between the way disc-based character
 and block nodes and devfs entries make the connection between an entry
 in /dev and the actual device driver.
 
 With the current 8 bit major and minor numbers the connection between
 disc-based c&b nodes and per-major drivers is done through a
 fixed-length table of 128 entries. The various filesystem types set
 the inode operations for c&b nodes to {chr,blk}dev_inode_operations,
 so when a device is opened a few quick levels of indirection bring us
 to the driver file_operations.
 
 For miscellaneous character devices a second step is required: there
 is a scan for the driver entry with the same minor number as the file
 that was opened, and the appropriate minor open method is called. This
 scanning is done *every time* you open a device node. Potentially, you
 may be searching through dozens of misc. entries before you find your
 open method. While not an enormous performance overhead, this does
 seem pointless.
 
 Linux *must* move beyond the 8 bit major and minor barrier,
 somehow. If we simply increase each to 16 bits, then the indexing
 scheme used for major driver lookup becomes untenable, because the
 major tables (one each for character and block devices) would need to
 be 64 k entries long (512 kBytes on x86, 1 MByte for 64 bit
 systems). So we would have to use a scheme like that used for
 miscellaneous character devices, which means the search time goes up
 linearly with the average number of major device drivers on your
 system. Not all "devices" are hardware, some are higher-level drivers
 like KGI, so you can get more "devices" without adding hardware
 You can improve this by creating an ordered (balanced:-)
 binary tree, in which case your search time becomes log(N).
 Alternatively, you can use hashing to speed up the search.
 But why do that search at all if you don't have to? Once again, it
 seems pointless.
 
 Note thate devfs doesn't use the major&minor system. For devfs
 entries, the connection is done when you lookup the /dev entry. When
 devfs_register() is called, an internal table is appended which has
 the entry name and the file_operations. If the dentry cache doesn't
 have the /dev entry already, this internal table is scanned to get the
 file_operations, and an inode is created. If the dentry cache already
 has the entry, there is *no lookup time* (other than the dentry scan
 itself, but we can't avoid that anyway, and besides Linux dentries
 cream other OS's which don't have them:-). Furthermore, the number of
 node entries in a devfs is only the number of available device
 entries, not the number of *conceivable* entries. Even if you remove
 unnecessary entries in a disc-based /dev, the number of conceivable
 entries remains the same: you just limit yourself in order to save
 space.
 
 Devfs provides a fast connection between a VFS node and the device
 driver, in a scalable way.
 
 /dev as a system administration tool
 
 Right now /dev contains a list of conceivable devices, most of which I
 don't have. A devfs would only show those devices available on my
 system. This means that listing /dev would be a handy way of checking
 what devices were available.
 
 Major&minor size
 
 Existing major and minor numbers are limited to 8 bits each. This is
 now a limiting factor for some drivers, particularly the SCSI disc
 driver, which consumes a single major number. Only 16 discs are
 supported, and each disc may have only 15 partitions. Maybe this isn't
 a problem for you, but some of us are building huge Linux systems with
 disc arrays. With devfs an arbitrary pointer can be associated with
 each device entry, which can be used to give an effective 32 bit
 device identifier (i.e. that's like having a 32 bit minor
 number). Since this is private to the kernel, there are no C library
 compatibility which you would have with increasing major and minor
 number sizes. See the section on "Allocation of Device Numbers" for
 details on maintaining compatibility with userspace.
 
 Solving this requires a kernel change.
 
 Since writing this, the kernel has been modified so that the SCSI disc
 driver has more major numbers allocated to it and now supports up to
 128 discs. Since these major numbers are non-contiguous (a result of
 unplanned expansion), the implementation is a little more cumbersome
 than originally.
 
 Just like the changes to IPv4 to fix impending limitations in the
 address space, people find ways around the limitations. In the long
 run, however, solutions like IPv6 or devfs can't be put off forever.
 
 Read-only root filesystem
 
 Having your device nodes on the root filesystem means that you can't
 operate properly with a read-only root filesystem. This is because you
 want to change ownerships and protections of tty devices. Existing
 practice prevents you using a CD-ROM as your root filesystem for a
 *real* system. Sure, you can boot off a CD-ROM, but you can't change
 tty ownerships, so it's only good for installing.
 
 Also, you can't use a shared NFS root filesystem for a cluster of
 discless Linux machines (having tty ownerships changed on a common
 /dev is not good). Nor can you embed your root filesystem in a
 ROM-FS.
 
 You can get around this by creating a RAMDISC at boot time, making
 an ext2 filesystem in it, mounting it somewhere and copying the
 contents of /dev into it, then unmounting it and mounting it over
 /dev.
 
 A devfs is a cleaner way of solving this.
 
 Non-Unix root filesystem
 
 Non-Unix filesystems (such as NTFS) can't be used for a root
 filesystem because they variously don't support character and block
 special files or symbolic links. You can't have a separate disc-based
 or RAMDISC-based filesystem mounted on /dev because you need device
 nodes before you can mount these. Devfs can be mounted without any
 device nodes. Devlinks won't work because symlinks aren't supported.
 An alternative solution is to use initrd to mount a RAMDISC initial
 root filesystem (which is populated with a minimal set of device
 nodes), and then construct a new /dev in another RAMDISC, and finally
 switch to your non-Unix root filesystem. This requires clever boot
 scripts and a fragile and conceptually complex boot procedure.
 
 Devfs solves this in a robust and conceptually simple way.
 
 PTY security
 
 Current pseudo-tty (pty) devices are owned by root and read-writable
 by everyone. The user of a pty-pair cannot change
 ownership/protections without being suid-root.
 
 This could be solved with a secure user-space daemon which runs as
 root and does the actual creation of pty-pairs. Such a daemon would
 require modification to *every* programme that wants to use this new
 mechanism. It also slows down creation of pty-pairs.
 
