Low Level Serial API¶

This document is meant as a brief overview of some aspects of the new serial driver. It is not complete, any questions you have should be directed to <rmk@arm.linux.org.uk>

The reference implementation is contained within amba-pl011.c.

Low Level Serial Hardware Driver¶

The low level serial hardware driver is responsible for supplying port information (defined by uart_port) and a set of control methods (defined by uart_ops) to the core serial driver. The low level driver is also responsible for handling interrupts for the port, and providing any console support.

Console Support¶

The serial core provides a few helper functions. This includes identifying the correct port structure (via uart_get_console()) and decoding command line arguments (uart_parse_options()).

There is also a helper function (uart_console_write()) which performs a character by character write, translating newlines to CRLF sequences. Driver writers are recommended to use this function rather than implementing their own version.

Locking¶

It is the responsibility of the low level hardware driver to perform the necessary locking using port->lock. There are some exceptions (which are described in the struct uart_ops listing below.)

There are two locks. A per-port spinlock, and an overall semaphore.

From the core driver perspective, the port->lock locks the following data:

port->mctrl
port->icount
port->state->xmit.head (circ_buf->head)
port->state->xmit.tail (circ_buf->tail)

The low level driver is free to use this lock to provide any additional locking.

The port_sem semaphore is used to protect against ports being added/ removed or reconfigured at inappropriate times. Since v2.6.27, this semaphore has been the ‘mutex’ member of the tty_port struct, and commonly referred to as the port mutex.

uart_ops¶

struct uart_ops¶

interface between serial_core and the driver

Definition:

struct uart_ops {
    unsigned int    (*tx_empty)(struct uart_port *);
    void (*set_mctrl)(struct uart_port *, unsigned int mctrl);
    unsigned int    (*get_mctrl)(struct uart_port *);
    void (*stop_tx)(struct uart_port *);
    void (*start_tx)(struct uart_port *);
    void (*throttle)(struct uart_port *);
    void (*unthrottle)(struct uart_port *);
    void (*send_xchar)(struct uart_port *, char ch);
    void (*stop_rx)(struct uart_port *);
    void (*start_rx)(struct uart_port *);
    void (*enable_ms)(struct uart_port *);
    void (*break_ctl)(struct uart_port *, int ctl);
    int (*startup)(struct uart_port *);
    void (*shutdown)(struct uart_port *);
    void (*flush_buffer)(struct uart_port *);
    void (*set_termios)(struct uart_port *, struct ktermios *new, const struct ktermios *old);
    void (*set_ldisc)(struct uart_port *, struct ktermios *);
    void (*pm)(struct uart_port *, unsigned int state, unsigned int oldstate);
    const char      *(*type)(struct uart_port *);
    void (*release_port)(struct uart_port *);
    int (*request_port)(struct uart_port *);
    void (*config_port)(struct uart_port *, int);
    int (*verify_port)(struct uart_port *, struct serial_struct *);
    int (*ioctl)(struct uart_port *, unsigned int, unsigned long);
#ifdef CONFIG_CONSOLE_POLL;
    int (*poll_init)(struct uart_port *);
    void (*poll_put_char)(struct uart_port *, unsigned char);
    int (*poll_get_char)(struct uart_port *);
#endif;
};

Members

tx_empty

unsigned int ()(struct uart_port *port)

This function tests whether the transmitter fifo and shifter for the port is empty. If it is empty, this function should return TIOCSER_TEMT, otherwise return 0. If the port does not support this operation, then it should return TIOCSER_TEMT.

Locking: none. Interrupts: caller dependent. This call must not sleep

set_mctrl

void ()(struct uart_port *port, unsigned int mctrl)

This function sets the modem control lines for port to the state described by mctrl. The relevant bits of mctrl are:

TIOCM_RTS RTS signal.

TIOCM_DTR DTR signal.

TIOCM_OUT1 OUT1 signal.

TIOCM_OUT2 OUT2 signal.

TIOCM_LOOP Set the port into loopback mode.

