iio: proximity: isl29501: Fix return type of isl29501_register_writegrafted

isl29501_register_write() was returning a u32 instead of an int.
Since the function returns negative error codes (such as -ERANGE or
return values from i2c_smbus_write_byte_data()), returning an unsigned
integer type prevents callers from correctly checking for negative error
conditions.

Fix this by changing the function return type from u32 to int.

This was found through manual code review.

Fixes: 1c28799257bc ("iio: light: isl29501: Add support for the ISL29501 ToF sensor.")
Signed-off-by: Salah Triki <salah.triki@gmail.com>
Reviewed-by: Joshua Crofts <joshua.crofts1@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>

12 files changed, 988 insertions, 0 deletions

diff --git a/Documentation/driver-api/tty/console.rst b/Documentation/driver-api/tty/console.rst
new file mode 100644
index 000000000..4348e36cd
--- /dev/null
+++ b/Documentation/driver-api/tty/console.rst
@@ -0,0 +1,45 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=======
+Console
+=======
+
+.. contents:: :local:
+
+Struct Console
+==============
+
+.. kernel-doc:: include/linux/console.h
+   :identifiers: console cons_flags
+
+Internals
+---------
+
+.. kernel-doc:: include/linux/console.h
+   :identifiers: nbcon_state nbcon_prio nbcon_context nbcon_write_context
+
+Struct Consw
+============
+
+.. kernel-doc:: include/linux/console.h
+   :identifiers: consw
+
+Console functions
+=================
+
+.. kernel-doc:: include/linux/console.h
+   :identifiers: console_srcu_read_flags console_srcu_write_flags
+        console_is_registered for_each_console_srcu for_each_console
+
+.. kernel-doc:: drivers/tty/vt/selection.c
+   :export:
+.. kernel-doc:: drivers/tty/vt/vt.c
+   :export:
+
+Internals
+---------
+
+.. kernel-doc:: drivers/tty/vt/selection.c
+   :internal:
+.. kernel-doc:: drivers/tty/vt/vt.c
+   :internal:
diff --git a/Documentation/driver-api/tty/index.rst b/Documentation/driver-api/tty/index.rst
new file mode 100644
index 000000000..c1ffe3d1e
--- /dev/null
+++ b/Documentation/driver-api/tty/index.rst
@@ -0,0 +1,75 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===
+TTY
+===
+
+Teletypewriter (TTY) layer takes care of all those serial devices. Including
+the virtual ones like pseudoterminal (PTY).
+
+TTY structures
+==============
+
+There are several major TTY structures. Every TTY device in a system has a
+corresponding struct tty_port. These devices are maintained by a TTY driver
+which is struct tty_driver. This structure describes the driver but also
+contains a reference to operations which could be performed on the TTYs. It is
+struct tty_operations. Then, upon open, a struct tty_struct is allocated and
+lives until the final close. During this time, several callbacks from struct
+tty_operations are invoked by the TTY layer.
+
+Every character received by the kernel (both from devices and users) is passed
+through a preselected :doc:`tty_ldisc` (in
+short ldisc; in C, struct tty_ldisc_ops). Its task is to transform characters
+as defined by a particular ldisc or by user too. The default one is n_tty,
+implementing echoes, signal handling, jobs control, special characters
+processing, and more. The transformed characters are passed further to
+user/device, depending on the source.
+
+In-detail description of the named TTY structures is in separate documents:
+
+.. toctree::
+   :maxdepth: 2
+
+   tty_driver
+   tty_port
+   tty_struct
+   tty_ldisc
+   tty_buffer
+   tty_ioctl
+   tty_internals
+   console
+
+Writing TTY Driver
+==================
+
+Before one starts writing a TTY driver, they must consider
+:doc:`Serial <../serial/driver>` and :doc:`USB Serial <../../usb/usb-serial>`
+layers first. Drivers for serial devices can often use one of these specific
+layers to implement a serial driver. Only special devices should be handled
+directly by the TTY Layer. If you are about to write such a driver, read on.
+
+A *typical* sequence a TTY driver performs is as follows:
+
+#. Allocate and register a TTY driver (module init)
+#. Create and register TTY devices as they are probed (probe function)
+#. Handle TTY operations and events like interrupts (TTY core invokes the
+   former, the device the latter)
+#. Remove devices as they are going away (remove function)
+#. Unregister and free the TTY driver (module exit)
+
+Steps regarding driver, i.e. 1., 3., and 5. are described in detail in
+:doc:`tty_driver`. For the other two (devices handling), look into
+:doc:`tty_port`.