 An alternative is to create a new open_pty() syscall which does much
 the same thing as the user-space daemon. Once again, this requires
 modifications to pty-handling programmes.
 
 The devfs solution allows a device driver to "tag" certain device
 files so that when an unopened device is opened, the ownerships are
 changed to the current euid and egid of the opening process, and the
 protections are changed to the default registered by the driver. When
 the device is closed ownership is set back to root and protections are
 set back to read-write for everybody. No programme need be changed.
 The devpts filesystem provides this auto-ownership feature for Unix98
 ptys. It doesn't support old-style pty devices, nor does it have all
 the other features of devfs.
 
 Intelligent device management
 
 Devfs implements a simple yet powerful protocol for communication with
 a device management daemon (devfsd) which runs in user space. It is
 possible to send a message (either synchronously or asynchronously) to
 devfsd on any event, such as registration/unregistration of device
 entries, opening and closing devices, looking up inodes, scanning
 directories and more. This has many possibilities. Some of these are
 already implemented.
 
 See: 
 http://www.atnf.csiro.au/~rgooch/linux/
 
 Device entry registration events can be used by devfsd to change
 permissions of newly-created device nodes. This is one mechanism to
 control device permissions.
 
 Device entry registration/unregistration events can be used to run
 programmes or scripts. This can be used to provide automatic mounting
 of filesystems when a new block device media is inserted into the
 drive.
 
 Asynchronous device open and close events can be used to implement
 clever permissions management. For example, the default permissions on
 /dev/dsp do not allow everybody to read from the device. This is
 sensible, as you don't want some remote user recording what you say at
 your console. However, the console user is also prevented from
 recording. This behaviour is not desirable. With asynchronous device
 open and close events, you can have devfsd run a programme or script
 when console devices are opened to change the ownerships for *other*
 device nodes (such as /dev/dsp). On closure, you can run a different
 script to restore permissions. An advantage of this scheme over
 modifying the C library tty handling is that this works even if your
 programme crashes (how many times have you seen the utmp database with
 lingering entries for non-existent logins?).
 
 Synchronous device open events can be used to perform intelligent
 device access protections. Before the device driver open() method is
 called, the daemon must first validate the open attempt, by running an
 external programme or script. This is far more flexible than access
 control lists, as access can be determined on the basis of other
 system conditions instead of just the UID and GID.
 
 Inode lookup events can be used to authenticate module autoload
 requests. Instead of using kmod directly, the event is sent to
 devfsd which can implement an arbitrary authentication before loading
 the module itself.
 Inode lookup events can also be used to construct arbitrary
 namespaces, without having to resort to populating devfs with symlinks
 to devices that don't exist.
 
 Speculative Device Scanning
 
 Consider an application (like cdparanoia) that wants to find all
 CD-ROM devices on the system (SCSI, IDE and other types), whether or
 not their respective modules are loaded. The application must
 speculatively open certain device nodes (such as /dev/sr0 for the SCSI
 CD-ROMs) in order to make sure the module is loaded. This requires
 that all Linux distributions follow the standard device naming scheme
 (last time I looked RedHat did things differently). Devfs solves the
 naming problem.
 
 The same application also wants to see which devices are actually
 available on the system. With the existing system it needs to read the
 /dev directory and speculatively open each /dev/sr* device to
 determine if the device exists or not. With a large /dev this is an
 inefficient operation, especially if there are many /dev/sr* nodes. A
 solution like scsidev could reduce the number of /dev/sr* entries (but
 of course that also requires all that inefficient directory scanning).
 
 With devfs, the application can open the /dev/sr directory
 (which triggers the module autoloading if required), and proceed to
 read /dev/sr. Since only the available devices will have
 entries, there are no inefficencies in directory scanning or device
 openings.
 
 -----------------------------------------------------------------------------
 
 Who else does it?
 
 FreeBSD has a devfs implementation. Solaris 2 has a pseudo-devfs
 (something akin to scsidev but for all devices, with some unspecified
 kernel support). BeOS, Plan9 and QNX also have it. SGI's IRIX 6.4 and
 above also have a device filesystem.
 
 While we shouldn't just automatically do something because others do
 it, we should not ignore the work of others either. FreeBSD has a lot
 of competent people working on it, so their opinion should not be
 blithely ignored.
 
 -----------------------------------------------------------------------------
 
 
 How it works
 
 Registering device entries
 
 For every entry (device node) in a devfs-based /dev a driver must call
 devfs_register(). This adds the name of the device entry, the
 file_operations structure pointer and a few other things to an
 internal table. Device entries may be added and removed at any
 time. When a device entry is registered, it automagically appears in
 any mounted devfs'.
 
 Inode lookup
 
 When a lookup operation on an entry is performed and if there is no
 driver information for that entry devfs will attempt to call
 devfsd. If still no driver information can be found then a negative
 dentry is yielded and the next stage operation will be called by the
 VFS (such as create() or mknod() inode methods). If driver information
 can be found, an inode is created (if one does not exist already) and
 all is well.
 
 Manually creating device nodes
 
 The mknod() method allows you to create an ordinary named pipe in the
 devfs, or you can create a character or block special inode if one
 does not already exist. You may wish to create a character or block
 special inode so that you can set permissions and ownership. Later, if
 a device driver registers an entry with the same name, the
 permissions, ownership and times are retained. This is how you can set
 the protections on a device even before the driver is loaded. Once you
 create an inode it appears in the directory listing.
 
 Unregistering device entries
 
 A device driver calls devfs_unregister() to unregister an entry.
 