If the appropriate bit is set, the signal should be driven active. If the bit is clear, the signal should be driven inactive.

Locking: port->lock taken. Interrupts: locally disabled. This call must not sleep

get_mctrl

unsigned int ()(struct uart_port *port)

Returns the current state of modem control inputs of port. The state of the outputs should not be returned, since the core keeps track of their state. The state information should include:

TIOCM_CAR state of DCD signal

TIOCM_CTS state of CTS signal

TIOCM_DSR state of DSR signal

TIOCM_RI state of RI signal

The bit is set if the signal is currently driven active. If the port does not support CTS, DCD or DSR, the driver should indicate that the signal is permanently active. If RI is not available, the signal should not be indicated as active.

Locking: port->lock taken. Interrupts: locally disabled. This call must not sleep

stop_tx

void ()(struct uart_port *port)

Stop transmitting characters. This might be due to the CTS line becoming inactive or the tty layer indicating we want to stop transmission due to an XOFF character.

The driver should stop transmitting characters as soon as possible.

Locking: port->lock taken. Interrupts: locally disabled. This call must not sleep

start_tx

void ()(struct uart_port *port)

Start transmitting characters.

Locking: port->lock taken. Interrupts: locally disabled. This call must not sleep

throttle

void ()(struct uart_port *port)

Notify the serial driver that input buffers for the line discipline are close to full, and it should somehow signal that no more characters should be sent to the serial port. This will be called only if hardware assisted flow control is enabled.

Locking: serialized with unthrottle() and termios modification by the tty layer.

unthrottle

void ()(struct uart_port *port)

Notify the serial driver that characters can now be sent to the serial port without fear of overrunning the input buffers of the line disciplines.

This will be called only if hardware assisted flow control is enabled.

Locking: serialized with throttle() and termios modification by the tty layer.

send_xchar

void ()(struct uart_port *port, char ch)

Transmit a high priority character, even if the port is stopped. This is used to implement XON/XOFF flow control and tcflow(). If the serial driver does not implement this function, the tty core will append the character to the circular buffer and then call start_tx() / stop_tx() to flush the data out.

Do not transmit if ch == ‘0’ (__DISABLED_CHAR).

Locking: none. Interrupts: caller dependent.

stop_rx

void ()(struct uart_port *port)

Stop receiving characters; the port is in the process of being closed.

Locking: port->lock taken. Interrupts: locally disabled. This call must not sleep

start_rx

void ()(struct uart_port *port)

Start receiving characters.

Locking: port->lock taken. Interrupts: locally disabled. This call must not sleep

enable_ms

void ()(struct uart_port *port)

Enable the modem status interrupts.

This method may be called multiple times. Modem status interrupts should be disabled when the shutdown() method is called.

Locking: port->lock taken. Interrupts: locally disabled. This call must not sleep

break_ctl

void ()(struct uart_port *port, int ctl)

Control the transmission of a break signal. If ctl is nonzero, the break signal should be transmitted. The signal should be terminated when another call is made with a zero ctl.

Locking: caller holds tty_port->mutex

startup

int ()(struct uart_port *port)

Grab any interrupt resources and initialise any low level driver state. Enable the port for reception. It should not activate RTS nor DTR; this will be done via a separate call to set_mctrl().

This method will only be called when the port is initially opened.

Locking: port_sem taken. Interrupts: globally disabled.

shutdown

void ()(struct uart_port *port)

Disable the port, disable any break condition that may be in effect, and free any interrupt resources. It should not disable RTS nor DTR; this will have already been done via a separate call to set_mctrl().

Drivers must not access port->state once this call has completed.

This method will only be called when there are no more users of this port.

Locking: port_sem taken. Interrupts: caller dependent.

flush_buffer

void ()(struct uart_port *port)

Flush any write buffers, reset any DMA state and stop any ongoing DMA transfers.

This will be called whenever the port->state->xmit circular buffer is cleared.

