1. Introduction to Linux Device Drivers
Class 3: Serial Drivers using Linux
Aug 6, 2014 Khem Raj

2. Introduction Serial port is commonly used communication channel
Implemented using UART
On PCs UART is part of SuperIO chipset
Kernel Drivers for serial devices are generic

3. Introduction Serial subsystem is use when
Terminal session over serial console
Using modem or dialup
Touch controllers, IR devices, bluetooth
USB-2-serial converters

4. Serial Subsystem Architecture
Low level drivers to interact with UART
Tty driver layer above driver
Data exchange core layer talks to tty driver
Shapes behavior of serial subsystem

5. Serial Drivers Serial over USB Char drivers
USB driver talks in a layers setup
Tty driver interacts with USB drivers and insulated the serial data exchange (data discipline)
Char drivers
But do not expose syscall APIs like regular char drivers
Services tty layer

6. UART Driver 3 main Data structures uart_driver – Per UART
uart_port – Per port owned by UART
uart_ops – Common tty file operations
Each UART driver must support
Used by tty layer
Defines methods to access hardware

7. UART Driver Register with serial subsystem
uart_register_driver(struct uart_driver *);
Register every port on device
uart_add_one_port(struct uart_driver * , struct uart_port *)
serial8250_register_port(struct uart_port *)
Internally calls uart_add_one_port

8. USB UART Platform Driver
Implemented at platform Driver
Platform drivers used for platform devices
Autonomous, appear on port-based devices, host-bridges, controllers on SOCs
Directly accessible from CPU bus
Has a name, driver bindings, resources list

9. USB UART Platform Driver
Standard Driver Model
Device discovery handled outside driver
Provides probe() and remove() functions
Support Power management and notifications on shutdown

10. USB UART Platform Driver
Registers with USB core
Uses uart_register_driver()
New tty entries /proc/tty/drivers/
Registers Platform devices
Uses platform_device_register_simple()
Platform Driver entry points ( functions )
Registers probe(), remove(), suspend(), resume() functions

11. tty Drivers Registration tty_struct tty_flip_buffer tty_driver
Contains information about opening tty
tty_flip_buffer
Data collection
Serial drivers use it.
tty_driver
Tty driver API used by upper layers
Uses tty_register_driver() and tty_register_device () for devices

12. Line Discipline Registered with tty_register_ldisc()
Removed with tty_unregister_ldisc()
TTY side APIs
Used to call into tty driver
Driver access
Calls tty driver functions e.g. write(), read() .. Via function pointers
Driver Side Interfaces
Talks between line discipline and driver
receive_buf, write_wakeup, dcd_change

13. Line Discipline
Callers from tty layer as well as driver should take locks If
More info in Documentation/serial/tty.txt

14. Thanks
Questions?