+
+Other Documentation
+===================
+
+Miscellaneous documentation can be further found in these documents:
+
+.. toctree::
+   :maxdepth: 2
+
+   moxa-smartio
+   n_gsm
+   n_tty
diff --git a/Documentation/driver-api/tty/moxa-smartio.rst b/Documentation/driver-api/tty/moxa-smartio.rst
new file mode 100644
index 000000000..af25bc5cc
--- /dev/null
+++ b/Documentation/driver-api/tty/moxa-smartio.rst
@@ -0,0 +1,197 @@
+=============================================================
+MOXA Smartio/Industio Family Device Driver Installation Guide
+=============================================================
+
+Copyright (C) 2008, Moxa Inc.
+Copyright (C) 2021, Jiri Slaby
+
+.. Content
+
+   1. Introduction
+   2. System Requirement
+   3. Installation
+      3.1 Hardware installation
+      3.2 Device naming convention
+   4. Utilities
+   5. Setserial
+   6. Troubleshooting
+
+1. Introduction
+^^^^^^^^^^^^^^^
+
+   The Smartio/Industio/UPCI family Linux driver supports following multiport
+   boards:
+
+    - 2 ports multiport board
+	CP-102U, CP-102UL, CP-102UF
+	CP-132U-I, CP-132UL,
+	CP-132, CP-132I, CP132S, CP-132IS,
+	(CP-102, CP-102S)
+
+    - 4 ports multiport board
+	CP-104EL,
+	CP-104UL, CP-104JU,
+	CP-134U, CP-134U-I,
+	C104H/PCI, C104HS/PCI,
+	CP-114, CP-114I, CP-114S, CP-114IS, CP-114UL,
+	(C114HI, CT-114I),
+	POS-104UL,
+	CB-114,
+	CB-134I
+
+    - 8 ports multiport board
+	CP-118EL, CP-168EL,
+	CP-118U, CP-168U,
+	C168H/PCI,
+	CB-108
+
+   If a compatibility problem occurs, please contact Moxa at
+   support@moxa.com.tw.
+
+   In addition to device driver, useful utilities are also provided in this
+   version. They are:
+
+    - msdiag
+		 Diagnostic program for displaying installed Moxa
+                 Smartio/Industio boards.
+    - msmon
+		 Monitor program to observe data count and line status signals.
+    - msterm     A simple terminal program which is useful in testing serial
+	         ports.
+
+   All the drivers and utilities are published in form of source code under
+   GNU General Public License in this version. Please refer to GNU General
+   Public License announcement in each source code file for more detail.
+
+   In Moxa's Web sites, you may always find the latest driver at
+   https://www.moxa.com/.
+
+   This version of driver can be installed as a Loadable Module (Module driver)
+   or built-in into kernel (Static driver). Before you install the driver,
+   please refer to hardware installation procedure in the User's Manual.
+
+   We assume the user should be familiar with following documents:
+
+   - Serial-HOWTO
+   - Kernel-HOWTO
+
+2. System Requirement
+^^^^^^^^^^^^^^^^^^^^^
+
+   - Maximum 4 boards can be installed in combination
+
+3. Installation
+^^^^^^^^^^^^^^^
+
+3.1 Hardware installation
+=========================
+
+PCI/UPCI board
+--------------
+
+   You may need to adjust IRQ usage in BIOS to avoid IRQ conflict with other
+   ISA devices. Please refer to hardware installation procedure in User's
+   Manual in advance.
+
+PCI IRQ Sharing
+---------------
+
+   Each port within the same multiport board shares the same IRQ. Up to
+   4 Moxa Smartio/Industio PCI Family multiport boards can be installed
+   together on one system and they can share the same IRQ.
+
+
+
+3.2 Device naming convention
+============================
+
+   The device node is named "ttyMxx".
+
+Device naming when more than 2 boards installed
+-----------------------------------------------
+
+   Naming convention for each Smartio/Industio multiport board is
+   pre-defined as below.
+
+   ============ ===============
+   Board Num.	Device node
+   1st board	ttyM0  - ttyM7
+   2nd board	ttyM8  - ttyM15
+   3rd board	ttyM16 - ttyM23
+   4th board	ttyM24 - ttyM31
+   ============ ===============
+
+4. Utilities
+^^^^^^^^^^^^
+
+   There are 3 utilities contained in this driver. They are msdiag, msmon and
+   msterm. These 3 utilities are released in form of source code. They should
+   be compiled into executable file and copied into /usr/bin.
+
+msdiag - Diagnostic
+===================
+
+   This utility provides the function to display what Moxa Smartio/Industio
+   board was found by the driver in the system.