Locking: port->lock taken. Interrupts: locally disabled. This call must not sleep

set_termios

void ()(struct uart_port *port, struct ktermios *new, struct ktermios *old)

Change the port parameters, including word length, parity, stop bits. Update port->read_status_mask and port->ignore_status_mask to indicate the types of events we are interested in receiving. Relevant ktermios::c_cflag bits are:

CSIZE - word size

CSTOPB - 2 stop bits

PARENB - parity enable

PARODD - odd parity (when PARENB is in force)

ADDRB - address bit (changed through uart_port::rs485_config()).

CREAD - enable reception of characters (if not set, still receive characters from the port, but throw them away).

CRTSCTS - if set, enable CTS status change reporting.

CLOCAL - if not set, enable modem status change reporting.

Relevant ktermios::c_iflag bits are:

INPCK - enable frame and parity error events to be passed to the TTY layer.

BRKINT / PARMRK - both of these enable break events to be passed to the TTY layer.

IGNPAR - ignore parity and framing errors.

IGNBRK - ignore break errors. If IGNPAR is also set, ignore overrun errors as well.

The interaction of the ktermios::c_iflag bits is as follows (parity error given as an example):

Parity error

INPCK

IGNPAR

n/a

0

n/a

character received, marked as TTY_NORMAL

None

1

n/a

character received, marked as TTY_NORMAL

Yes

1

0

character received, marked as TTY_PARITY

Yes

1

1

character discarded

Other flags may be used (eg, xon/xoff characters) if your hardware supports hardware “soft” flow control.

Locking: caller holds tty_port->mutex Interrupts: caller dependent. This call must not sleep

set_ldisc

void ()(struct uart_port *port, struct ktermios *termios)

Notifier for discipline change. See TTY Line Discipline.

Locking: caller holds tty_port->mutex

pm

void ()(struct uart_port *port, unsigned int state, unsigned int oldstate)

Perform any power management related activities on the specified port. state indicates the new state (defined by enum uart_pm_state), oldstate indicates the previous state.

This function should not be used to grab any resources.

This will be called when the port is initially opened and finally closed, except when the port is also the system console. This will occur even if CONFIG_PM is not set.

Locking: none. Interrupts: caller dependent.

type

const char *()(struct uart_port *port)

Return a pointer to a string constant describing the specified port, or return NULL, in which case the string ‘unknown’ is substituted.

Locking: none. Interrupts: caller dependent.

release_port

void ()(struct uart_port *port)

Release any memory and IO region resources currently in use by the port.

Locking: none. Interrupts: caller dependent.

request_port

int ()(struct uart_port *port)

Request any memory and IO region resources required by the port. If any fail, no resources should be registered when this function returns, and it should return -EBUSY on failure.

Locking: none. Interrupts: caller dependent.

config_port

void ()(struct uart_port *port, int type)

Perform any autoconfiguration steps required for the port. type contains a bit mask of the required configuration. UART_CONFIG_TYPE indicates that the port requires detection and identification. port->type should be set to the type found, or PORT_UNKNOWN if no port was detected.

UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal, which should be probed using standard kernel autoprobing techniques. This is not necessary on platforms where ports have interrupts internally hard wired (eg, system on a chip implementations).

Locking: none. Interrupts: caller dependent.

verify_port

int ()(struct uart_port *port, struct serial_struct *serinfo)

Verify the new serial port information contained within serinfo is suitable for this port type.

Locking: none. Interrupts: caller dependent.

ioctl

int ()(struct uart_port *port, unsigned int cmd, unsigned long arg)

Perform any port specific IOCTLs. IOCTL commands must be defined using the standard numbering system found in <asm/ioctl.h>.

Locking: none. Interrupts: caller dependent.

poll_init

int ()(struct uart_port *port)

Called by kgdb to perform the minimal hardware initialization needed to support poll_put_char() and poll_get_char(). Unlike startup(), this should not request interrupts.

Locking: tty_mutex and tty_port->mutex taken. Interrupts: n/a.

poll_put_char

void ()(struct uart_port *port, unsigned char ch)

Called by kgdb to write a single character ch directly to the serial port. It can and should block until there is space in the TX FIFO.

