550 lines
15 KiB
ReStructuredText
550 lines
15 KiB
ReStructuredText
====================
|
|
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 identifing
|
|
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 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
|
|
--------
|
|
|
|
The uart_ops structure is the main interface between serial_core and the
|
|
hardware specific driver. It contains all the methods to control the
|
|
hardware.
|
|
|
|
tx_empty(port)
|
|
This function tests whether the transmitter fifo and shifter
|
|
for the port described by '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(port, mctrl)
|
|
This function sets the modem control lines for port described
|
|
by '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(port)
|
|
Returns the current state of modem control inputs. 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(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(port)
|
|
Start transmitting characters.
|
|
|
|
Locking: port->lock taken.
|
|
|
|
Interrupts: locally disabled.
|
|
|
|
This call must not sleep
|
|
|
|
throttle(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(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(port,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(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
|
|
|
|
enable_ms(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(port,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(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(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(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(port,termios,oldtermios)
|
|
Change the port parameters, including word length, parity, stop
|
|
bits. Update read_status_mask and ignore_status_mask to indicate
|
|
the types of events we are interested in receiving. Relevant
|
|
termios->c_cflag bits are:
|
|
|
|
CSIZE
|
|
- word size
|
|
CSTOPB
|
|
- 2 stop bits
|
|
PARENB
|
|
- parity enable
|
|
PARODD
|
|
- odd parity (when PARENB is in force)
|
|
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 termios->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 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(port,termios)
|
|
Notifier for discipline change. See Documentation/driver-api/serial/tty.rst.
|
|
|
|
Locking: caller holds tty_port->mutex
|
|
|
|
pm(port,state,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(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(port)
|
|
Release any memory and IO region resources currently in use by
|
|
the port.
|
|
|
|
Locking: none.
|
|
|
|
Interrupts: caller dependent.
|
|
|
|
request_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(port,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(port,serinfo)
|
|
Verify the new serial port information contained within serinfo is
|
|
suitable for this port type.
|
|
|
|
Locking: none.
|
|
|
|
Interrupts: caller dependent.
|
|
|
|
ioctl(port,cmd,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(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(port,ch)
|
|
Called by kgdb to write a single character 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(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
|
|
|
|
Other functions
|
|
---------------
|
|
|
|
uart_update_timeout(port,cflag,baud)
|
|
Update the FIFO drain timeout, port->timeout, according to the
|
|
number of bits, parity, stop bits and baud rate.
|
|
|
|
Locking: caller is expected to take port->lock
|
|
|
|
Interrupts: n/a
|
|
|
|
uart_get_baud_rate(port,termios,old,min,max)
|
|
Return the numeric baud rate for the specified termios, taking
|
|
account of the special 38400 baud "kludge". The B0 baud rate
|
|
is mapped to 9600 baud.
|
|
|
|
If the baud rate is not within min..max, then if old is non-NULL,
|
|
the original baud rate will be tried. If that exceeds the
|
|
min..max constraint, 9600 baud will be returned. termios will
|
|
be updated to the baud rate in use.
|
|
|
|
Note: min..max must always allow 9600 baud to be selected.
|
|
|
|
Locking: caller dependent.
|
|
|
|
Interrupts: n/a
|
|
|
|
uart_get_divisor(port,baud)
|
|
Return the divisor (baud_base / baud) for the specified baud
|
|
rate, appropriately rounded.
|
|
|
|
If 38400 baud and custom divisor is selected, return the
|
|
custom divisor instead.
|
|
|
|
Locking: caller dependent.
|
|
|
|
Interrupts: n/a
|
|
|
|
uart_match_port(port1,port2)
|
|
This utility function can be used to determine whether two
|
|
uart_port structures describe the same port.
|
|
|
|
Locking: n/a
|
|
|
|
Interrupts: n/a
|
|
|
|
uart_write_wakeup(port)
|
|
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.
|
|
|
|
Interrupts: n/a
|
|
|
|
uart_register_driver(drv)
|
|
Register a uart driver with the core driver. We in turn register
|
|
with the tty layer, and initialise the core driver per-port state.
|
|
|
|
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
|
|
|
|
uart_unregister_driver()
|
|
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().
|
|
|
|
Locking: none
|
|
|
|
Interrupts: enabled
|
|
|
|
**uart_suspend_port()**
|
|
|
|
**uart_resume_port()**
|
|
|
|
**uart_add_one_port()**
|
|
|
|
**uart_remove_one_port()**
|
|
|
|
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.
|
|
|
|
mctrl_gpio_init(port, idx):
|
|
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.
|
|
|
|
mctrl_gpio_free(dev, gpios):
|
|
This will free the requested gpios in mctrl_gpio_init().
|
|
As `devm_*` functions are used, there's generally no need to call
|
|
this function.
|
|
|
|
mctrl_gpio_to_gpiod(gpios, gidx)
|
|
This returns the gpio_desc structure associated to the modem line
|
|
index.
|
|
|
|
mctrl_gpio_set(gpios, mctrl):
|
|
This will sets the gpios according to the mctrl state.
|
|
|
|
mctrl_gpio_get(gpios, mctrl):
|
|
This will update mctrl with the gpios values.
|
|
|
|
mctrl_gpio_enable_ms(gpios):
|
|
Enables irqs and handling of changes to the ms lines.
|
|
|
|
mctrl_gpio_disable_ms(gpios):
|
|
Disables irqs and handling of changes to the ms lines.
|