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1.. SPDX-License-Identifier: GPL-2.02 3===4TTY5===6 7Teletypewriter (TTY) layer takes care of all those serial devices. Including8the virtual ones like pseudoterminal (PTY).9 10TTY structures11==============12 13There are several major TTY structures. Every TTY device in a system has a14corresponding struct tty_port. These devices are maintained by a TTY driver15which is struct tty_driver. This structure describes the driver but also16contains a reference to operations which could be performed on the TTYs. It is17struct tty_operations. Then, upon open, a struct tty_struct is allocated and18lives until the final close. During this time, several callbacks from struct19tty_operations are invoked by the TTY layer.20 21Every character received by the kernel (both from devices and users) is passed22through a preselected :doc:`tty_ldisc` (in23short ldisc; in C, struct tty_ldisc_ops). Its task is to transform characters24as defined by a particular ldisc or by user too. The default one is n_tty,25implementing echoes, signal handling, jobs control, special characters26processing, and more. The transformed characters are passed further to27user/device, depending on the source.28 29In-detail description of the named TTY structures is in separate documents:30 31.. toctree::32 :maxdepth: 233 34 tty_driver35 tty_port36 tty_struct37 tty_ldisc38 tty_buffer39 tty_ioctl40 tty_internals41 console42 43Writing TTY Driver44==================45 46Before one starts writing a TTY driver, they must consider47:doc:`Serial <../serial/driver>` and :doc:`USB Serial <../../usb/usb-serial>`48layers first. Drivers for serial devices can often use one of these specific49layers to implement a serial driver. Only special devices should be handled50directly by the TTY Layer. If you are about to write such a driver, read on.51 52A *typical* sequence a TTY driver performs is as follows:53 54#. Allocate and register a TTY driver (module init)55#. Create and register TTY devices as they are probed (probe function)56#. Handle TTY operations and events like interrupts (TTY core invokes the57 former, the device the latter)58#. Remove devices as they are going away (remove function)59#. Unregister and free the TTY driver (module exit)60 61Steps regarding driver, i.e. 1., 3., and 5. are described in detail in62:doc:`tty_driver`. For the other two (devices handling), look into63:doc:`tty_port`.64 65Other Documentation66===================67 68Miscellaneous documentation can be further found in these documents:69 70.. toctree::71 :maxdepth: 272 73 moxa-smartio74 n_gsm75 n_tty76