6 This document is meant as a brief overview of some aspects of the new serial
7 driver. It is not complete, any questions you have should be directed to
10 The reference implementation is contained within amba_pl011.c.
14 Low Level Serial Hardware Driver
15 --------------------------------
17 The low level serial hardware driver is responsible for supplying port
18 information (defined by uart_port) and a set of control methods (defined
19 by uart_ops) to the core serial driver. The low level driver is also
20 responsible for handling interrupts for the port, and providing any
27 The serial core provides a few helper functions. This includes identifing
28 the correct port structure (via uart_get_console) and decoding command line
29 arguments (uart_parse_options).
31 There is also a helper function (uart_write_console) which performs a
32 character by character write, translating newlines to CRLF sequences.
33 Driver writers are recommended to use this function rather than implementing
40 It is the responsibility of the low level hardware driver to perform the
41 necessary locking using port->lock. There are some exceptions (which
42 are described in the uart_ops listing below.)
44 There are three locks. A per-port spinlock, a per-port tmpbuf semaphore,
45 and an overall semaphore.
47 From the core driver perspective, the port->lock locks the following
52 info->xmit.head (circ->head)
53 info->xmit.tail (circ->tail)
55 The low level driver is free to use this lock to provide any additional
58 The core driver uses the info->tmpbuf_sem lock to prevent multi-threaded
59 access to the info->tmpbuf bouncebuffer used for port writes.
61 The port_sem semaphore is used to protect against ports being added/
62 removed or reconfigured at inappropriate times.
68 The uart_ops structure is the main interface between serial_core and the
69 hardware specific driver. It contains all the methods to control the
73 This function tests whether the transmitter fifo and shifter
74 for the port described by 'port' is empty. If it is empty,
75 this function should return TIOCSER_TEMT, otherwise return 0.
76 If the port does not support this operation, then it should
80 Interrupts: caller dependent.
81 This call must not sleep
83 set_mctrl(port, mctrl)
84 This function sets the modem control lines for port described
85 by 'port' to the state described by mctrl. The relevant bits
87 - TIOCM_RTS RTS signal.
88 - TIOCM_DTR DTR signal.
89 - TIOCM_OUT1 OUT1 signal.
90 - TIOCM_OUT2 OUT2 signal.
91 - TIOCM_LOOP Set the port into loopback mode.
92 If the appropriate bit is set, the signal should be driven
93 active. If the bit is clear, the signal should be driven
96 Locking: port->lock taken.
97 Interrupts: locally disabled.
98 This call must not sleep
101 Returns the current state of modem control inputs. The state
102 of the outputs should not be returned, since the core keeps
103 track of their state. The state information should include:
104 - TIOCM_DCD state of DCD signal
105 - TIOCM_CTS state of CTS signal
106 - TIOCM_DSR state of DSR signal
107 - TIOCM_RI state of RI signal
108 The bit is set if the signal is currently driven active. If
109 the port does not support CTS, DCD or DSR, the driver should
110 indicate that the signal is permanently active. If RI is
111 not available, the signal should not be indicated as active.
113 Locking: port->lock taken.
114 Interrupts: locally disabled.
115 This call must not sleep
118 Stop transmitting characters. This might be due to the CTS
119 line becoming inactive or the tty layer indicating we want
120 to stop transmission due to an XOFF character.
122 The driver should stop transmitting characters as soon as
125 Locking: port->lock taken.
126 Interrupts: locally disabled.
127 This call must not sleep
130 Start transmitting characters.
132 Locking: port->lock taken.
133 Interrupts: locally disabled.
134 This call must not sleep
137 Stop receiving characters; the port is in the process of
140 Locking: port->lock taken.
141 Interrupts: locally disabled.
142 This call must not sleep
145 Enable the modem status interrupts.
147 This method may be called multiple times. Modem status
148 interrupts should be disabled when the shutdown method is
151 Locking: port->lock taken.
152 Interrupts: locally disabled.
153 This call must not sleep
156 Control the transmission of a break signal. If ctl is
157 nonzero, the break signal should be transmitted. The signal
158 should be terminated when another call is made with a zero
162 Interrupts: caller dependent.
163 This call must not sleep
166 Grab any interrupt resources and initialise any low level driver
167 state. Enable the port for reception. It should not activate
168 RTS nor DTR; this will be done via a separate call to set_mctrl.
170 This method will only be called when the port is initially opened.
172 Locking: port_sem taken.
173 Interrupts: globally disabled.
176 Disable the port, disable any break condition that may be in
177 effect, and free any interrupt resources. It should not disable
178 RTS nor DTR; this will have already been done via a separate
181 Drivers must not access port->info once this call has completed.
183 This method will only be called when there are no more users of
186 Locking: port_sem taken.
187 Interrupts: caller dependent.
190 Flush any write buffers, reset any DMA state and stop any
191 ongoing DMA transfers.