Locking: none. Interrupts: caller dependent. This call must not sleep

poll_get_char

int ()(struct uart_port *port)

Called by kgdb to read a single character directly from the serial port. If data is available, it should be returned; otherwise the function should return NO_POLL_CHAR immediately.

Locking: none. Interrupts: caller dependent. This call must not sleep

Description

This structure describes all the operations that can be done on the physical hardware.

Other functions¶

void uart_write_wakeup(struct uart_port *port)¶: schedule write processing

Parameters

struct uart_port *port: port to be processed

Description

This routine is used by the interrupt handler to schedule processing in the software interrupt portion of the driver. A driver is expected to call this function when the number of characters in the transmit buffer have dropped below a threshold.

Locking: port->lock should be held

void uart_update_timeout(struct uart_port *port, unsigned int cflag, unsigned int baud)¶: update per-port frame timing information

Parameters

struct uart_port *port: uart_port structure describing the port
unsigned int cflag: termios cflag value
unsigned int baud: speed of the port

Description

Set the port frame timing information from which the FIFO timeout value is derived. The cflag value should reflect the actual hardware settings as number of bits, parity, stop bits and baud rate is taken into account here.

Locking: caller is expected to take port->lock

unsigned int uart_get_baud_rate(struct uart_port *port, struct ktermios *termios, const struct ktermios *old, unsigned int min, unsigned int max)¶: return baud rate for a particular port

Parameters

struct uart_port *port: uart_port structure describing the port in question.
struct ktermios *termios: desired termios settings
const struct ktermios *old: old termios (or NULL)
unsigned int min: minimum acceptable baud rate
unsigned int max: maximum acceptable baud rate

Description

Decode the termios structure into a numeric baud rate, taking account of the magic 38400 baud rate (with spd_* flags), and mapping the B0 rate to 9600 baud.

If the new baud rate is invalid, try the old termios setting. If it’s still invalid, we try 9600 baud. If that is also invalid 0 is returned.

The termios structure is updated to reflect the baud rate we’re actually going to be using. Don’t do this for the case where B0 is requested (“hang up”).

Locking: caller dependent

unsigned int uart_get_divisor(struct uart_port *port, unsigned int baud)¶: return uart clock divisor

Parameters

struct uart_port *port: uart_port structure describing the port
unsigned int baud: desired baud rate

Description

Calculate the divisor (baud_base / baud) for the specified baud, appropriately rounded.

If 38400 baud and custom divisor is selected, return the custom divisor instead.

Locking: caller dependent

int uart_get_lsr_info(struct tty_struct *tty, struct uart_state *state, unsigned int __user *value)¶: get line status register info

Parameters

struct tty_struct *tty: tty associated with the UART
struct uart_state *state: UART being queried
unsigned int __user *value: returned modem value

void uart_console_write(struct uart_port *port, const char *s, unsigned int count, void (*putchar)(struct uart_port*, unsigned char))¶: write a console message to a serial port

Parameters

struct uart_port *port: the port to write the message
const char *s: array of characters
unsigned int count: number of characters in string to write
void (*putchar)(struct uart_port *, unsigned char): function to write character to port

struct uart_port *uart_get_console(struct uart_port *ports, int nr, struct console *co)¶: get uart port for console

Parameters

struct uart_port *ports: ports to search in
int nr: number of ports
struct console *co: console to search for

Return

uart_port for the console co

Description

Check whether an invalid uart number has been specified (as co->index), and if so, search for the first available port that does have console support.

int uart_parse_earlycon(char *p, unsigned char *iotype, resource_size_t *addr, char **options)¶: Parse earlycon options

Parameters

char *p: ptr to 2nd field (ie., just beyond ‘<name>,’)
unsigned char *iotype: ptr for decoded iotype (out)
resource_size_t *addr: ptr for decoded mapbase/iobase (out)
char **options: ptr for <options> field; NULL if not present (out)

Description

Decodes earlycon kernel command line parameters of the form:

earlycon=<name>,io|mmio|mmio16|mmio32|mmio32be|mmio32native,<addr>,<options>
console=<name>,io|mmio|mmio16|mmio32|mmio32be|mmio32native,<addr>,<options>

The optional form:

earlycon=<name>,0x<addr>,<options>
console=<name>,0x<addr>,<options>

is also accepted; the returned iotype will be UPIO_MEM.