+
+msmon - Port Monitoring
+=======================
+
+   This utility gives the user a quick view about all the MOXA ports'
+   activities. One can easily learn each port's total received/transmitted
+   (Rx/Tx) character count since the time when the monitoring is started.
+
+   Rx/Tx throughputs per second are also reported in interval basis (e.g.
+   the last 5 seconds) and in average basis (since the time the monitoring
+   is started). You can reset all ports' count by <HOME> key. <+> <->
+   (plus/minus) keys to change the displaying time interval. Press <ENTER>
+   on the port, that cursor stay, to view the port's communication
+   parameters, signal status, and input/output queue.
+
+msterm - Terminal Emulation
+===========================
+
+   This utility provides data sending and receiving ability of all tty ports,
+   especially for MOXA ports. It is quite useful for testing simple
+   application, for example, sending AT command to a modem connected to the
+   port or used as a terminal for login purpose. Note that this is only a
+   dumb terminal emulation without handling full screen operation.
+
+5. Setserial
+^^^^^^^^^^^^
+
+   Supported Setserial parameters are listed as below.
+
+   ============== =============================================================
+   uart		  set UART type(16450 --> disable FIFO, 16550A --> enable FIFO)
+   close_delay	  set the amount of time (in 1/100 of a second) that DTR
+		  should be kept low while being closed.
+   closing_wait   set the amount of time (in 1/100 of a second) that the
+		  serial port should wait for data to be drained while
+		  being closed, before the receiver is disabled.
+   spd_hi	  Use 57.6kb when the application requests 38.4kb.
+   spd_vhi	  Use 115.2kb when the application requests 38.4kb.
+   spd_shi	  Use 230.4kb when the application requests 38.4kb.
+   spd_warp	  Use 460.8kb when the application requests 38.4kb.
+   spd_normal	  Use 38.4kb when the application requests 38.4kb.
+   spd_cust	  Use the custom divisor to set the speed when the
+		  application requests 38.4kb.
+   divisor	  This option sets the custom division.
+   baud_base	  This option sets the base baud rate.
+   ============== =============================================================
+
+6. Troubleshooting
+^^^^^^^^^^^^^^^^^^
+
+   The boot time error messages and solutions are stated as clearly as
+   possible. If all the possible solutions fail, please contact our technical
+   support team to get more help.
+
+
+   Error msg:
+	      More than 4 Moxa Smartio/Industio family boards found. Fifth board
+              and after are ignored.
+
+   Solution:
+   To avoid this problem, please unplug fifth and after board, because Moxa
+   driver supports up to 4 boards.
diff --git a/Documentation/driver-api/tty/n_gsm.rst b/Documentation/driver-api/tty/n_gsm.rst
new file mode 100644
index 000000000..120317ec9
--- /dev/null
+++ b/Documentation/driver-api/tty/n_gsm.rst
@@ -0,0 +1,192 @@
+==============================
+GSM 0710 tty multiplexor HOWTO
+==============================
+
+.. contents:: :local:
+
+This line discipline implements the GSM 07.10 multiplexing protocol
+detailed in the following 3GPP document:
+
+	https://www.3gpp.org/ftp/Specs/archive/07_series/07.10/0710-720.zip
+
+This document give some hints on how to use this driver with GPRS and 3G
+modems connected to a physical serial port.
+
+How to use it
+=============
+
+Config Initiator
+----------------
+
+#. Initialize the modem in 0710 mux mode (usually ``AT+CMUX=`` command) through
+   its serial port. Depending on the modem used, you can pass more or less
+   parameters to this command.
+
+#. Switch the serial line to using the n_gsm line discipline by using
+   ``TIOCSETD`` ioctl.
+
+#. Configure the mux using ``GSMIOC_GETCONF_EXT``/``GSMIOC_SETCONF_EXT`` ioctl if needed.
+
+#. Configure the mux using ``GSMIOC_GETCONF``/``GSMIOC_SETCONF`` ioctl.
+
+#. Configure DLCs using ``GSMIOC_GETCONF_DLCI``/``GSMIOC_SETCONF_DLCI`` ioctl for non-defaults.
+
+#. Obtain base gsmtty number for the used serial port.
