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-Devfs (Device File System) FAQ
-Linux Devfs (Device File System) FAQ
-Richard Gooch
-20-AUG-2002
-Document languages:
-----------------------------------------------------------------------------
-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. A mirror site is available at:
-http://www.ras.ucalgary.ca/~rgooch/linux/docs/devfs.html
-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/
-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
-To unsubscribe, send the message body:
-unsubscribe devfs
-instead. 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 across reboots
-Dealing with drivers without devfs support
-All the way with Devfs
-Other Issues
-Kernel Naming Scheme
-Devfsd Naming Scheme
-Old Compatibility Names
-SCSI Host Probing Issues
-Device drivers currently ported
-Allocation of Device Numbers
-Questions and Answers
-Making things work
-Alternatives to devfs
-What I don't like about devfs
-How to report bugs
-Strange kernel messages
-Compilation problems with devfsd
-Other resources
-Translations of this document
-----------------------------------------------------------------------------
-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. Devfs has been in the kernel since 2.3.46.
-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 (naturally) 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 that 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. Devfs only shows those devices available on my
-system. This means that listing /dev is a handy way of checking what
-devices are 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 issues 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 and AIX each have 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 should ensure that /usr/src/linux contains a
-recent kernel source tree. Then 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:
-vc/1
-vc/2
-vc/3
-vc/4
-vc/5
-vc/6
-vc/7
-vc/8
-This will not weaken security. If you have a version of util-linux
-earlier than 2.10.h, please upgrade to 2.10.h or later. If you
-absolutely cannot upgrade, then also 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. Note that not all distributions have this file (e.g. Debian),
-so if it's not present, don't worry about it.
--- /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]* vc/[0-9][0-9]* :[0-9]\.[0-9] :[0-9] 
- # device classes -- these are shell-style globs 
- <floppy>=/dev/fd[0-1]* 
-If the patch does not apply, then change the line:
-<console>=tty[0-9][0-9]* :[0-9]\.[0-9] :[0-9]
-with:
-<console>=tty[0-9][0-9]* vc/[0-9][0-9]* :[0-9]\.[0-9] :[0-9]
-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 devpts 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_EXPERIMENTAL=y, CONFIG_DEVFS_FS=y and CONFIG_DEVFS_MOUNT=y by
-using favourite configuration tool (i.e. make config or
-make xconfig) and then make clean and then recompile your kernel and 
-modules. At boot, devfs will be mounted onto /dev.
-If you encounter problems booting (for example if you forgot a
-configuration step), you can pass devfs=nomount at the kernel
-boot command line. This will prevent the kernel from mounting devfs at
-boot time onto /dev.
-In general, a kernel built with CONFIG_DEVFS_FS=y but without mounting
-devfs onto /dev is completely safe, and requires no
-configuration changes. One exception to take note of is when
-LABEL= directives are used in /etc/fstab. In this
-case you will be unable to boot properly. This is because the
-mount(8) programme uses /proc/partitions as part of
-the volume label search process, and the device names it finds are not
-available, because setting CONFIG_DEVFS_FS=y changes the names in
-/proc/partitions, irrespective of whether devfs is mounted.
-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 saves
-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
-make sure you have a correct /dev/console entry in your
-root file-system (where your disc-based /dev lives)
-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        ^pt[sy]         IGNORE
-CREATE          ^pt[sy]         IGNORE
-CHANGE          ^pt[sy]         IGNORE
-DELETE          ^pt[sy]         IGNORE
-REGISTER        .*              COPY    /dev-state/$devname $devpath
-CREATE          .*              COPY    $devpath /dev-state/$devname
-CHANGE          .*              COPY    $devpath /dev-state/$devname
-DELETE          .*              CFUNCTION GLOBAL unlink /dev-state/$devname
-RESTORE         /dev-state
-Note that the sample devfsd.conf file contains these lines,
-as well as other sample configurations you may find useful. See the
-devfsd distribution
-reboot.
