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_write_console) 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 three locks. A per-port spinlock, a per-port tmpbuf semaphore,
and an overall semaphore.
From the core driver perspective, the port->lock locks the following
data:
port->mctrl
port->icount
info->xmit.head (circ->head)
info->xmit.tail (circ->tail)
The low level driver is free to use this lock to provide any additional
locking.
The core driver uses the info->tmpbuf_sem lock to prevent multi-threaded
access to the info->tmpbuf bouncebuffer used for port writes.
The port_sem semaphore is used to protect against ports being added/
removed or reconfigured at inappropriate times.
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
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: none.
Interrupts: caller dependent.
This call must not sleep
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->info 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->info->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: none.
Interrupts: caller dependent.
This call must not sleep
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 divsor (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(dev, 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().
mctrl_gpio_free(dev, gpios):
This will free the requested gpios in mctrl_gpio_init().
As devm_* function are used, there's generally no need to call
this function.
mctrl_gpio_to_gpiod(gpios, gidx)
This returns the gpio 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.