1.. SPDX-License-Identifier: GPL-2.0 2 3=== 4TTY 5=== 6 7Teletypewriter (TTY) layer takes care of all those serial devices. Including 8the virtual ones like pseudoterminal (PTY). 9 10TTY structures 11============== 12 13There are several major TTY structures. Every TTY device in a system has a 14corresponding struct tty_port. These devices are maintained by a TTY driver 15which is struct tty_driver. This structure describes the driver but also 16contains a reference to operations which could be performed on the TTYs. It is 17struct tty_operations. Then, upon open, a struct tty_struct is allocated and 18lives until the final close. During this time, several callbacks from struct 19tty_operations are invoked by the TTY layer. 20 21Every character received by the kernel (both from devices and users) is passed 22through a preselected :doc:`tty_ldisc` (in 23short ldisc; in C, struct tty_ldisc_ops). Its task is to transform characters 24as defined by a particular ldisc or by user too. The default one is n_tty, 25implementing echoes, signal handling, jobs control, special characters 26processing, and more. The transformed characters are passed further to 27user/device, depending on the source. 28 29In-detail description of the named TTY structures is in separate documents: 30 31.. toctree:: 32 :maxdepth: 2 33 34 tty_driver 35 tty_port 36 tty_struct 37 tty_ldisc 38 tty_buffer 39 tty_internals 40 41Writing TTY Driver 42================== 43 44Before one starts writing a TTY driver, they must consider 45:doc:`Serial <../serial/driver>` and :doc:`USB Serial <../../usb/usb-serial>` 46layers first. Drivers for serial devices can often use one of these specific 47layers to implement a serial driver. Only special devices should be handled 48directly by the TTY Layer. If you are about to write such a driver, read on. 49 50A *typical* sequence a TTY driver performs is as follows: 51 52#. Allocate and register a TTY driver (module init) 53#. Create and register TTY devices as they are probed (probe function) 54#. Handle TTY operations and events like interrupts (TTY core invokes the 55 former, the device the latter) 56#. Remove devices as they are going away (remove function) 57#. Unregister and free the TTY driver (module exit) 58 59Steps regarding driver, i.e. 1., 3., and 5. are described in detail in 60:doc:`tty_driver`. For the other two (devices handling), look into 61:doc:`tty_port`. 62 63Other Documentation 64=================== 65 66Miscellaneous documentation can be further found in these documents: 67 68.. toctree:: 69 :maxdepth: 2 70 71 moxa-smartio 72 n_gsm 73 n_tty 74