193 This will be called whenever the port->info->xmit circular
196 Locking: port->lock taken.
197 Interrupts: locally disabled.
198 This call must not sleep
200 set_termios(port,termios,oldtermios)
201 Change the port parameters, including word length, parity, stop
202 bits. Update read_status_mask and ignore_status_mask to indicate
203 the types of events we are interested in receiving. Relevant
204 termios->c_cflag bits are:
207 PARENB - parity enable
208 PARODD - odd parity (when PARENB is in force)
209 CREAD - enable reception of characters (if not set,
210 still receive characters from the port, but
212 CRTSCTS - if set, enable CTS status change reporting
213 CLOCAL - if not set, enable modem status change
215 Relevant termios->c_iflag bits are:
216 INPCK - enable frame and parity error events to be
217 passed to the TTY layer.
219 PARMRK - both of these enable break events to be
220 passed to the TTY layer.
222 IGNPAR - ignore parity and framing errors
223 IGNBRK - ignore break errors, If IGNPAR is also
224 set, ignore overrun errors as well.
225 The interaction of the iflag bits is as follows (parity error
226 given as an example):
227 Parity error INPCK IGNPAR
228 n/a 0 n/a character received, marked as
230 None 1 n/a character received, marked as
232 Yes 1 0 character received, marked as
234 Yes 1 1 character discarded
236 Other flags may be used (eg, xon/xoff characters) if your
237 hardware supports hardware "soft" flow control.
240 Interrupts: caller dependent.
241 This call must not sleep
243 pm(port,state,oldstate)
244 Perform any power management related activities on the specified
245 port. State indicates the new state (defined by ACPI D0-D3),
246 oldstate indicates the previous state. Essentially, D0 means
247 fully on, D3 means powered down.
249 This function should not be used to grab any resources.
251 This will be called when the port is initially opened and finally
252 closed, except when the port is also the system console. This
253 will occur even if CONFIG_PM is not set.
256 Interrupts: caller dependent.
259 Return a pointer to a string constant describing the specified
260 port, or return NULL, in which case the string 'unknown' is
264 Interrupts: caller dependent.
267 Release any memory and IO region resources currently in use by
271 Interrupts: caller dependent.
274 Request any memory and IO region resources required by the port.
275 If any fail, no resources should be registered when this function
276 returns, and it should return -EBUSY on failure.
279 Interrupts: caller dependent.
281 config_port(port,type)
282 Perform any autoconfiguration steps required for the port. `type`
283 contains a bit mask of the required configuration. UART_CONFIG_TYPE
284 indicates that the port requires detection and identification.
285 port->type should be set to the type found, or PORT_UNKNOWN if
286 no port was detected.
288 UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal,
289 which should be probed using standard kernel autoprobing techniques.
290 This is not necessary on platforms where ports have interrupts
291 internally hard wired (eg, system on a chip implementations).
294 Interrupts: caller dependent.
296 verify_port(port,serinfo)
297 Verify the new serial port information contained within serinfo is
298 suitable for this port type.
301 Interrupts: caller dependent.
304 Perform any port specific IOCTLs. IOCTL commands must be defined
305 using the standard numbering system found in <asm/ioctl.h>
308 Interrupts: caller dependent.
313 uart_update_timeout(port,cflag,baud)
314 Update the FIFO drain timeout, port->timeout, according to the
315 number of bits, parity, stop bits and baud rate.
317 Locking: caller is expected to take port->lock
320 uart_get_baud_rate(port,termios,old,min,max)
321 Return the numeric baud rate for the specified termios, taking
322 account of the special 38400 baud "kludge". The B0 baud rate
323 is mapped to 9600 baud.
325 If the baud rate is not within min..max, then if old is non-NULL,
326 the original baud rate will be tried. If that exceeds the
327 min..max constraint, 9600 baud will be returned. termios will
328 be updated to the baud rate in use.
330 Note: min..max must always allow 9600 baud to be selected.
332 Locking: caller dependent.
335 uart_get_divisor(port,baud)
336 Return the divsor (baud_base / baud) for the specified baud
337 rate, appropriately rounded.
339 If 38400 baud and custom divisor is selected, return the
340 custom divisor instead.
342 Locking: caller dependent.
345 uart_match_port(port1,port2)
346 This utility function can be used to determine whether two
347 uart_port structures describe the same port.
352 uart_write_wakeup(port)
353 A driver is expected to call this function when the number of
354 characters in the transmit buffer have dropped below a threshold.
356 Locking: port->lock should be held.
359 uart_register_driver(drv)
360 Register a uart driver with the core driver. We in turn register
361 with the tty layer, and initialise the core driver per-port state.
363 drv->port should be NULL, and the per-port structures should be
364 registered using uart_add_one_port after this call has succeeded.
369 uart_unregister_driver()
370 Remove all references to a driver from the core driver. The low
371 level driver must have removed all its ports via the
372 uart_remove_one_port() if it registered them with uart_add_one_port().
383 uart_remove_one_port()
388 It is intended some day to drop the 'unused' entries from uart_port, and
389 allow low level drivers to register their own individual uart_port's with
390 the core. This will allow drivers to use uart_port as a pointer to a
391 structure containing both the uart_port entry with their own extensions,
395 struct uart_port port;