 Chroot() gaols
 
 2.2.x kernels
 
 The semantics of inode creation are different when devfs is mounted
 with the "explicit" option. Now, when a device entry is registered, it
 will not appear until you use mknod() to create the device. It doesn't
 matter if you mknod() before or after the device is registered with
 devfs_register(). The purpose of this behaviour is to support
 chroot(2) gaols, where you want to mount a minimal devfs inside the
 gaol. Only the devices you specifically want to be available (through
 your mknod() setup) will be accessible.
 
 2.4.x kernels
 
 As of kernel 2.3.99, the VFS has had the ability to rebind parts of
 the global filesystem namespace into another part of the namespace.
 This now works even at the leaf-node level, which means that
 individual files and device nodes may be bound into other parts of the
 namespace. This is like making links, but better, because it works
 across filesystems (unlike hard links) and works through chroot()
 gaols (unlike symbolic links).
 
 Because of these improvements to the VFS, the multi-mount capability
 in devfs is no longer needed. The administrator may create a minimal
 device tree inside a chroot(2) gaol by using VFS bindings. As this
 provides most of the features of the devfs multi-mount capability, I
 removed the multi-mount support code (after issuing an RFC). This
 yielded code size reductions and simplifications.
 
 If you want to construct a minimal chroot() gaol, the following
 command should suffice:
 
 mount --bind /dev/null /gaol/dev/null
 
 
 Repeat for other device nodes you want to expose. Simple!
 
 -----------------------------------------------------------------------------
 
 
 Operational issues
 
 
 Instructions for the impatient
 
 Nobody likes reading documentation. People just want to get in there
 and play. So this section tells you quickly the steps you need to take
 to run with devfs mounted over /dev. Skip these steps and you will end
 up with a nearly unbootable system. Subsequent sections describe the
 issues in more detail, and discuss non-essential configuration
 options.
 
 Devfsd
 OK, if you're reading this, I assume you want to play with
 devfs. First you need to compile devfsd, the device management daemon,
 available at 
 http://www.atnf.csiro.au/~rgooch/linux/.
 Because the kernel has a naming scheme
 which is quite different from the old naming scheme, you need to
 install devfsd so that software and configuration files that use the
 old naming scheme will not break.
 
 Compile and install devfsd. You will be provided with a default
 configuration file /etc/devfsd.conf which will provide
 compatibility symlinks for the old naming scheme. Don't change this
 config file unless you know what you're doing. Even if you think you
 do know what you're doing, don't change it until you've followed all
 the steps below and booted a devfs-enabled system and verified that it
 works.
 
 Now edit your main system boot script so that devfsd is started at the
 very beginning (before any filesystem
 checks). /etc/rc.d/rc.sysinit is often the main boot script
 on systems with SysV-style boot scripts. On systems with BSD-style
 boot scripts it is often /etc/rc. Also check
 /sbin/rc.
 
 NOTE that the line you put into the boot
 script should be exactly:
 
 /sbin/devfsd /dev
 
 DO NOT use some special daemon-launching
 programme, otherwise the boot script may not wait for devfsd to finish
 initialising.
 
 System Libraries
 There may still be some problems because of broken software making
 assumptions about device names. In particular, some software does not
 handle devices which are symbolic links. If you are running a libc 5
 based system, install libc 5.4.44 (if you have libc 5.4.46, go back to
 libc 5.4.44, which is actually correct). If you are running a glibc
 based system, make sure you have glibc 2.1.3 or later.
 
 /etc/securetty
 PAM (Pluggable Authentication Modules) is supposed to be a flexible
 mechanism for providing better user authentication and access to
 services. Unfortunately, it's also fragile, complex and undocumented
 (check out RedHat 6.1, and probably other distributions as well). PAM
 has problems with symbolic links. Append the following lines to your
 /etc/securetty file:
 
 1
 2
 3
 4
 5
 6
 7
 8
 
 This may potentially weaken security by allowing root logins over the
 network (a password is still required, though). However, since there
 are problems with dealing with symlinks, I'm suspicious of the level
 of security offered in any case.
 
 XFree86
 While not essential, it's probably a good idea to upgrade to XFree86
 4.0, as patches went in to make it more devfs-friendly. If you don't,
 you'll probably need to apply the following patch to
 /etc/security/console.perms so that ordinary users can run
 startx.
 
 --- /etc/security/console.perms.orig    Sat Apr 17 16:26:47 1999 
 +++ /etc/security/console.perms Fri Feb 25 23:53:55 2000 
 @@ -14,7 +14,7 @@ 
  # man 5 console.perms 
 
  # file classes -- these are regular expressions 
 -<console>=tty[0-9][0-9]* :[0-9]\.[0-9] :[0-9] 
 +<console>=tty[0-9][0-9]* [0-9][0-9]* :[0-9]\.[0-9] :[0-9] 
 
  # device classes -- these are shell-style globs 
  <floppy>=/dev/fd[0-1]* 
 
 
 Disable devpts
 I've had a report of devpts mounted on /dev/pts not working
 correctly. Since devfs will also manage /dev/pts, there is no
 need to mount devpts as well. You should either edit your
 /etc/fstab so devpts is not mounted, or disable devfs from
 your kernel configuration.
 
 Unsupported drivers
 Not all drivers have devfs support. If you depend on one of these
 drivers, you will need to create a script or tarfile that you can use
 at boot time to create device nodes as appropriate. There is a
 section which describes this. Another
 section lists the drivers which have
 devfs support.
 
 /dev/mouse
 
 Many disributions configure /dev/mouse to be the mouse device
 for XFree86 and GPM. I actually think this is a bad idea, because it
 adds another level of indirection. When looking at a config file, if
 you see /dev/mouse you're left wondering which mouse
 is being referred to. Hence I recommend putting the actual mouse
 device (for example /dev/psaux) into your
 /etc/X11/XF86Config file (and similarly for the GPM
 configuration file).
 
 Alternatively, use the same technique used for unsupported drivers
 described above.
 