+
+   Major parts of the initialization program
+   (a good starting point is util-linux-ng/sys-utils/ldattach.c)::
+
+      #include <stdio.h>
+      #include <stdint.h>
+      #include <linux/gsmmux.h>
+      #include <linux/tty.h>
+
+      #define DEFAULT_SPEED	B115200
+      #define SERIAL_PORT	/dev/ttyS0
+
+      int ldisc = N_GSM0710;
+      struct gsm_config c;
+      struct gsm_config_ext ce;
+      struct gsm_dlci_config dc;
+      struct termios configuration;
+      uint32_t first;
+
+      /* open the serial port connected to the modem */
+      fd = open(SERIAL_PORT, O_RDWR | O_NOCTTY | O_NDELAY);
+
+      /* configure the serial port : speed, flow control ... */
+
+      /* send the AT commands to switch the modem to CMUX mode
+         and check that it's successful (should return OK) */
+      write(fd, "AT+CMUX=0\r", 10);
+
+      /* experience showed that some modems need some time before
+         being able to answer to the first MUX packet so a delay
+         may be needed here in some case */
+      sleep(3);
+
+      /* use n_gsm line discipline */
+      ioctl(fd, TIOCSETD, &ldisc);
+
+      /* get n_gsm extended configuration */
+      ioctl(fd, GSMIOC_GETCONF_EXT, &ce);
+      /* use keep-alive once every 5s for modem connection supervision */
+      ce.keep_alive = 500;
+      /* set the new extended configuration */
+      ioctl(fd, GSMIOC_SETCONF_EXT, &ce);
+      /* get n_gsm configuration */
+      ioctl(fd, GSMIOC_GETCONF, &c);
+      /* we are initiator and need encoding 0 (basic) */
+      c.initiator = 1;
+      c.encapsulation = 0;
+      /* our modem defaults to a maximum size of 127 bytes */
+      c.mru = 127;
+      c.mtu = 127;
+      /* set the new configuration */
+      ioctl(fd, GSMIOC_SETCONF, &c);
+      /* get DLC 1 configuration */
+      dc.channel = 1;
+      ioctl(fd, GSMIOC_GETCONF_DLCI, &dc);
+      /* the first user channel gets a higher priority */
+      dc.priority = 1;
+      /* set the new DLC 1 specific configuration */
+      ioctl(fd, GSMIOC_SETCONF_DLCI, &dc);
+      /* get first gsmtty device node */
+      ioctl(fd, GSMIOC_GETFIRST, &first);
+      printf("first muxed line: /dev/gsmtty%i\n", first);
+
+      /* and wait for ever to keep the line discipline enabled */
+      daemon(0,0);
+      pause();
+
+#. Use these devices as plain serial ports.
+
+   For example, it's possible:
+
+   - to use *gnokii* to send / receive SMS on ``ttygsm1``
+   - to use *ppp* to establish a datalink on ``ttygsm2``
+
+#. First close all virtual ports before closing the physical port.
+
+   Note that after closing the physical port the modem is still in multiplexing
+   mode. This may prevent a successful re-opening of the port later. To avoid
+   this situation either reset the modem if your hardware allows that or send
+   a disconnect command frame manually before initializing the multiplexing mode
+   for the second time. The byte sequence for the disconnect command frame is::
+
+      0xf9, 0x03, 0xef, 0x03, 0xc3, 0x16, 0xf9
+
+Config Requester
+----------------
+
+#. Receive ``AT+CMUX=`` command through its serial port, initialize mux mode
+   config.
+
+#. Switch the serial line to using the *n_gsm* line discipline by using
+   ``TIOCSETD`` ioctl.
+
+#. Configure the mux using ``GSMIOC_GETCONF_EXT``/``GSMIOC_SETCONF_EXT``
+   ioctl if needed.
+
+#. Configure the mux using ``GSMIOC_GETCONF``/``GSMIOC_SETCONF`` ioctl.
+
+#. Configure DLCs using ``GSMIOC_GETCONF_DLCI``/``GSMIOC_SETCONF_DLCI`` ioctl for non-defaults.
