Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.. _admin_devices:
+
+Linux allocated devices (4.x+ version)
+======================================
+
+This list is the Linux Device List, the official registry of allocated
+device numbers and ``/dev`` directory nodes for the Linux operating
+system.
+
+The version of this document at lanana.org is no longer maintained.  This
+version in the mainline Linux kernel is the master document.  Updates
+shall be sent as patches to the kernel maintainers (see the
+:ref:`Documentation/process/submitting-patches.rst <submittingpatches>` document).
+Specifically explore the sections titled "CHAR and MISC DRIVERS", and
+"BLOCK LAYER" in the MAINTAINERS file to find the right maintainers
+to involve for character and block devices.
+
+This document is included by reference into the Filesystem Hierarchy
+Standard (FHS).	 The FHS is available from https://www.pathname.com/fhs/.
+
+Allocations marked (68k/Amiga) apply to Linux/68k on the Amiga
+platform only.	Allocations marked (68k/Atari) apply to Linux/68k on
+the Atari platform only.
+
+This document is in the public domain.	The authors requests, however,
+that semantically altered versions are not distributed without
+permission of the authors, assuming the authors can be contacted without
+an unreasonable effort.
+
+
+.. attention::
+
+  DEVICE DRIVERS AUTHORS PLEASE READ THIS
+
+  Linux now has extensive support for dynamic allocation of device numbering
+  and can use ``sysfs`` and ``udev`` (``systemd``) to handle the naming needs.
+  There are still some exceptions in the serial and boot device area. Before
+  asking   for a device number make sure you actually need one.
+
+  To have a major number allocated, or a minor number in situations
+  where that applies (e.g. busmice), please submit a patch and send to
+  the authors as indicated above.
+
+  Keep the description of the device *in the same format
+  as this list*. The reason for this is that it is the only way we have
+  found to ensure we have all the requisite information to publish your
+  device and avoid conflicts.
+
+  Finally, sometimes we have to play "namespace police."  Please don't be
+  offended.  We often get submissions for ``/dev`` names that would be bound
+  to cause conflicts down the road.  We are trying to avoid getting in a
+  situation where we would have to suffer an incompatible forward
+  change.  Therefore, please consult with us **before** you make your
+  device names and numbers in any way public, at least to the point
+  where it would be at all difficult to get them changed.
+
+  Your cooperation is appreciated.
+
+.. include:: devices.txt
+   :literal:
+
+Additional ``/dev/`` directory entries
+--------------------------------------
+
+This section details additional entries that should or may exist in
+the /dev directory.  It is preferred that symbolic links use the same
+form (absolute or relative) as is indicated here.  Links are
+classified as "hard" or "symbolic" depending on the preferred type of
+link; if possible, the indicated type of link should be used.
+
+Compulsory links
+++++++++++++++++
+
+These links should exist on all systems:
+
+=============== =============== =============== ===============================
+/dev/fd		/proc/self/fd	symbolic	File descriptors
+/dev/stdin	fd/0		symbolic	stdin file descriptor
+/dev/stdout	fd/1		symbolic	stdout file descriptor
+/dev/stderr	fd/2		symbolic	stderr file descriptor
+/dev/nfsd	socksys		symbolic	Required by iBCS-2
+/dev/X0R	null		symbolic	Required by iBCS-2
+=============== =============== =============== ===============================
+
+Note: ``/dev/X0R`` is <letter X>-<digit 0>-<letter R>.
+
+Recommended links
++++++++++++++++++
+
+It is recommended that these links exist on all systems:
+
+
+=============== =============== =============== ===============================
+/dev/core	/proc/kcore	symbolic	Backward compatibility
+/dev/ramdisk	ram0		symbolic	Backward compatibility
+/dev/ftape	qft0		symbolic	Backward compatibility
+/dev/bttv0	video0		symbolic	Backward compatibility
+/dev/radio	radio0		symbolic	Backward compatibility
+/dev/i2o*	/dev/i2o/*	symbolic	Backward compatibility
+/dev/scd?	sr?		hard		Alternate SCSI CD-ROM name
+=============== =============== =============== ===============================
+
+Locally defined links
++++++++++++++++++++++
+
+The following links may be established locally to conform to the
+configuration of the system.  This is merely a tabulation of existing
+practice, and does not constitute a recommendation.  However, if they
+exist, they should have the following uses.