 The Kernel
 Finally, you need to make sure devfs is compiled into your
 kernel. Set CONFIG_DEVFS_FS=y and recompile your kernel. Next, you
 need to make sure devfs is mounted. The best solution is to pass
 devfs=mount at the kernel boot command line. You can edit
 /etc/lilo.conf and add the line:
 
 append = "devfs=mount"
 
 
 This will make the kernel mount devfs at boot time onto /dev.
 
 Now you've finished all the steps required. You're now ready to boot
 your shiny new kernel. Enjoy.
 
 Changing the configuration
 
 OK, you've now booted a devfs-enabled system, and everything works.
 Now you may feel like changing the configuration (common targets are
 /etc/fstab and /etc/devfsd.conf). Since you have a
 system that works, if you make any changes and it doesn't work, you
 now know that you only have to restore your configuration files to the
 default and it will work again.
 
 
 Permissions persistence across reboots
 
 If you don't use mknod(2) to create a device file, nor use chmod(2) or
 chown(2) to change the ownerships/permissions, the inode ctime will
 remain at 0 (the epoch, 12 am, 1-JAN-1970, GMT). Anything with a ctime
 later than this has had it's ownership/permissions changed. Hence, a
 simple script or programme may be used to tar up all changed inodes,
 prior to shutdown. Although effective, many consider this approach a
 kludge.
 
 A much better approach is to use devfsd to save and restore
 permissions. It may be configured to record changes in permissions and
 will save them in a database (in fact a directory tree), and restore
 these upon boot. This is an efficient method and results in immediate
 saving of current permissions (unlike the tar approach, which save
 permissions at some unspecified future time).
 
 The default configuration file supplied with devfsd has config entries
 which you may uncomment to enable persistence management.
 
 If you decide to use the tar approach anyway, be aware that tar will
 first unlink(2) an inode before creating a new device node. The
 unlink(2) has the effect of breaking the connection between a devfs
 entry and the device driver. If you use the "devfs=only" boot option,
 you lose access to the device driver, requiring you to reload the
 module. I consider this a bug in tar (there is no real need to
 unlink(2) the inode first).
 
 Alternatively, you can use devfsd to provide more sophisticated
 management of device permissions. You can use devfsd to store
 permissions for whole groups of devices with a single configuration
 entry, rather than the conventional single entry per device entry.
 
 Permissions database stored in mounted-over /dev
 
 If you wish to save and restore your device permissions into the
 disc-based /dev while still mounting devfs onto /dev
 you may do so. This requires a 2.4.x kernel (in fact, 2.3.99 or
 later), which has the VFS binding facility. You need to do the
 following to set this up:
 
 
 
 make sure the kernel does not mount devfs at boot time
 
 
 create the /dev-state directory
 
 
 add the following lines near the very beginning of your boot
 scripts:
 
 mount --bind /dev /dev-state
 mount -t devfs none /dev
 devfsd /dev
 
 
 
 add the following lines to your /etc/devfsd.conf file:
 
 REGISTER        .*              COPY    /dev-state/$devname $devpath
 CHANGE          .*              COPY    $devpath /dev-state/$devname
 CREATE          .*              COPY    $devpath /dev-state/$devname
 
 
 
 reboot.
 
 
 
 
 
 Dealing with drivers without devfs support
 
 Currently, not all device drivers in the kernel have been modified to
 use devfs. Device drivers which do not yet have devfs support will not
 automagically appear in devfs. The simplest way to create device nodes
 for these drivers is to unpack a tarfile containing the required
 device nodes. You can do this in your boot scripts. All your drivers
 will now work as before.
 
 Hopefully for most people devfs will have enough support so that they
 can mount devfs directly over /dev without loosing most functionality
 (i.e. loosing access to various devices). As of 22-JAN-1998 (devfs
 patch version 10) I am now running this way. All the devices I have
 are available in devfs, so I don't lose anything.
 
 WARNING: if your configuration requires the old-style device names
 (i.e. /dev/hda1 or /dev/sda1), you must install devfsd and configure
 it to maintain compatibility entries. It is almost certain that you
 will require this. Note that the kernel creates a compatibility entry
 for the root device, so you don't need initrd.
 
 Note that you no longer need to mount devpts if you use Unix98 PTYs,
 as devfs can manage /dev/pts itself. This saves you some RAM, as you
 don't need to compile and install devpts. Note that some versions of
 glibc have a bug with Unix98 pty handling on devfs systems. Contact
 the glibc maintainers for a fix. Glibc 2.1.3 has the fix.
 
 Note also that apart from editing /etc/fstab, other things will need
 to be changed if you *don't* install devfsd. Some software (like the X
 server) hard-wire device names in their source. It really is much
 easier to install devfsd so that compatibility entries are created.
 You can then slowly migrate your system to using the new device names
 (for example, by starting with /etc/fstab), and then limiting the
 compatibility entries that devfsd creates.
 
 MAKE SURE YOU INSTALL DEVFSD BEFORE YOU BOOT A DEVFS-ENABLED KERNEL!
 
 Now that devfs has gone into the 2.3.46 kernel, I'm getting a lot of
 reports back. Many of these are because people are trying to run
 without devfsd, and hence some things break. Please just run devfsd if
 things break. I want to concentrate on real bugs rather than
 misconfiguration problems at the moment. If people are willing to fix
 bugs/false assumptions in other code (i.e. glibc, X server) and submit
 that to the respective maintainers, that would be great.
 
 
 All the way with Devfs
 
 The devfs kernel patch creates a rationalised device tree. As stated
 above, if you want to keep using the old /dev naming scheme,
 you just need to configure devfsd appopriately (see the man
 page). People who prefer the old names can ignore this section. For
 those of us who like the rationalised names and an uncluttered
 /dev, read on.
 