+
+#. Obtain base gsmtty number for the used serial port::
+
+        #include <stdio.h>
+        #include <stdint.h>
+        #include <linux/gsmmux.h>
+        #include <linux/tty.h>
+        #define DEFAULT_SPEED	B115200
+        #define SERIAL_PORT	/dev/ttyS0
+
+	int ldisc = N_GSM0710;
+	struct gsm_config c;
+	struct gsm_config_ext ce;
+	struct gsm_dlci_config dc;
+	struct termios configuration;
+	uint32_t first;
+
+	/* open the serial port */
+	fd = open(SERIAL_PORT, O_RDWR | O_NOCTTY | O_NDELAY);
+
+	/* configure the serial port : speed, flow control ... */
+
+	/* get serial data and check "AT+CMUX=command" parameter ... */
+
+	/* use n_gsm line discipline */
+	ioctl(fd, TIOCSETD, &ldisc);
+
+	/* get n_gsm extended configuration */
+	ioctl(fd, GSMIOC_GETCONF_EXT, &ce);
+	/* use keep-alive once every 5s for peer connection supervision */
+	ce.keep_alive = 500;
+	/* set the new extended configuration */
+	ioctl(fd, GSMIOC_SETCONF_EXT, &ce);
+	/* get n_gsm configuration */
+	ioctl(fd, GSMIOC_GETCONF, &c);
+	/* we are requester and need encoding 0 (basic) */
+	c.initiator = 0;
+	c.encapsulation = 0;
+	/* our modem defaults to a maximum size of 127 bytes */
+	c.mru = 127;
+	c.mtu = 127;
+	/* set the new configuration */
+	ioctl(fd, GSMIOC_SETCONF, &c);
+	/* get DLC 1 configuration */
+	dc.channel = 1;
+	ioctl(fd, GSMIOC_GETCONF_DLCI, &dc);
+	/* the first user channel gets a higher priority */
+	dc.priority = 1;
+	/* set the new DLC 1 specific configuration */
+	ioctl(fd, GSMIOC_SETCONF_DLCI, &dc);
+	/* get first gsmtty device node */
+	ioctl(fd, GSMIOC_GETFIRST, &first);
+	printf("first muxed line: /dev/gsmtty%i\n", first);
+
+	/* and wait for ever to keep the line discipline enabled */
+	daemon(0,0);
+	pause();
+
+11-03-08 - Eric Bénard - <eric@eukrea.com>
diff --git a/Documentation/driver-api/tty/n_tty.rst b/Documentation/driver-api/tty/n_tty.rst
new file mode 100644
index 000000000..15b70faee
--- /dev/null
+++ b/Documentation/driver-api/tty/n_tty.rst
@@ -0,0 +1,22 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====
+N_TTY
+=====
+
+.. contents:: :local:
+
+The default (and fallback) :doc:`TTY line discipline <tty_ldisc>`. It tries to
+handle characters as per POSIX.
+
+External Functions
+==================
+
+.. kernel-doc:: drivers/tty/n_tty.c
+   :export:
+
+Internal Functions
+==================
+
+.. kernel-doc:: drivers/tty/n_tty.c
+   :internal:
diff --git a/Documentation/driver-api/tty/tty_buffer.rst b/Documentation/driver-api/tty/tty_buffer.rst
new file mode 100644
index 000000000..4b5ca1776
--- /dev/null
+++ b/Documentation/driver-api/tty/tty_buffer.rst
@@ -0,0 +1,49 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==========
+TTY Buffer
+==========
+
+.. contents:: :local:
+
+Here, we document functions for taking care of tty buffer and their flipping.
+Drivers are supposed to fill the buffer by one of those functions below and
+then flip the buffer, so that the data are passed to :doc:`line discipline
+<tty_ldisc>` for further processing.
+
+Flip Buffer Management
+======================
+
+.. kernel-doc:: drivers/tty/tty_buffer.c
+   :identifiers: tty_prepare_flip_string
+           tty_flip_buffer_push tty_ldisc_receive_buf
+
+.. kernel-doc:: include/linux/tty_flip.h
+   :identifiers: tty_insert_flip_string_fixed_flag tty_insert_flip_string_flags
+           tty_insert_flip_char
+
+----
+
+Other Functions
+===============
+
+.. kernel-doc:: drivers/tty/tty_buffer.c
+   :identifiers: tty_buffer_space_avail tty_buffer_set_limit
+
+----
+
+Buffer Locking
+==============
+
+These are used only in special circumstances. Avoid them.
+
+.. kernel-doc:: drivers/tty/tty_buffer.c
+   :identifiers: tty_buffer_lock_exclusive tty_buffer_unlock_exclusive
+
+----
+
+Internal Functions
+==================
+
+.. kernel-doc:: drivers/tty/tty_buffer.c
+   :internal:
diff --git a/Documentation/driver-api/tty/tty_driver.rst b/Documentation/driver-api/tty/tty_driver.rst
new file mode 100644
index 000000000..7138222a7
--- /dev/null
+++ b/Documentation/driver-api/tty/tty_driver.rst
@@ -0,0 +1,130 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============================
+TTY Driver and TTY Operations
+=============================
+
+.. contents:: :local:
+
+Allocation
+==========
+
+The first thing a driver needs to do is to allocate a struct tty_driver. This
+is done by tty_alloc_driver() (or __tty_alloc_driver()). Next, the newly
+allocated structure is filled with information. See `TTY Driver Reference`_ at
+the end of this document on what actually shall be filled in.
+
+The allocation routines expect a number of devices the driver can handle at
+most and flags. Flags are those starting ``TTY_DRIVER_`` listed and described
+in `TTY Driver Flags`_ below.