+
+=============== =============== =============== ===============================
+/dev/mouse	mouse port	symbolic	Current mouse device
+/dev/tape	tape device	symbolic	Current tape device
+/dev/cdrom	CD-ROM device	symbolic	Current CD-ROM device
+/dev/cdwriter	CD-writer	symbolic	Current CD-writer device
+/dev/scanner	scanner		symbolic	Current scanner device
+/dev/modem	modem port	symbolic	Current dialout device
+/dev/root	root device	symbolic	Current root filesystem
+/dev/swap	swap device	symbolic	Current swap device
+=============== =============== =============== ===============================
+
+``/dev/modem`` should not be used for a modem which supports dialin as
+well as dialout, as it tends to cause lock file problems.  If it
+exists, ``/dev/modem`` should point to the appropriate primary TTY device
+(the use of the alternate callout devices is deprecated).
+
+For SCSI devices, ``/dev/tape`` and ``/dev/cdrom`` should point to the
+*cooked* devices (``/dev/st*`` and ``/dev/sr*``, respectively), whereas
+``/dev/cdwriter`` and /dev/scanner should point to the appropriate generic
+SCSI devices (/dev/sg*).
+
+``/dev/mouse`` may point to a primary serial TTY device, a hardware mouse
+device, or a socket for a mouse driver program (e.g. ``/dev/gpmdata``).
+
+Sockets and pipes
++++++++++++++++++
+
+Non-transient sockets and named pipes may exist in /dev.  Common entries are:
+
+=============== =============== ===============================================
+/dev/printer	socket		lpd local socket
+/dev/log	socket		syslog local socket
+/dev/gpmdata	socket		gpm mouse multiplexer
+=============== =============== ===============================================
+
+Mount points
+++++++++++++
+
+The following names are reserved for mounting special filesystems
+under /dev.  These special filesystems provide kernel interfaces that
+cannot be provided with standard device nodes.
+
+=============== =============== ===============================================
+/dev/pts	devpts		PTY slave filesystem
+/dev/shm	tmpfs		POSIX shared memory maintenance access
+=============== =============== ===============================================
+
+Terminal devices
+----------------
+
+Terminal, or TTY devices are a special class of character devices.  A
+terminal device is any device that could act as a controlling terminal
+for a session; this includes virtual consoles, serial ports, and
+pseudoterminals (PTYs).
+
+All terminal devices share a common set of capabilities known as line
+disciplines; these include the common terminal line discipline as well
+as SLIP and PPP modes.
+
+All terminal devices are named similarly; this section explains the
+naming and use of the various types of TTYs.  Note that the naming
+conventions include several historical warts; some of these are
+Linux-specific, some were inherited from other systems, and some
+reflect Linux outgrowing a borrowed convention.
+
+A hash mark (``#``) in a device name is used here to indicate a decimal
+number without leading zeroes.
+
+Virtual consoles and the console device
++++++++++++++++++++++++++++++++++++++++
+
+Virtual consoles are full-screen terminal displays on the system video
+monitor.  Virtual consoles are named ``/dev/tty#``, with numbering
+starting at ``/dev/tty1``; ``/dev/tty0`` is the current virtual console.
+``/dev/tty0`` is the device that should be used to access the system video
+card on those architectures for which the frame buffer devices
+(``/dev/fb*``) are not applicable. Do not use ``/dev/console``
+for this purpose.
