Documentation/driver-api/tty/index.rst
.. SPDX-License-Identifier: GPL-2.0
Teletypewriter (TTY) layer takes care of all those serial devices. Including the virtual ones like pseudoterminal (PTY).
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
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.
Miscellaneous documentation can be further found in these documents:
.. toctree:: :maxdepth: 2
moxa-smartio n_gsm n_tty