+
+When the driver is about to be freed, tty_driver_kref_put() is called on that.
+It will decrements the reference count and if it reaches zero, the driver is
+freed.
+
+For reference, both allocation and deallocation functions are explained here in
+detail:
+
+.. kernel-doc:: include/linux/tty_driver.h
+   :identifiers: tty_alloc_driver
+.. kernel-doc:: drivers/tty/tty_io.c
+   :identifiers: __tty_alloc_driver tty_driver_kref_put
+
+TTY Driver Flags
+----------------
+
+Here comes the documentation of flags accepted by tty_alloc_driver() (or
+__tty_alloc_driver()):
+
+.. kernel-doc:: include/linux/tty_driver.h
+   :identifiers: tty_driver_flag
+
+----
+
+Registration
+============
+
+When a struct tty_driver is allocated and filled in, it can be registered using
+tty_register_driver(). It is recommended to pass ``TTY_DRIVER_DYNAMIC_DEV`` in
+flags of tty_alloc_driver(). If it is not passed, *all* devices are also
+registered during tty_register_driver() and the following paragraph of
+registering devices can be skipped for such drivers. However, the struct
+tty_port part in `Registering Devices`_ is still relevant there.
+
+.. kernel-doc:: drivers/tty/tty_io.c
+   :identifiers: tty_register_driver tty_unregister_driver
+
+Registering Devices
+-------------------
+
+Every TTY device shall be backed by a struct tty_port. Usually, TTY drivers
+embed tty_port into device's private structures. Further details about handling
+tty_port can be found in :doc:`tty_port`. The driver is also recommended to use
+tty_port's reference counting by tty_port_get() and tty_port_put(). The final
+put is supposed to free the tty_port including the device's private struct.
+
+Unless ``TTY_DRIVER_DYNAMIC_DEV`` was passed as flags to tty_alloc_driver(),
+TTY driver is supposed to register every device discovered in the system
+(the latter is preferred). This is performed by tty_register_device(). Or by
+tty_register_device_attr() if the driver wants to expose some information
+through struct attribute_group. Both of them register ``index``'th device and
+upon return, the device can be opened. There are also preferred tty_port
+variants described in `Linking Devices to Ports`_ later. It is up to driver to
+manage free indices and choosing the right one. The TTY layer only refuses to
+register more devices than passed to tty_alloc_driver().
+
+When the device is opened, the TTY layer allocates struct tty_struct and starts
+calling operations from :c:member:`tty_driver.ops`, see `TTY Operations
+Reference`_.
+
+The registration routines are documented as follows:
+
+.. kernel-doc:: drivers/tty/tty_io.c
+   :identifiers: tty_register_device tty_register_device_attr
+        tty_unregister_device
+
+----
+
+Linking Devices to Ports
+------------------------
+As stated earlier, every TTY device shall have a struct tty_port assigned to
+it. It must be known to the TTY layer at :c:member:`tty_driver.ops.install()`
+at latest.  There are few helpers to *link* the two. Ideally, the driver uses
+tty_port_register_device() or tty_port_register_device_attr() instead of
+tty_register_device() and tty_register_device_attr() at the registration time.
+This way, the driver needs not care about linking later on.
+
+If that is not possible, the driver still can link the tty_port to a specific
+index *before* the actual registration by tty_port_link_device(). If it still
+does not fit, tty_port_install() can be used from the
+:c:member:`tty_driver.ops.install` hook as a last resort. The last one is
+dedicated mostly for in-memory devices like PTY where tty_ports are allocated
+on demand.
+
+The linking routines are documented here:
+
+.. kernel-doc::  drivers/tty/tty_port.c
+   :identifiers: tty_port_link_device tty_port_register_device
+        tty_port_register_device_attr
+
+----
+
+TTY Driver Reference
+====================
+
+All members of struct tty_driver are documented here. The required members are
+noted at the end. struct tty_operations are documented next.