-Permissions database stored in normal directory
-If you are using an older kernel which doesn't support VFS binding,
-then you won't be able to have the permissions database in a
-mounted-over /dev. However, you can still use a regular
-directory to store the database. The sample /etc/devfsd.conf
-file above may still be used. You will need to create the
-/dev-state directory prior to installing devfsd. If you have
-old permissions in /dev, then just copy (or move) the device
-nodes over to the new directory.
-Which method is better?
-The best method is to have the permissions database stored in the
-mounted-over /dev. This is because you will not need to copy
-device nodes over to /dev-state, and because it allows you to
-switch between devfs and non-devfs kernels, without requiring you to
-copy permissions between /dev-state (for devfs) and
-/dev (for non-devfs).
-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 losing most functionality
-(i.e. losing 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.
-IF YOU CONFIGURE TO MOUNT DEVFS AT BOOT, 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. The way module autoloading work with devfs is:
-a process attempts to lookup a device node (e.g. /dev/fred)
-if that device node does not exist, the full pathname is passed to
-devfsd as a string
-devfsd will pass the string to the modprobe programme (provided the
-configuration line shown above is present), and specifies that
-/etc/modules.devfs is the configuration file
-/etc/modules.devfs includes /etc/modules.conf to
-access local configurations
-modprobe will search it's configuration files, looking for an alias
-that translates the pathname into a module name
-the translated pathname is then used to load the module.
-If you wanted a lookup of /dev/fred to load the
-mymod module, you would require the following configuration
-line in /etc/modules.conf:
-alias    /dev/fred    mymod
-The /etc/modules.devfs configuration file provides many such
-aliases for standard device names. If you look closely at this file,
-you will note that some modules require multiple alias configuration
-lines. This is required to support module autoloading for old and new
-device names.
-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=<device>" 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 <device> 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.
-Below is a list of the naming schemes for the most common drivers. A
-list of reserved device names is
-available for reference. Please send email to
-rgooch@atnf.csiro.au to obtain an allocation. Please be
-patient (the maintainer is busy). An alternative name may be allocated
-instead of the requested name, at the discretion of the maintainer.
-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                  /dev/tty                   Current virtual console
-  /dev/vc/{1,2,...}          /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). These are referred to as "new compatibility entries".
-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
-The generic (aka. raw) interface for all SCSI devices 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.
-Old Compatibility Names
-The old compatibility names are the legacy device names, such as
-/dev/hda, /dev/sda, /dev/rtc and so on.
-Devfsd can be configured to create compatibility symlinks so that you
-may continue to use the old names in your configuration files and so
-that old applications will continue to function correctly.
-In order to configure devfsd to create these legacy names, the
-following lines should be placed in your /etc/devfsd.conf:
-REGISTER        .*              MKOLDCOMPAT
-UNREGISTER      .*              RMOLDCOMPAT
-This will cause devfsd to create (and destroy) symbolic links which
-point to the kernel-supplied names.
-----------------------------------------------------------------------------
-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.
-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
-What I don't like about devfs
-How to report bugs
-Strange kernel messages
-Compilation problems with devfsd
-Making things work
-Here are some common questions and answers.
-Devfsd doesn't start
-Make sure you have compiled and installed devfsd
-Make sure devfsd is being started from your boot
-scripts
-Make sure you have configured your kernel to enable devfs (see
-below)
-Make sure devfs is mounted (see below)
-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.
-How can I test if I have devfs compiled into my kernel?
-All filesystems built-in or currently loaded are listed in
-/proc/filesystems. If you see a devfs entry, then
-you know that devfs was compiled into your kernel. If you have
-correctly configured and rebuilt your kernel, then devfs will be
-built-in. If you think you've configured it in, but
-/proc/filesystems doesn't show it, you've made a mistake.
