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Diffstat (limited to 'Documentation/serial/driver')
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diff --git a/Documentation/serial/driver b/Documentation/serial/driver new file mode 100644 index 00000000000..e9c0178cd20 --- /dev/null +++ b/Documentation/serial/driver @@ -0,0 +1,330 @@ + + Low Level Serial API + -------------------- + + + $Id: driver,v 1.10 2002/07/22 15:27:30 rmk Exp $ + + +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 serial_amba.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). + + +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. + 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_DCD 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: none. + Interrupts: caller dependent. + This call must not sleep + + stop_tx(port,tty_stop) + Stop transmitting characters. This might be due to the CTS + line becoming inactive or the tty layer indicating we want + to stop transmission. + + tty_stop: 1 if this call is due to the TTY layer issuing a + TTY stop to the driver (equiv to rs_stop). + + Locking: port->lock taken. + Interrupts: locally disabled. + This call must not sleep + + start_tx(port,tty_start) + start transmitting characters. (incidentally, nonempty will + always be nonzero, and shouldn't be used - it will be dropped). + + tty_start: 1 if this call was due to the TTY layer issuing + a TTY start to the driver (equiv to rs_start) + + Locking: port->lock taken. + Interrupts: locally disabled. + This call must not sleep + + 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. + + 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. + + 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. + + Locking: port_sem taken. + Interrupts: caller dependent. + + 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 + None n/a n/a character received + Yes n/a 0 character discarded + Yes 0 1 character received, marked as + TTY_NORMAL + Yes 1 1 character received, marked as + TTY_PARITY + + 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 ACPI D0-D3), + oldstate indicates the previous state. Essentially, D0 means + fully on, D3 means powered down. + + 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. + +Other functions +--------------- + +uart_update_timeout(port,cflag,quot) + Update the FIFO drain timeout, port->timeout, according to the + number of bits, parity, stop bits and quotient. + + Locking: caller is expected to take port->lock + Interrupts: n/a + +uart_get_baud_rate(port,termios) + 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. + + Locking: caller dependent. + Interrupts: n/a + +uart_get_divisor(port,termios,oldtermios) + Return the divsor (baud_base / baud) for the selected baud rate + specified by termios. If the baud rate is out of range, try + the original baud rate specified by oldtermios (if non-NULL). + If that fails, try 9600 baud. + + If 38400 baud and custom divisor is selected, return the + custom divisor instead. + + Locking: caller dependent. + Interrupts: n/a + +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; + }; |