+
+.. kernel-doc:: include/linux/tty_driver.h
+   :identifiers: tty_driver
+
+----
+
+TTY Operations Reference
+========================
+
+When a TTY is registered, these driver hooks can be invoked by the TTY layer:
+
+.. kernel-doc:: include/linux/tty_driver.h
+   :identifiers: tty_operations
+
diff --git a/Documentation/driver-api/tty/tty_internals.rst b/Documentation/driver-api/tty/tty_internals.rst
new file mode 100644
index 000000000..d0d415820
--- /dev/null
+++ b/Documentation/driver-api/tty/tty_internals.rst
@@ -0,0 +1,31 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============
+TTY Internals
+=============
+
+.. contents:: :local:
+
+Kopen
+=====
+
+These functions serve for opening a TTY from the kernelspace:
+
+.. kernel-doc:: drivers/tty/tty_io.c
+      :identifiers: tty_kopen_exclusive tty_kopen_shared tty_kclose
+
+----
+
+Exported Internal Functions
+===========================
+
+.. kernel-doc:: drivers/tty/tty_io.c
+   :identifiers: tty_release_struct tty_dev_name_to_number tty_get_icount
+
+----
+
+Internal Functions
+==================
+
+.. kernel-doc:: drivers/tty/tty_io.c
+   :internal:
diff --git a/Documentation/driver-api/tty/tty_ioctl.rst b/Documentation/driver-api/tty/tty_ioctl.rst
new file mode 100644
index 000000000..3ff1ac5e0
--- /dev/null
+++ b/Documentation/driver-api/tty/tty_ioctl.rst
@@ -0,0 +1,10 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=================
+TTY IOCTL Helpers
+=================
+
+.. kernel-doc:: drivers/tty/tty_ioctl.c
+
+.. kernel-doc:: include/linux/tty.h
+   :identifiers: tty_get_baud_rate
diff --git a/Documentation/driver-api/tty/tty_ldisc.rst b/Documentation/driver-api/tty/tty_ldisc.rst
new file mode 100644
index 000000000..d034e117c
--- /dev/null
+++ b/Documentation/driver-api/tty/tty_ldisc.rst
@@ -0,0 +1,85 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===================
+TTY Line Discipline
+===================
+
+.. contents:: :local:
+
+TTY line discipline process all incoming and outgoing character from/to a tty
+device. The default line discipline is :doc:`N_TTY <n_tty>`. It is also a
+fallback if establishing any other discipline for a tty fails. If even N_TTY
+fails, N_NULL takes over. That never fails, but also does not process any
+characters -- it throws them away.
+
+Registration
+============
+
+Line disciplines are registered with tty_register_ldisc() passing the ldisc
+structure. At the point of registration the discipline must be ready to use and
+it is possible it will get used before the call returns success. If the call
+returns an error then it won’t get called. Do not reuse ldisc numbers as they
+are part of the userspace ABI and writing over an existing ldisc will cause
+demons to eat your computer. You must not re-register over the top of the line
+discipline even with the same data or your computer again will be eaten by
+demons. In order to remove a line discipline call tty_unregister_ldisc().
+
+Heed this warning: the reference count field of the registered copies of the
+tty_ldisc structure in the ldisc table counts the number of lines using this
+discipline. The reference count of the tty_ldisc structure within a tty counts
+the number of active users of the ldisc at this instant. In effect it counts
+the number of threads of execution within an ldisc method (plus those about to
+enter and exit although this detail matters not).
+
+.. kernel-doc:: drivers/tty/tty_ldisc.c
+   :identifiers: tty_register_ldisc tty_unregister_ldisc
+
+Other Functions
+===============
+
+.. kernel-doc:: drivers/tty/tty_ldisc.c
+   :identifiers: tty_set_ldisc tty_ldisc_flush
+
+Line Discipline Operations Reference
+====================================
+
+.. kernel-doc:: include/linux/tty_ldisc.h
+   :identifiers: tty_ldisc_ops
+
+Driver Access
+=============
+
+Line discipline methods can call the methods of the underlying hardware driver.
+These are documented as a part of struct tty_operations.
+
+TTY Flags
+=========
+
+Line discipline methods have access to :c:member:`tty_struct.flags` field. See
+:doc:`tty_struct`.
+
+Locking
+=======
+
+Callers to the line discipline functions from the tty layer are required to
+take line discipline locks. The same is true of calls from the driver side
+but not yet enforced.
+
+.. kernel-doc:: drivers/tty/tty_ldisc.c
+   :identifiers: tty_ldisc_ref_wait tty_ldisc_ref tty_ldisc_deref
+
+While these functions are slightly slower than the old code they should have
+minimal impact as most receive logic uses the flip buffers and they only
+need to take a reference when they push bits up through the driver.
+
+A caution: The :c:member:`tty_ldisc_ops.open()`,
+:c:member:`tty_ldisc_ops.close()` and :c:member:`tty_driver.set_ldisc()`
+functions are called with the ldisc unavailable. Thus tty_ldisc_ref() will fail
+in this situation if used within these functions.  Ldisc and driver code
+calling its own functions must be careful in this case.
