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_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: 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 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. 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 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,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; };