Return

0 on success or -EINVAL on failure

void uart_parse_options(const char *options, int *baud, int *parity, int *bits, int *flow)¶: Parse serial port baud/parity/bits/flow control.

Parameters

const char *options: pointer to option string
int *baud: pointer to an ‘int’ variable for the baud rate.
int *parity: pointer to an ‘int’ variable for the parity.
int *bits: pointer to an ‘int’ variable for the number of data bits.
int *flow: pointer to an ‘int’ variable for the flow control character.

Description

uart_parse_options() decodes a string containing the serial console options. The format of the string is <baud><parity><bits><flow>, eg: 115200n8r

int uart_set_options(struct uart_port *port, struct console *co, int baud, int parity, int bits, int flow)¶: setup the serial console parameters

Parameters

struct uart_port *port: pointer to the serial ports uart_port structure
struct console *co: console pointer
int baud: baud rate
int parity: parity character - ‘n’ (none), ‘o’ (odd), ‘e’ (even)
int bits: number of data bits
int flow: flow control character - ‘r’ (rts)

Description

Locking: Caller must hold console_list_lock in order to serialize early initialization of the serial-console lock.

int uart_register_driver(struct uart_driver *drv)¶: register a driver with the uart core layer

Parameters

struct uart_driver *drv: low level driver structure

Description

Register a uart driver with the core driver. We in turn register with the tty layer, and initialise the core driver per-port state.

We have a proc file in /proc/tty/driver which is named after the normal driver.

drv->port should be NULL, and the per-port structures should be registered using uart_add_one_port() after this call has succeeded.

Locking: none, Interrupts: enabled

void uart_unregister_driver(struct uart_driver *drv)¶: remove a driver from the uart core layer

Parameters

struct uart_driver *drv: low level driver structure

Description

Remove all references to a driver from the core driver. The low level driver must have removed all its ports via the uart_remove_one_port() if it registered them with uart_add_one_port(). (I.e. drv->port is NULL.)

Locking: none, Interrupts: enabled

bool uart_match_port(const struct uart_port *port1, const struct uart_port *port2)¶: are the two ports equivalent?

Parameters

const struct uart_port *port1: first port
const struct uart_port *port2: second port

Description

This utility function can be used to determine whether two uart_port structures describe the same port.

void uart_handle_dcd_change(struct uart_port *uport, bool active)¶: handle a change of carrier detect state

Parameters

struct uart_port *uport: uart_port structure for the open port
bool active: new carrier detect status

Description

Caller must hold uport->lock.

void uart_handle_cts_change(struct uart_port *uport, bool active)¶: handle a change of clear-to-send state

Parameters

struct uart_port *uport: uart_port structure for the open port
bool active: new clear-to-send status

Description

Caller must hold uport->lock.

bool uart_try_toggle_sysrq(struct uart_port *port, u8 ch)¶: Enables SysRq from serial line

Parameters

struct uart_port *port: uart_port structure where char(s) after BREAK met
u8 ch: new character in the sequence after received BREAK

Description

Enables magic SysRq when the required sequence is met on port (see CONFIG_MAGIC_SYSRQ_SERIAL_SEQUENCE).

Return

false if ch is out of enabling sequence and should be handled some other way, true if ch was consumed.

uart_port_tx_limited¶

uart_port_tx_limited (port, ch, count, tx_ready, put_char, tx_done)

transmit helper for uart_port with count limiting

Parameters

port: uart port
ch: variable to store a character to be written to the HW
count: a limit of characters to send
tx_ready: can HW accept more data function
put_char: function to write a character
tx_done: function to call after the loop is done

Description

This helper transmits characters from the xmit buffer to the hardware using put_char(). It does so until count characters are sent and while tx_ready evaluates to true.