 If you don't run devfsd, or don't enable compatibility entry
 management, then you will have to configure your system to use the new
 names. For example, you will then need to edit your
 /etc/fstab to use the new disc naming scheme. If you want to
 be able to boot non-devfs kernels, you will need compatibility
 symlinks in the underlying disc-based /dev pointing back to
 the old-style names for when you boot a kernel without devfs.
 
 You can selectively decide which devices you want compatibility
 entries for. For example, you may only want compatibility entries for
 BSD pseudo-terminal devices (otherwise you'll have to patch you C
 library or use Unix98 ptys instead). It's just a matter of putting in
 the correct regular expression into /dev/devfsd.conf.
 
 There are other choices of naming schemes that you may prefer. For
 example, I don't use the kernel-supplied
 names, because they are too verbose. A common misconception is
 that the kernel-supplied names are meant to be used directly in
 configuration files. This is not the case. They are designed to
 reflect the layout of the devices attached and to provide easy
 classification.
 
 If you like the kernel-supplied names, that's fine. If you don't then
 you should be using devfsd to construct a namespace more to your
 liking. Devfsd has built-in code to construct a
 namespace that is both logical and easy to
 manage. In essence, it creates a convenient abbreviation of the
 kernel-supplied namespace.
 
 You are of course free to build your own namespace. Devfsd has all the
 infrastructure required to make this easy for you. All you need do is
 write a script. You can even write some C code and devfsd can load the
 shared object as a callable extension.
 
 
 Other Issues
 
 The init programme
 Another thing to take note of is whether your init programme
 creates a Unix socket /dev/telinit. Some versions of init
 create /dev/telinit so that the telinit programme can
 communicate with the init process. If you have such a system you need
 to make sure that devfs is mounted over /dev *before* init
 starts. In other words, you can't leave the mounting of devfs to
 /etc/rc, since this is executed after init. Other
 versions of init require a named pipe /dev/initctl
 which must exist *before* init starts. Once again, you need to
 mount devfs and then create the named pipe *before* init
 starts.
 
 The default behaviour now is not to mount devfs onto /dev at
 boot time for 2.3.x and later kernels. You can correct this with the
 "devfs=mount" boot option. This solves any problems with init,
 and also prevents the dreaded:
 
 Cannot open initial console
 
 message. For 2.2.x kernels where you need to apply the devfs patch,
 the default is to mount.
 
 If you have automatic mounting of devfs onto /dev then you
 may need to create /dev/initctl in your boot scripts. The
 following lines should suffice:
 
 mknod /dev/initctl p
 kill -SIGUSR1 1       # tell init that /dev/initctl now exists
 
 Alternatively, if you don't want the kernel to mount devfs onto
 /dev then you could use the following procedure is a
 guideline for how to get around /dev/initctl problems:
 
 # cd /sbin
 # mv init init.real
 # cat > init
 #! /bin/sh
 mount -n -t devfs none /dev
 mknod /dev/initctl p
 exec /sbin/init.real $*
 [control-D]
 # chmod a+x init
 
 Note that newer versions of init create /dev/initctl
 automatically, so you don't have to worry about this.
 
 Module autoloading
 You will need to configure devfsd to enable module
 autoloading. The following lines should be placed in your
 /etc/devfsd.conf file:
 
 LOOKUP  .*              MODLOAD
 
 
 As of devfsd-v1.3.10, a generic /etc/modules.devfs
 configuration file is installed, which is used by the MODLOAD
 action. This should be sufficient for most configurations. If you
 require further configuration, edit your /etc/modules.conf
 file.
 
 Mounting root off a devfs device
 If you wish to mount root off a devfs device when you pass the
 "devfs=only" boot option, then you need to pass in the "root="
 option to the kernel when booting. If you use LILO, then you must have
 this in lilo.conf:
 
 append = "root=<device>"
 
 Surprised? Yep, so was I. It turns out if you have (as most people
 do):
 
 root = <device>
 
 
 then LILO will determine the device number of  and will write
 that device number into a special place in the kernel image before
 starting the kernel, and the kernel will use that device number to
 mount the root filesystem. So, using the "append" variety ensures that
 LILO passes the root filesystem device as a string, which devfs can
 then use.
 
 Note that this isn't an issue if you don't pass "devfs=only".
 
 TTY issues
 The ttyname(3) function in some versions of the C library makes
 false assumptions about device entries which are symbolic links.  The
 tty(1) programme is one that depends on this function.  I've
 written a patch to libc 5.4.43 which fixes this. This has been
 included in libc 5.4.44 and a similar fix is in glibc 2.1.3.
 
 
 Kernel Naming Scheme
 
 The kernel provides a default naming scheme. This scheme is designed
 to make it easy to search for specific devices or device types, and to
 view the available devices. Some device types (such as hard discs),
 have a directory of entries, making it easy to see what devices of
 that class are available. Often, the entries are symbolic links into a
 directory tree that reflects the topology of available devices. The
 topological tree is useful for finding how your devices are arranged.
 
 Disc Devices
 
 All discs, whether SCSI, IDE or whatever, are placed under the
 /dev/discs hierarchy:
 
         /dev/discs/disc0        first disc
         /dev/discs/disc1        second disc
 
 
 Each of these entries is a symbolic link to the directory for that
 device. The device directory contains:
 
         disc    for the whole disc
         part*   for individual partitions
 
 
 CD-ROM Devices
 
 All CD-ROMs, whether SCSI, IDE or whatever, are placed under the
 /dev/cdroms hierarchy:
 
         /dev/cdroms/cdrom0      first CD-ROM
         /dev/cdroms/cdrom1      second CD-ROM
 
 
 Each of these entries is a symbolic link to the real device entry for
 that device.
 