+
+The console device, ``/dev/console``, is the device to which system
+messages should be sent, and on which logins should be permitted in
+single-user mode.  Starting with Linux 2.1.71, ``/dev/console`` is managed
+by the kernel; for previous versions it should be a symbolic link to
+either ``/dev/tty0``, a specific virtual console such as ``/dev/tty1``, or to
+a serial port primary (``tty*``, not ``cu*``) device, depending on the
+configuration of the system.
+
+Serial ports
+++++++++++++
+
+Serial ports are RS-232 serial ports and any device which simulates
+one, either in hardware (such as internal modems) or in software (such
+as the ISDN driver.)  Under Linux, each serial ports has two device
+names, the primary or callin device and the alternate or callout one.
+Each kind of device is indicated by a different letter.	 For any
+letter X, the names of the devices are ``/dev/ttyX#`` and ``/dev/cux#``,
+respectively; for historical reasons, ``/dev/ttyS#`` and ``/dev/ttyC#``
+correspond to ``/dev/cua#`` and ``/dev/cub#``. In the future, it should be
+expected that multiple letters will be used; all letters will be upper
+case for the "tty" device (e.g. ``/dev/ttyDP#``) and lower case for the
+"cu" device (e.g. ``/dev/cudp#``).
+
+The names ``/dev/ttyQ#`` and ``/dev/cuq#`` are reserved for local use.
+
+The alternate devices provide for kernel-based exclusion and somewhat
+different defaults than the primary devices.  Their main purpose is to
+allow the use of serial ports with programs with no inherent or broken
+support for serial ports.  Their use is deprecated, and they may be
+removed from a future version of Linux.
+
+Arbitration of serial ports is provided by the use of lock files with
+the names ``/var/lock/LCK..ttyX#``. The contents of the lock file should
+be the PID of the locking process as an ASCII number.
+
+It is common practice to install links such as /dev/modem
+which point to serial ports.  In order to ensure proper locking in the
+presence of these links, it is recommended that software chase
+symlinks and lock all possible names; additionally, it is recommended
+that a lock file be installed with the corresponding alternate
+device.	 In order to avoid deadlocks, it is recommended that the locks
+are acquired in the following order, and released in the reverse:
+
+	1. The symbolic link name, if any (``/var/lock/LCK..modem``)
+	2. The "tty" name (``/var/lock/LCK..ttyS2``)
+	3. The alternate device name (``/var/lock/LCK..cua2``)
+
+In the case of nested symbolic links, the lock files should be
+installed in the order the symlinks are resolved.
+
+Under no circumstances should an application hold a lock while waiting
+for another to be released.  In addition, applications which attempt
+to create lock files for the corresponding alternate device names
+should take into account the possibility of being used on a non-serial
+port TTY, for which no alternate device would exist.
+
+Pseudoterminals (PTYs)
+++++++++++++++++++++++
+
+Pseudoterminals, or PTYs, are used to create login sessions or provide
+other capabilities requiring a TTY line discipline (including SLIP or
+PPP capability) to arbitrary data-generation processes.	 Each PTY has
+a master side, named ``/dev/pty[p-za-e][0-9a-f]``, and a slave side, named
+``/dev/tty[p-za-e][0-9a-f]``.  The kernel arbitrates the use of PTYs by
+allowing each master side to be opened only once.
+
+Once the master side has been opened, the corresponding slave device
+can be used in the same manner as any TTY device.  The master and
+slave devices are connected by the kernel, generating the equivalent
+of a bidirectional pipe with TTY capabilities.
+
+Recent versions of the Linux kernels and GNU libc contain support for
+the System V/Unix98 naming scheme for PTYs, which assigns a common
+device, ``/dev/ptmx``, to all the masters (opening it will automatically
+give you a previously unassigned PTY) and a subdirectory, ``/dev/pts``,
+for the slaves; the slaves are named with decimal integers (``/dev/pts/#``
+in our notation).  This removes the problem of exhausting the
+namespace and enables the kernel to automatically create the device
+nodes for the slaves on demand using the "devpts" filesystem.