Devfs (Device File System) FAQ

# Linux Devfs (Device File System) FAQ

## Richard Gooch

#### 20-JAN-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 accross 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 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 dep; 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
- 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
      CHANGE		^pt[sy]/.*	IGNORE
      REGISTER	.*		COPY	/dev-state/$devname $devpath
      CHANGE		.*		COPY	$devpath /dev-state/$devname
      CREATE		.*		COPY	$devpath /dev-state/$devname

- 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.

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

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`.

## 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.

## 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 /dev/scsi/host0/bus#/target#/lun#
    - first parallel port ZIP    - will be /dev/scsi/host1/bus#/target#/lun#
    - second aha1542 controller  - will be /dev/scsi/host2/bus#/target#/lun#
    - first NCR53C7xx controller - will be /dev/scsi/host4/bus#/target#/lun#
    - any extra controller       - will be /dev/scsi/host5/bus#/target#/lun#,
                                           /dev/scsi/host6/bus#/target#/lun#, etc
    - if any of above controllers will not be found - the reserved names will
      not be used by any other device.
    - /dev/scsi/host3/bus#/target#/lun# 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
- 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 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.

## 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 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:

- 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
- 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
- if you get an Oops, run `ksymoops` to decode it so that the names of the offending functions are provided. A non-decoded Oops is pretty useless
- 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/scsihosts.html which discusses the `scsihosts=` boot option
- Douglas Gilbert has written yet 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://home.nownuri.net/~viatoris/devfs/devfs.html
  A newer version is under construcation at http://viatoris.new21.org/devfs/devfs.html

Most flags courtesy of ITA's Flags of All Countries used with permission.