 Tape Devices
 
 All tapes, whether SCSI, IDE or whatever, are placed under the
 /dev/tapes hierarchy:
 
         /dev/tapes/tape0        first tape
         /dev/tapes/tape1        second tape
 
 
 Each of these entries is a symbolic link to the directory for that
 device. The device directory contains:
 
         mt                      for mode 0
         mtl                     for mode 1
         mtm                     for mode 2
         mta                     for mode 3
         mtn                     for mode 0, no rewind
         mtln                    for mode 1, no rewind
         mtmn                    for mode 2, no rewind
         mtan                    for mode 3, no rewind
 
 
 SCSI Devices
 
 To uniquely identify any SCSI device requires the following
 information:
 
   controller    (host adapter)
   bus           (SCSI channel)
   target        (SCSI ID)
   unit          (Logical Unit Number)
 
 
 All SCSI devices are placed under /dev/scsi (assuming devfs
 is mounted on /dev). Hence, a SCSI device with the following
 parameters: c=1,b=2,t=3,u=4 would appear as:
 
         /dev/scsi/host1/bus2/target3/lun4       device directory
 
 
 Inside this directory, a number of device entries may be created,
 depending on which SCSI device-type drivers were installed.
 
 See the section on the disc naming scheme to see what entries the SCSI
 disc driver creates.
 
 See the section on the tape naming scheme to see what entries the SCSI
 tape driver creates.
 
 The SCSI CD-ROM driver creates:
 
         cd
 
 
 The SCSI generic driver creates:
 
         generic
 
 
 IDE Devices
 
 To uniquely identify any IDE device requires the following
 information:
 
   controller
   bus           (aka. primary/secondary)
   target        (aka. master/slave)
   unit
 
 
 All IDE devices are placed under /dev/ide, and uses a similar
 naming scheme to the SCSI subsystem.
 
 XT Hard Discs
 
 All XT discs are placed under /dev/xd. The first XT disc has
 the directory /dev/xd/disc0.
 
 TTY devices
 
 The tty devices now appear as:
 
   New name                   Old-name                   Device Type
   --------                   --------                   -----------
   /dev/tts/{0,1,...}         /dev/ttyS{0,1,...}         Serial ports
   /dev/cua/{0,1,...}         /dev/cua{0,1,...}          Call out devices
   /dev/vc/{0,1,...}          /dev/tty{1...63}           Virtual consoles
   /dev/vcc/{0,1,...}         /dev/vcs{1...63}           Virtual consoles
   /dev/pty/m{0,1,...}        /dev/ptyp??                PTY masters
   /dev/pty/s{0,1,...}        /dev/ttyp??                PTY slaves
 
 
 RAMDISCS
 
 The RAMDISCS are placed in their own directory, and are named thus:
 
   /dev/rd/{0,1,2,...}
 
 
 Meta Devices
 
 The meta devices are placed in their own directory, and are named
 thus:
 
   /dev/md/{0,1,2,...}
 
 
 Floppy discs
 
 Floppy discs are placed in the /dev/floppy directory.
 
 Loop devices
 
 Loop devices are placed in the /dev/loop directory.
 
 Sound devices
 
 Sound devices are placed in the /dev/sound directory
 (audio, sequencer, ...).
 
 
 Devfsd Naming Scheme
 
 Devfsd provides a naming scheme which is a convenient abbreviation of
 the kernel-supplied namespace. In some
 cases, the kernel-supplied naming scheme is quite convenient, so
 devfsd does not provide another naming scheme. The convenience names
 that devfsd creates are in fact the same names as the original devfs
 kernel patch created (before Linus mandated the Big Name Change).
 
 In order to configure devfsd to create these convenience names, the
 following lines should be placed in your /etc/devfsd.conf:
 
 REGISTER        .*              MKNEWCOMPAT
 UNREGISTER      .*              RMNEWCOMPAT
 
 This will cause devfsd to create (and destroy) symbolic links which
 point to the kernel-supplied names.
 
 SCSI Hard Discs
 
 All SCSI discs are placed under /dev/sd (assuming devfs is
 mounted on /dev). Hence, a SCSI disc with the following
 parameters: c=1,b=2,t=3,u=4 would appear as:
 
         /dev/sd/c1b2t3u4        for the whole disc
         /dev/sd/c1b2t3u4p5      for the 5th partition
         /dev/sd/c1b2t3u4p5s6    for the 6th slice in the 5th partition
 
 
 SCSI Tapes
 
 All SCSI tapes are placed under /dev/st. A similar naming
 scheme is used as for SCSI discs. A SCSI tape with the
 parameters:c=1,b=2,t=3,u=4 would appear as:
 
         /dev/st/c1b2t3u4m0      for mode 0
         /dev/st/c1b2t3u4m1      for mode 1
         /dev/st/c1b2t3u4m2      for mode 2
         /dev/st/c1b2t3u4m3      for mode 3
         /dev/st/c1b2t3u4m0n     for mode 0, no rewind
         /dev/st/c1b2t3u4m1n     for mode 1, no rewind
         /dev/st/c1b2t3u4m2n     for mode 2, no rewind
         /dev/st/c1b2t3u4m3n     for mode 3, no rewind
 
 
 SCSI CD-ROMs
 
 All SCSI CD-ROMs are placed under /dev/sr. A similar naming
 scheme is used as for SCSI discs. A SCSI CD-ROM with the
 parameters:c=1,b=2,t=3,u=4 would appear as:
 
         /dev/sr/c1b2t3u4
 
 
 SCSI Generic Devices
 
 All SCSI CD-ROMs are placed under /dev/sg. A similar naming
 scheme is used as for SCSI discs. A SCSI generic device with the
 parameters:c=1,b=2,t=3,u=4 would appear as:
 
         /dev/sg/c1b2t3u4
 
 
 IDE Hard Discs
 
 All IDE discs are placed under /dev/ide/hd, using a similar
 convention to SCSI discs. The following mappings exist between the new
 and the old names:
 
         /dev/hda        /dev/ide/hd/c0b0t0u0
         /dev/hdb        /dev/ide/hd/c0b0t1u0
         /dev/hdc        /dev/ide/hd/c0b1t0u0
         /dev/hdd        /dev/ide/hd/c0b1t1u0
 
 
 IDE Tapes
 
 A similar naming scheme is used as for IDE discs. The entries will
 appear in the /dev/ide/mt directory.
 