-Common mistakes include:
-Using a 2.2.x kernel without applying the devfs patch (if you
-don't know how to patch your kernel, use 2.4.x instead, don't bother
-asking me how to patch)
-Forgetting to set CONFIG_EXPERIMENTAL=y
-Forgetting to set CONFIG_DEVFS_FS=y
-Forgetting to set CONFIG_DEVFS_MOUNT=y (if you want devfs
-to be automatically mounted at boot)
-Editing your .config manually, instead of using make
-config or make xconfig
-Forgetting to run make dep; make clean after changing the
-configuration and before compiling
-Forgetting to compile your kernel and modules
-Forgetting to install your kernel
-Forgetting to install your modules
-Please check twice that you've done all these steps before sending in
-a bug report.
-How can I test if devfs is mounted on /dev?
-The device filesystem will always create an entry called
-".devfsd", which is used to communicate with the daemon. Even
-if the daemon is not running, this entry will exist. Testing for the
-existence of this entry is the approved method of determining if devfs
-is mounted or not. Note that the type of entry (i.e. regular file,
-character device, named pipe, etc.) may change without notice. Only
-the existence of the entry should be relied upon.
-When I start devfsd, I see the error:
-Error opening file: ".devfsd"   No such file or directory?
-This means that devfs is not mounted. Make sure you have devfs mounted.
-How do I mount devfs?
-First make sure you have devfs compiled into your kernel (see
-above). Then you will either need to:
-set CONFIG_DEVFS_MOUNT=y in your kernel config
-pass devfs=mount to your boot loader
-mount devfs manually in your boot scripts with:
-mount -t none devfs /dev
-Mount by volume LABEL=<label> doesn't work with
-devfs
-Most probably you are not mounting devfs onto /dev. What
-happens is that if your kernel config has CONFIG_DEVFS_FS=y
-then the contents of /proc/partitions will have the devfs
-names (such as scsi/host0/bus0/target0/lun0/part1). The
-contents of /proc/partitions are used by mount(8) when
-mounting by volume label. If devfs is not mounted on /dev,
-then mount(8) will fail to find devices. The solution is to
-make sure that devfs is mounted on /dev. See above for how to
-do that.
-I have extra or incorrect entries in /dev
-You may have stale entries in your dev-state area. Check for a
-RESTORE configuration line in your devfsd configuration
-(typically /etc/devfsd.conf). If you have this line, check
-the contents of the specified directory for stale entries. Remove
-any entries which are incorrect, then reboot.
-I get "Unable to open initial console" messages at boot
-This usually happens when you don't have devfs automounted onto
-/dev at boot time, and there is no valid
-/dev/console entry on your root file-system. Create a valid
-/dev/console device node.
-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
-simply 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
-What I don't like about devfs
-Here are some common complaints about devfs, and some suggestions and
-solutions that may make it more palatable for you. I can't please
-everybody, but I do try :-)
-I hate the naming scheme
-First, remember that no naming scheme will please everybody. You hate
-the scheme, others love it. Who's to say who's right and who's wrong?
-Ultimately, the person who writes the code gets to choose, and what
-exists now is a combination of the choices made by the
-devfs author and the
-kernel maintainer (Linus).
-However, not all is lost. If you want to create your own naming
-scheme, it is a simple matter to write a standalone script, hack
-devfsd, or write a script called by devfsd. You can create whatever
-naming scheme you like.
-Further, if you want to remove all traces of the devfs naming scheme
-from /dev, you can mount devfs elsewhere (say
-/devfs) and populate /dev with links into
-/devfs. This population can be automated using devfsd if you
-wish.
-You can even use the VFS binding facility to make the links, rather
-than using symbolic links. This way, you don't even have to see the
-"destination" of these symbolic links.
-Devfs puts policy into the kernel
-There's already policy in the kernel. Device numbers are in fact
-policy (why should the kernel dictate what device numbers I use?).
-Face it, some policy has to be in the kernel. The real difference
-between device names as policy and device numbers as policy is that
-no one will use device numbers directly, because device
-numbers are devoid of meaning to humans and are ugly. At least with
-the devfs device names, (even though you can add your own naming
-scheme) some people will use the devfs-supplied names directly. This
-offends some people :-)
-Devfs is bloatware
-This is not even remotely true. As shown above,
-both code and data size are quite modest.
