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linux-2.6.15.6

/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 <[e-mail]>

- Loop devices (/dev/loop?)
 
- TTY devices (console, serial ports, terminals and pseudo-terminals)
  Thanks to C. Scott Ananian <[e-mail]>

- 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 <[e-mail]> and
  C. Scott Ananian <[e-mail]>

- 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 <[e-mail]>

- 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)