+
+Internal Functions
+==================
+
+.. kernel-doc:: drivers/tty/tty_ldisc.c
+   :internal:
diff --git a/Documentation/driver-api/tty/tty_port.rst b/Documentation/driver-api/tty/tty_port.rst
new file mode 100644
index 000000000..504a353f2
--- /dev/null
+++ b/Documentation/driver-api/tty/tty_port.rst
@@ -0,0 +1,71 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+========
+TTY Port
+========
+
+.. contents:: :local:
+
+The TTY drivers are advised to use struct tty_port helpers as much as possible.
+If the drivers implement :c:member:`tty_port.ops.activate()` and
+:c:member:`tty_port.ops.shutdown()`, they can use tty_port_open(),
+tty_port_close(), and tty_port_hangup() in respective
+:c:member:`tty_struct.ops` hooks.
+
+The reference and details are contained in the `TTY Port Reference`_ and `TTY
+Port Operations Reference`_ sections at the bottom.
+
+TTY Port Functions
+==================
+
+Init & Destroy
+--------------
+
+.. kernel-doc::  drivers/tty/tty_port.c
+   :identifiers: tty_port_init tty_port_destroy
+        tty_port_get tty_port_put
+
+Open/Close/Hangup Helpers
+-------------------------
+
+.. kernel-doc::  drivers/tty/tty_port.c
+   :identifiers: tty_port_install tty_port_open tty_port_block_til_ready
+        tty_port_close tty_port_close_start tty_port_close_end tty_port_hangup
+        tty_port_shutdown
+
+TTY Refcounting
+---------------
+
+.. kernel-doc::  drivers/tty/tty_port.c
+   :identifiers: tty_port_tty_get tty_port_tty_set
+
+TTY Helpers
+-----------
+
+.. kernel-doc::  include/linux/tty_port.h
+   :identifiers: tty_port_tty_hangup tty_port_tty_vhangup
+.. kernel-doc::  drivers/tty/tty_port.c
+   :identifiers: tty_port_tty_wakeup
+
+Modem Signals
+-------------
+
+.. kernel-doc::  drivers/tty/tty_port.c
+   :identifiers: tty_port_carrier_raised tty_port_raise_dtr_rts
+        tty_port_lower_dtr_rts
+
+----
+
+TTY Port Reference
+==================
+
+.. kernel-doc:: include/linux/tty_port.h
+   :identifiers: tty_port
+
+----
+
+TTY Port Operations Reference
+=============================
+
+.. kernel-doc:: include/linux/tty_port.h
+   :identifiers: tty_port_operations
diff --git a/Documentation/driver-api/tty/tty_struct.rst b/Documentation/driver-api/tty/tty_struct.rst
new file mode 100644
index 000000000..29caf1c1c
--- /dev/null
+++ b/Documentation/driver-api/tty/tty_struct.rst
@@ -0,0 +1,81 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==========
+TTY Struct
+==========
+
+.. contents:: :local:
+
+struct tty_struct is allocated by the TTY layer upon the first open of the TTY
+device and released after the last close. The TTY layer passes this structure
+to most of struct tty_operation's hooks. Members of tty_struct are documented
+in `TTY Struct Reference`_ at the bottom.
+
+Initialization
+==============
+
+.. kernel-doc:: drivers/tty/tty_io.c
+   :identifiers: tty_init_termios
+
+Name
+====
+
+.. kernel-doc:: drivers/tty/tty_io.c
+   :identifiers: tty_name
+
+Reference counting
+==================
+
+.. kernel-doc:: include/linux/tty.h
+   :identifiers: tty_kref_get
+
+.. kernel-doc:: drivers/tty/tty_io.c
+   :identifiers: tty_kref_put
+
+Install
+=======
+
+.. kernel-doc:: drivers/tty/tty_io.c
+   :identifiers: tty_standard_install
+
+Read & Write
+============
+
+.. kernel-doc:: drivers/tty/tty_io.c
+   :identifiers: tty_put_char
+
+Start & Stop
+============
+
+.. kernel-doc:: drivers/tty/tty_io.c
+   :identifiers: start_tty stop_tty
+
+Wakeup
+======
+
+.. kernel-doc:: drivers/tty/tty_io.c
+   :identifiers: tty_wakeup
+
+Hangup
+======
+
+.. kernel-doc:: drivers/tty/tty_io.c
+   :identifiers: tty_hangup tty_vhangup tty_hung_up_p
+
+Misc
+====
+
+.. kernel-doc:: drivers/tty/tty_io.c
+   :identifiers: tty_do_resize
+
+TTY Struct Flags
+================
+
+.. kernel-doc:: include/linux/tty.h
+   :identifiers: tty_struct_flags
+
+TTY Struct Reference
+====================
+
+.. kernel-doc:: include/linux/tty.h
+   :identifiers: tty_struct