 IDE CD-ROM
 
 A similar naming scheme is used as for IDE discs. The entries will
 appear in the /dev/ide/cd directory.
 
 IDE Floppies
 
 A similar naming scheme is used as for IDE discs. The entries will
 appear in the /dev/ide/fd directory.
 
 XT Hard Discs
 
 All XT discs are placed under /dev/xd. The first XT disc
 would appear as /dev/xd/c0t0.
 
 
 SCSI Host Probing Issues
 
 Devfs allows you to identify SCSI discs based in part on SCSI host
 numbers. If you have only one SCSI host (card) in your computer, then
 clearly it will be given host number 0. Life is not always that easy
 is you have multiple SCSI hosts. Unfortunately, it can sometimes be
 difficult to guess what the probing order of SCSI hosts is. You need
 to know the probe order before you can use device names. To make this
 easy, there is a kernel boot parameter called "scsihosts". This allows
 you to specify the probe order for different types of SCSI hosts. The
 syntax of this parameter is:
 
 scsihosts=<name_1>:<name_2>:<name_3>:...:<name_n>
 
 where <name_1>,<name_2>,...,<name_n> are the names
 of drivers used in the /proc filesystem. For example:
 
     scsihosts=aha1542:ppa:aha1542::ncr53c7xx
 
 
 means that devices connected to
 
 - first aha1542 controller   - will be c0b#t#u#
 - first parallel port ZIP    - will be c1b#t#u#
 - second aha1542 controller  - will be c2b#t#u#
 - first NCR53C7xx controller - will be c4b#t#u#
 - any extra controller       - will be c5b#t#u#, c6b#t#u#, etc
 - if any of above controllers will not be found - the reserved names will
   not be used by any other device.
 - c3b#t#u# names will never be used
 
 
 You can use ',' instead of ':' as the separator character if you
 wish. I have used the devfsd naming scheme
 here.
 
 Note that this scheme does not address the SCSI host order if you have
 multiple cards of the same type (such as NCR53c8xx). In this case you
 need to use the driver-specific boot parameters to control this.
 
 -----------------------------------------------------------------------------
 
 
 Device drivers currently ported
 
 - All miscellaneous character devices support devfs (this is done
   transparently through misc_register())
 
 - SCSI discs and generic hard discs
 
 - Character memory devices (null, zero, full and so on)
   Thanks to C. Scott Ananian <cananian@alumni.princeton.edu>
 
 - Loop devices (/dev/loop?)
  
 - TTY devices (console, serial ports, terminals and pseudo-terminals)
   Thanks to C. Scott Ananian <cananian@alumni.princeton.edu>
 
 - SCSI tapes (/dev/scsi and /dev/tapes)
 
 - SCSI CD-ROMs (/dev/scsi and /dev/cdroms)
 
 - SCSI generic devices (/dev/scsi)
 
 - RAMDISCS (/dev/ram?)
 
 - Meta Devices (/dev/md*)
 
 - Floppy discs (/dev/floppy)
 
 - Parallel port printers (/dev/printers)
 
 - Sound devices (/dev/sound)
   Thanks to Eric Dumas <dumas@linux.eu.org> and
   C. Scott Ananian <cananian@alumni.princeton.edu>
 
 - Joysticks (/dev/joysticks)
 
 - Sparc keyboard (/dev/kbd)
 
 - DSP56001 digital signal processor (/dev/dsp56k)
 
 - Apple Desktop Bus (/dev/adb)
 
 - Coda network file system (/dev/cfs*)
 
 - Virtual console capture devices (/dev/vcc)
   Thanks to Dennis Hou <smilax@mindmeld.yi.org>
 
 - Frame buffer devices (/dev/fb)
 
 - Video capture devices (/dev/v4l)
 
 
 -----------------------------------------------------------------------------
 
 
 Allocation of Device Numbers
 
 Devfs allows you to write a driver which doesn't need to allocate a
 device number (major&minor numbers) for the internal operation of the
 kernel. However, there are a number of userspace programmes that use
 the device number as a unique handle for a device. An example is the
 find programme, which uses device numbers to determine whether
 an inode is on a different filesystem than another inode. The device
 number used is the one for the block device which a filesystem is
 using. To preserve compatibility with userspace programmes, block
 devices using devfs need to have unique device numbers allocated to
 them. Furthermore, POSIX specifies device numbers, so some kind of
 device number needs to be presented to userspace.
 
 The simplest option (especially when porting drivers to devfs) is to
 keep using the old major and minor numbers. Devfs will take whatever
 values are given for major&minor and pass them onto userspace.
 
 Alternatively, you can have devfs choose unique device numbers for
 you. When you register a character or block device using
 devfs_register you can provide the optional
 DEVFS_FL_AUTO_DEVNUM flag, which will then automatically allocate a
 unique device number (the allocation is separated for the character
 and block devices).
 
 This device number is a 16 bit number, so this leaves plenty of space
 for large numbers of discs and partitions. This scheme can also be
 used for character devices, in particular the tty devices, which are
 currently limited to 256 pseudo-ttys (this limits the total number of
 simultaneous xterms and remote logins).  Note that the device number
 is limited to the range 36864-61439 (majors 144-239), in order to
 avoid any possible conflicts with existing official allocations.
 