The expression in macro parameters shall be designed as follows:

tx_ready: should evaluate to true if the HW can accept more data to be sent. This parameter can be true, which means the HW is always ready.
put_char: shall write ch to the device of port.
tx_done: when the write loop is done, this can perform arbitrary action before potential invocation of ops->stop_tx() happens. If the driver does not need to do anything, use e.g. ({}).

For all of them, port->lock is held, interrupts are locally disabled and the expressions must not sleep.

Return

the number of characters in the xmit buffer when done.

uart_port_tx¶

uart_port_tx (port, ch, tx_ready, put_char)

transmit helper for uart_port

Parameters

port: uart port
ch: variable to store a character to be written to the HW
tx_ready: can HW accept more data function
put_char: function to write a character

Description

See uart_port_tx_limited() for more details.

Other notes¶

It is intended some day to drop the ‘unused’ entries from uart_port, and allow low level drivers to register their own individual uart_port’s with the core. This will allow drivers to use uart_port as a pointer to a structure containing both the uart_port entry with their own extensions, thus:

struct my_port {
        struct uart_port        port;
        int                     my_stuff;
};

Modem control lines via GPIO¶

Some helpers are provided in order to set/get modem control lines via GPIO.

void mctrl_gpio_set(struct mctrl_gpios *gpios, unsigned int mctrl)¶: set gpios according to mctrl state

Parameters

struct mctrl_gpios *gpios: gpios to set
unsigned int mctrl: state to set

Description

Set the gpios according to the mctrl state.

struct gpio_desc *mctrl_gpio_to_gpiod(struct mctrl_gpios *gpios, enum mctrl_gpio_idx gidx)¶: obtain gpio_desc of modem line index

Parameters

struct mctrl_gpios *gpios: gpios to look into
enum mctrl_gpio_idx gidx: index of the modem line

Return

the gpio_desc structure associated to the modem line index

unsigned int mctrl_gpio_get(struct mctrl_gpios *gpios, unsigned int *mctrl)¶: update mctrl with the gpios values.

Parameters

struct mctrl_gpios *gpios: gpios to get the info from
unsigned int *mctrl: mctrl to set

Return

modified mctrl (the same value as in mctrl)

Description

Update mctrl with the gpios values.

struct mctrl_gpios *mctrl_gpio_init(struct uart_port *port, unsigned int idx)¶: initialize uart gpios

Parameters

struct uart_port *port: port to initialize gpios for
unsigned int idx: index of the gpio in the port’s device

Description

This will get the {cts,rts,...}-gpios from device tree if they are present and request them, set direction etc, and return an allocated structure. devm_* functions are used, so there’s no need to call mctrl_gpio_free(). As this sets up the irq handling, make sure to not handle changes to the gpio input lines in your driver, too.

void mctrl_gpio_free(struct device *dev, struct mctrl_gpios *gpios)¶: explicitly free uart gpios

Parameters

struct device *dev: uart port’s device
struct mctrl_gpios *gpios: gpios structure to be freed

Description

This will free the requested gpios in mctrl_gpio_init(). As devm_* functions are used, there’s generally no need to call this function.

void mctrl_gpio_enable_ms(struct mctrl_gpios *gpios)¶: enable irqs and handling of changes to the ms lines

Parameters

struct mctrl_gpios *gpios: gpios to enable

void mctrl_gpio_disable_ms(struct mctrl_gpios *gpios)¶: disable irqs and handling of changes to the ms lines

Parameters

struct mctrl_gpios *gpios: gpios to disable

The Linux Kernel

Contents

This Page

Low Level Serial API¶

Low Level Serial Hardware Driver¶

Console Support¶

Locking¶

uart_ops¶

Other functions¶

Other notes¶

Modem control lines via GPIO¶

Parity error	INPCK	IGNPAR
n/a	0	n/a	character received, marked as `TTY_NORMAL`
None	1	n/a	character received, marked as `TTY_NORMAL`
Yes	1	0	character received, marked as `TTY_PARITY`
Yes	1	1	character discarded