Low Level Serial API — The Linux Kernel documentation (2024)

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 theport is empty. If it is empty, this function should returnTIOCSER_TEMT, otherwise return 0. If the port does not support thisoperation, 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 statedescribed 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 drivenactive. If the bit is clear, the signal should be driveninactive.

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 stateof the outputs should not be returned, since the core keeps track oftheir 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. Ifthe port does not support CTS, DCD or DSR, the driver shouldindicate that the signal is permanently active. If RI isnot 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 linebecoming inactive or the tty layer indicating we want to stoptransmission 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 areclose to full, and it should somehow signal that no more charactersshould 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 thetty layer.

unthrottle

void ()(struct uart_port *port)

Notify the serial driver that characters can now be sent to the serialport without fear of overrunning the input buffers of the linedisciplines.

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

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

send_xchar

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

Transmit a high priority character, even if the port is stopped. Thisis used to implement XON/XOFF flow control and tcflow(). If the serialdriver does not implement this function, the tty core will append thecharacter 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 interruptsshould 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, thebreak signal should be transmitted. The signal should be terminatedwhen 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 thisport.

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 DMAtransfers.

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

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 toindicate the types of events we are interested in receiving. Relevantktermios::c_cflag bits are:

• CSIZE - word size

• CSTOPB - 2 stop bits

• PARENB - parity enable

  See Also

  serial: imx: add rx and tx led triggerserial: imx: support RS-485 Rx disable on Tx

• 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 receivecharacters 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 TTYlayer.

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

• IGNPAR - ignore parity and framing errors.

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

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

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

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

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

set_ldisc

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

Notifier for discipline change. SeeTTY 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 finallyclosed, except when the port is also the system console. This willoccur 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 theport.

Locking: none.Interrupts: caller dependent.

request_port

int ()(struct uart_port *port)

Request any memory and IO region resources required by the port. If anyfail, no resources should be registered when this function returns, andit 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. typecontains a bit mask of the required configuration. UART_CONFIG_TYPEindicates that the port requires detection and identification.port->type should be set to the type found, or PORT_UNKNOWN if noport 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 interruptsinternally 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 issuitable 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 usingthe 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 tosupport poll_put_char() and poll_get_char(). Unlike startup(), thisshould 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 serialport. 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 serialport. If data is available, it should be returned; otherwise thefunction should return NO_POLL_CHAR immediately.

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

Parameters

struct uart_port *port

port to be processed

Description

This routine is used by the interrupt handler to schedule processing in thesoftware interrupt portion of the driver. A driver is expected to call thisfunction when the number of characters in the transmit buffer have droppedbelow a threshold.

Locking: port->lock should be held

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 isderived. The cflag value should reflect the actual hardware settings asnumber of bits, parity, stop bits and baud rate is taken into account here.

Locking: caller is expected to take port->lock

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 themagic 38400 baud rate (with spd_* flags), and mapping the B0 rate to 9600baud.

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

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

Locking: caller dependent

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 divisorinstead.

Locking: caller dependent

Parameters

struct tty_struct *tty

tty associated with the UART

struct uart_state *state

UART being queried

unsigned int __user *value

returned modem value

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

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), andif so, search for the first available port that does have console support.

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

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 consoleoptions. The format of the string is <baud><parity><bits><flow>,eg: 115200n8r

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 serializeearly initialization of the serial-console lock.

Parameters

struct uart_driver *drv

low level driver structure

Description

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

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

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

Locking: none, Interrupts: enabled

Parameters

struct uart_driver *drv

low level driver structure

Description

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

Locking: none, Interrupts: enabled

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_portstructures describe the same port.

Parameters

struct uart_port *uport

uart_port structure for the open port

bool active

new carrier detect status

Description

Caller must hold uport->lock.

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.

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 behandled some other way, true if ch was consumed.

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 usingput_char(). It does so until count characters are sent and while tx_readyevaluates 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 tobe 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 arbitraryaction before potential invocation of ops->stop_tx() happens. If thedriver does not need to do anything, use e.g. ({}).

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

Return

the number of characters in the xmit buffer when done.

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.

Parameters

struct mctrl_gpios *gpios

gpios to set

unsigned int mctrl

state to set

Description

Set the gpios according to the mctrl state.

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

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.

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 presentand 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 thegpio input lines in your driver, too.

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.

Parameters

struct mctrl_gpios *gpios

gpios to enable

Parameters

struct mctrl_gpios *gpios

gpios to disable