 Please note that using dynamically allocated block device numbers may
 break the NFS daemons (both user and kernel mode), which expect dev_t
 for a given device to be constant over the lifetime of remote mounts.
 
 A final note on this scheme: since it doesn't increase the size of
 device numbers, there are no compatibility issues with userspace.
 
 -----------------------------------------------------------------------------
 
 
 Questions and Answers
 
 
 Making things work
 Alternatives to devfs
 
 
 
 Making things work
 
 Here are some common questions and answers.
 
 
 
 Devfsd is not managing all my permissions
 
 Make sure you are capturing the appropriate events. For example,
 device entries created by the kernel generate REGISTER events,
 but those created by devfsd generate CREATE events.
 
 
 Devfsd is not capturing all REGISTER events
 
 See the previous entry: you may need to capture CREATE events.
 
 
 X will not start
 
 Make sure you followed the steps 
 outlined above.
 
 
 Why don't my network devices appear in devfs?
 
 This is not a bug. Network devices have their own, completely separate
 namespace. They are accessed via socket(2) and
 setsockopt(2) calls, and thus require no device nodes. I have
 raised the possibilty of moving network devices into the device
 namespace, but have had no response.
 
 
 
 
 
 Alternatives to devfs
 
 I've attempted to collate all the anti-devfs proposals and explain
 their limitations. Under construction.
 
 
 Why not just pass device create/remove events to a daemon?
 
 Here the suggestion is to develop an API in the kernel so that devices
 can register create and remove events, and a daemon listens for those
 events. The daemon would then populate/depopulate /dev (which
 resides on disc).
 
 This has several limitations:
 
 
 it only works for modules loaded and unloaded (or devices inserted
 and removed) after the kernel has finished booting. Without a database
 of events, there is no way the daemon could fully populate
 /dev
 
 
 if you add a database to this scheme, the question is then how to
 present that database to user-space. If you make it a list of strings
 with embedded event codes which are passed through a pipe to the
 daemon, then this is only of use to the daemon. I would argue that the
 natural way to present this data is via a filesystem (since many of
 the events will be of a hierarchical nature), such as devfs.
 Presenting the data as a filesystem makes it easy for the user to see
 what is available and also makes it easy to write scripts to scan the
 "database"
 
 
 the tight binding between device nodes and drivers is no longer
 possible (requiring the otherwise perfectly avoidable
 table lookups)
 
 
 you cannot catch inode lookup events on /dev which means
 that module autoloading requires device nodes to be created. This is a
 problem, particularly for drivers where only a few inodes are created
 from a potentially large set
 
 
 this technique can't be used when the root FS is mounted
 read-only
 
 
 
 
 Just implement a better scsidev
 
 This suggestion involves taking the scsidev programme and
 extending it to scan for all devices, not just SCSI devices. The
 scsidev programme works by scanning /proc/scsi
 
 Problems:
 
 
 the kernel does not currently provide a list of all devices
 available. Not all drivers register entries in /proc or
 generate kernel messages
 
 
 there is no uniform mechanism to register devices other than the
 devfs API
 
 
 implementing such an API is then the same as the
 proposal above
 
 
 
 
 Put /dev on a ramdisc
 
 This suggestion involves creating a ramdisc and populating it with
 device nodes and then mounting it over /dev.
 
 Problems:
 
 
 
 this doesn't help when mounting the root filesystem, since you
 still need a device node to do that
 
 
 if you want to use this technique for the root device node as
 well, you need to use initrd. This complicates the booting sequence
 and makes it significantly harder to administer and configure. The
 initrd is essentially opaque, robbing the system administrator of easy
 configuration
 
 
 insufficient information is available to correctly populate the
 ramdisc. So we come back to the
 proposal above to "solve" this
 
 
 a ramdisc-based solution would take more kernel memory, since the
 backing store would be (at best) normal VFS inodes and dentries, which
 take 284 bytes and 112 bytes, respectively, for each entry. Compare
 that to 72 bytes for devfs
 
 
 
 
 Do nothing: there's no problem
 
 Sometimes people can be heard to claim that the existing scheme is
 fine. This is what they're ignoring:
 
 
 device number size (8 bits each for major and minor) is a real
 limitation, and must be fixed somehow. Systems with large numbers of
 SCSI devices, for example, will continue to consume the remaining
 unallocated major numbers. USB will also need to push beyond the 8 bit
 minor limitation
 
 
 simplying increasing the device number size is insufficient. Apart
 from causing a lot of pain, it doesn't solve the management issues
 of a /dev with thousands or more device nodes
 
 
 ignoring the problem of a huge /dev will not make it go
 away, and dismisses the legitimacy of a large number of people who
 want a dynamic /dev
 
 
 the standard response then becomes: "write a device management
 daemon", which brings us back to the
 proposal above
 
 
 
 -----------------------------------------------------------------------------
 
 
 Other resources
 
 
 
 Douglas Gilbert has written a useful document at
 
 http://www.torque.net/sg/devfs_scsi.html which
 explores the SCSI subsystem and how it interacts with devfs
 
 
 Douglas Gilbert has written another useful document at
 
 http://www.torque.net/scsi/scsihosts.html which
 discusses the scsihosts= boot option
 
 
 Douglas Gilbert has written yet another useful document at
 
 http://www.torque.net/scsi/linux_scsi_24/ which
 discusses the Linux SCSI subsystem in 2.4.
 
 
 Johannes Erdfelt has started a discussion paper on Linux and
 hot-swap devices, describing what the requirements are for a scalable
 solution and how and why he's used devfs+devfsd. Note that this is an
 early draft only, available in plain text form at:
 
 http://johannes.erdfelt.com/hotswap.txt.
 Johannes has promised a HTML version will follow.