Based on kernel version
5.17. Page generated on
2022-03-28 08:42 EST.
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.. 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 n_tty tty_internals Writing TTY Driver ================== Before one starts writing a TTY driver, they must consider :doc:`Serial <../driver-api/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`.