-How to report bugs
-If you have (or think you have) a bug with devfs, please follow the
-steps below:
-make sure you have enabled debugging output when configuring your
-kernel. You will need to set (at least) the following config options:
-CONFIG_DEVFS_DEBUG=y
-CONFIG_DEBUG_KERNEL=y
-CONFIG_DEBUG_SLAB=y
-please make sure you have the latest devfs patches applied. The
-latest kernel version might not have the latest devfs patches applied
-yet (Linus is very busy)
-save a copy of your complete kernel logs (preferably by
-using the dmesg programme) for later inclusion in your bug
-report. You may need to use the -s switch to increase the
-internal buffer size so you can capture all the boot messages.
-Don't edit or trim the dmesg output
-try booting with devfs=dall passed to the kernel boot
-command line (read the documentation on your bootloader on how to do
-this), and save the result to a file. This may be quite verbose, and
-it may overflow the messages buffer, but try to get as much of it as
-you can
-send a copy of your devfsd configuration file(s)
-send the bug report to me first.
-Don't expect that I will see it if you post it to the linux-kernel
-mailing list. Include all the information listed above, plus
-anything else that you think might be relevant. Put the string
-devfs somewhere in the subject line, so my mail filters mark
-it as urgent
-Here is a general guide on how to ask questions in a way that greatly
-improves your chances of getting a reply:
-http://www.tuxedo.org/~esr/faqs/smart-questions.html. If you have
-a bug to report, you should also read
-http://www.chiark.greenend.org.uk/~sgtatham/bugs.html.
-Strange kernel messages
-You may see devfs-related messages in your kernel logs. Below are some
-messages and what they mean (and what you should do about them, if
-anything).
-devfs_register(fred): could not append to parent, err: -17
-You need to check what the error code means, but usually 17 means
-EEXIST. This means that a driver attempted to create an entry
-fred in a directory, but there already was an entry with that
-name. This is often caused by flawed boot scripts which untar a bunch
-of inodes into /dev, as a way to restore permissions. This
-message is harmless, as the device nodes will still
-provide access to the driver (unless you use the devfs=only
-boot option, which is only for dedicated souls:-). If you want to get
-rid of these annoying messages, upgrade to devfsd-v1.3.20 and use the
-recommended RESTORE directive to restore permissions.
-devfs_mk_dir(bill): using old entry in dir: c1808724 ""
-This is similar to the message above, except that a driver attempted
-to create a directory named bill, and the parent directory
-has an entry with the same name. In this case, to ensure that drivers
-continue to work properly, the old entry is re-used and given to the
-driver. In 2.5 kernels, the driver is given a NULL entry, and thus,
-under rare circumstances, may not create the require device nodes.
-The solution is the same as above.
-Compilation problems with devfsd
-Usually, you can compile devfsd just by typing in
-make in the source directory, followed by a make
-install (as root). Sometimes, you may have problems, particularly
-on broken configurations.
-error messages relating to DEVFSD_NOTIFY_DELETE
-This happened because you have an ancient set of kernel headers
-installed in /usr/include/linux or /usr/src/linux.
-Install kernel 2.4.10 or later. You may need to pass the
-KERNEL_DIR variable to make (if you did not install
-the new kernel sources as /usr/src/linux), or you may copy
-the devfs_fs.h file in the kernel source tree into
-/usr/include/linux.
-----------------------------------------------------------------------------
-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/SCSI-2.4-HOWTO/ 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.
-I presented an invited 
-paper
-at the
-2nd Annual Storage Management Workshop held in Miamia, Florida,
-U.S.A. in October 2000.
-----------------------------------------------------------------------------
-Translations of this document
-This document has been translated into other languages.
-The document master (in English) by rgooch@atnf.csiro.au is
-available at
-http://www.atnf.csiro.au/~rgooch/linux/docs/devfs.html
-A Korean translation by viatoris@nownuri.net is available at
-http://your.destiny.pe.kr/devfs/devfs.html
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