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
struct tty_port. These devices are maintained by a TTY driver
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
tty_operations are invoked by the TTY layer.
Every character received by the kernel (both from devices and users) is passed
through a preselected TTY Line Discipline (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:
- TTY Driver and TTY Operations
- TTY Port
- TTY Struct
- TTY Line Discipline
- TTY Buffer
- TTY Internals
Writing TTY Driver¶
Before one starts writing a TTY driver, they must consider Serial and 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)