Serial Drivers

1. © 2010-17 SysPlay Workshops <workshop@sysplay.in>
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Serial Drivers

2. 2© 2010-17 SysPlay Workshops <workshop@sysplay.in>
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What to Expect?
Serial Prologue
UART
TTY Driver Use Case
TTY Serial Subsystem
Low Level Driver API
UART Data Structures

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Serial Prologue
Typically known as RS232 protocol
But, it covers many more. Namely,
Infrared (back bone is RS232)
RS485 (industry enhanced RS232)
Serial Modem
Console
…
From Linux kernel's perspective
Serial Drivers provides a generic abstract view to the user
for all of these

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UART
UART - Universal Asynchronous receiver
transmitter
Converts the parallel data from the CPU to serial
data to be transmitted on lines
Data Frame contains 5 to 9 data bits, start bit, 1
or more stop bits, an option parity bit

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TTY Driver Use Case
Terminal
Physical
Line
UART
UART
Driver
Line
Discipline
TTY
Driver
User
Process
User
Process
User
Process
KernelHardware
Software

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TTY Serial Subsystem
N_TTY
(n_tty.c)
N_IRDA
(irtty_sir.c)
TTY Core
(tty_io.c)
serial_core.c
usb-serial.c
(Core Module)
omap-serial.c
USB-Serial
Converter
Driver
User Applications (System Call Interface)
Line Discipline Layer
TTY Layer
Low-level Driver Layer
Physical Devices like Infrared, UART, USB-Serial Converter

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Low-level (UART) Driver related Data
Structures & APIs
Data Structures
struct uart_driver
struct uart_port
struct uart_ops
APIs
int uart_register_driver(struct uart_driver *);
int uart_unregister_driver(struct uart_driver *);
int uart_add_one_port(struct uart_driver *, struct uart_port);
int uart_remove_one_port(struct uart_driver *, struct uart_port);
Typically implemented as a platform driver
Header: <linux/serial_core.h>

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UART Driver structure
struct uart_driver
struct module *owner;
const char *driver_name; /* Name */
const char *dev_name; /* dev node names */
int major; /* Major number */
int minor; /* Minor number */
struct tty_driver *tty_driver; /* tty driver */

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UART Port structure
struct uart_port
spinlock_t lock; /* lock */
unsigned int iobase; /* in/out[bwl] */
unsigned char __iomem *membase; /* I/O membase */
unsigned int irq; /* irq number */
unsigned int uartclk; /* base uart clock */
unsigned char fifosize; /* tx fifo size */
unsigned char x_char; /* flow control */

10. 10© 2010-17 SysPlay Workshops <workshop@sysplay.in>
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UART Operations structure
struct uart_ops
uint (*tx_empty)(struct uart_port *); /* Is TX FIFO empty? */
void (*set_mctrl)(struct uart_port *, unsigned int mctrl); /* Set
modem control params */
uint (*get_mctrl)(struct uart_port *); /* Get modem control params */
void (*stop_tx)(struct uart_port *); /* Stop xmission */
void (*start_tx)(struct uart_port *); /* Start xmission */
void (*shutdown)(struct uart_port *); /* Disable the port */
void (*set_termios)(struct uart_port *, struct termios *new, struct
termios *old); /* Terminal interface parameters */
void (*config_port) (struct uart_port *, int);

11. 11© 2010-17 SysPlay Workshops <workshop@sysplay.in>
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TTY Driver related Data Structures &
APIs
Header: <linux/tty.h>
Data Structures
struct tty_struct
struct tty_flip_buffer
struct tty_driver
APIs
int tty_register_driver(struct tty_driver *);
int tty_unregister_driver(struct tty_driver *);
struct device *tty_register_device(struct tty_driver *, unsigned,
struct device);
void tty_unregister_device(struct tty_driver *, unsigned);

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What all have we learnt?
Serial Prologue
UART
TTY Driver Use Case
TTY Serial Subsystem
Low Level Driver API
UART Data Structures

13. 13© 2010-17 SysPlay Workshops <workshop@sysplay.in>
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Any Queries?