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1
2 Low Level Serial API
3 --------------------
4
5
6This document is meant as a brief overview of some aspects of the new serial
7driver. It is not complete, any questions you have should be directed to
8<rmk@arm.linux.org.uk>
9
10The reference implementation is contained within amba_pl011.c.
11
12
13
14Low Level Serial Hardware Driver
15--------------------------------
16
17The low level serial hardware driver is responsible for supplying port
18information (defined by uart_port) and a set of control methods (defined
19by uart_ops) to the core serial driver. The low level driver is also
20responsible for handling interrupts for the port, and providing any
21console support.
22
23
24Console Support
25---------------
26
27The serial core provides a few helper functions. This includes identifing
28the correct port structure (via uart_get_console) and decoding command line
29arguments (uart_parse_options).
30
31There is also a helper function (uart_write_console) which performs a
32character by character write, translating newlines to CRLF sequences.
33Driver writers are recommended to use this function rather than implementing
34their own version.
35
36
37Locking
38-------
39
40It is the responsibility of the low level hardware driver to perform the
41necessary locking using port->lock. There are some exceptions (which
42are described in the uart_ops listing below.)
43
44There are three locks. A per-port spinlock, a per-port tmpbuf semaphore,
45and an overall semaphore.
46
47From the core driver perspective, the port->lock locks the following
48data:
49
50 port->mctrl
51 port->icount
52 info->xmit.head (circ->head)
53 info->xmit.tail (circ->tail)
54
55The low level driver is free to use this lock to provide any additional
56locking.
57
58The core driver uses the info->tmpbuf_sem lock to prevent multi-threaded
59access to the info->tmpbuf bouncebuffer used for port writes.
60
61The port_sem semaphore is used to protect against ports being added/
62removed or reconfigured at inappropriate times.
63
64
65uart_ops
66--------
67
68The uart_ops structure is the main interface between serial_core and the
69hardware specific driver. It contains all the methods to control the
70hardware.
71
72 tx_empty(port)
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
77 return TIOCSER_TEMT.
78
79 Locking: none.
80 Interrupts: caller dependent.
81 This call must not sleep
82
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
86 of mctrl are:
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
94 inactive.
95
96 Locking: port->lock taken.
97 Interrupts: locally disabled.
98 This call must not sleep
99
100 get_mctrl(port)
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_CAR 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.
112
113 Locking: port->lock taken.
114 Interrupts: locally disabled.
115 This call must not sleep
116
117 stop_tx(port)
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.
121
122 The driver should stop transmitting characters as soon as
123 possible.
124
125 Locking: port->lock taken.
126 Interrupts: locally disabled.
127 This call must not sleep
128
129 start_tx(port)
130 Start transmitting characters.
131
132 Locking: port->lock taken.
133 Interrupts: locally disabled.
134 This call must not sleep
135
136 send_xchar(port,ch)
137 Transmit a high priority character, even if the port is stopped.
138 This is used to implement XON/XOFF flow control and tcflow(). If
139 the serial driver does not implement this function, the tty core
140 will append the character to the circular buffer and then call
141 start_tx() / stop_tx() to flush the data out.
142
143 Locking: none.
144 Interrupts: caller dependent.
145
146 stop_rx(port)
147 Stop receiving characters; the port is in the process of
148 being closed.
149
150 Locking: port->lock taken.
151 Interrupts: locally disabled.
152 This call must not sleep
153
154 enable_ms(port)
155 Enable the modem status interrupts.
156
157 This method may be called multiple times. Modem status
158 interrupts should be disabled when the shutdown method is
159 called.
160
161 Locking: port->lock taken.
162 Interrupts: locally disabled.
163 This call must not sleep
164
165 break_ctl(port,ctl)
166 Control the transmission of a break signal. If ctl is
167 nonzero, the break signal should be transmitted. The signal
168 should be terminated when another call is made with a zero
169 ctl.
170
171 Locking: none.
172 Interrupts: caller dependent.
173 This call must not sleep
174
175 startup(port)
176 Grab any interrupt resources and initialise any low level driver
177 state. Enable the port for reception. It should not activate
178 RTS nor DTR; this will be done via a separate call to set_mctrl.
179
180 This method will only be called when the port is initially opened.
181
182 Locking: port_sem taken.
183 Interrupts: globally disabled.
184
185 shutdown(port)
186 Disable the port, disable any break condition that may be in
187 effect, and free any interrupt resources. It should not disable
188 RTS nor DTR; this will have already been done via a separate
189 call to set_mctrl.
190
191 Drivers must not access port->info once this call has completed.
192
193 This method will only be called when there are no more users of
194 this port.
195
196 Locking: port_sem taken.
197 Interrupts: caller dependent.
198
199 flush_buffer(port)
200 Flush any write buffers, reset any DMA state and stop any
201 ongoing DMA transfers.
202
203 This will be called whenever the port->info->xmit circular
204 buffer is cleared.
205
206 Locking: port->lock taken.
207 Interrupts: locally disabled.
208 This call must not sleep
209
210 set_termios(port,termios,oldtermios)
211 Change the port parameters, including word length, parity, stop
212 bits. Update read_status_mask and ignore_status_mask to indicate
213 the types of events we are interested in receiving. Relevant
214 termios->c_cflag bits are:
215 CSIZE - word size
216 CSTOPB - 2 stop bits
217 PARENB - parity enable
218 PARODD - odd parity (when PARENB is in force)
219 CREAD - enable reception of characters (if not set,
220 still receive characters from the port, but
221 throw them away.
222 CRTSCTS - if set, enable CTS status change reporting
223 CLOCAL - if not set, enable modem status change
224 reporting.
225 Relevant termios->c_iflag bits are:
226 INPCK - enable frame and parity error events to be
227 passed to the TTY layer.
228 BRKINT
229 PARMRK - both of these enable break events to be
230 passed to the TTY layer.
231
232 IGNPAR - ignore parity and framing errors
233 IGNBRK - ignore break errors, If IGNPAR is also
234 set, ignore overrun errors as well.
235 The interaction of the iflag bits is as follows (parity error
236 given as an example):
237 Parity error INPCK IGNPAR
238 n/a 0 n/a character received, marked as
239 TTY_NORMAL
240 None 1 n/a character received, marked as
241 TTY_NORMAL
242 Yes 1 0 character received, marked as
243 TTY_PARITY
244 Yes 1 1 character discarded
245
246 Other flags may be used (eg, xon/xoff characters) if your
247 hardware supports hardware "soft" flow control.
248
249 Locking: none.
250 Interrupts: caller dependent.
251 This call must not sleep
252
253 pm(port,state,oldstate)
254 Perform any power management related activities on the specified
255 port. State indicates the new state (defined by
256 enum uart_pm_state), oldstate indicates the previous state.
257
258 This function should not be used to grab any resources.
259
260 This will be called when the port is initially opened and finally
261 closed, except when the port is also the system console. This
262 will occur even if CONFIG_PM is not set.
263
264 Locking: none.
265 Interrupts: caller dependent.
266
267 type(port)
268 Return a pointer to a string constant describing the specified
269 port, or return NULL, in which case the string 'unknown' is
270 substituted.
271
272 Locking: none.
273 Interrupts: caller dependent.
274
275 release_port(port)
276 Release any memory and IO region resources currently in use by
277 the port.
278
279 Locking: none.
280 Interrupts: caller dependent.
281
282 request_port(port)
283 Request any memory and IO region resources required by the port.
284 If any fail, no resources should be registered when this function
285 returns, and it should return -EBUSY on failure.
286
287 Locking: none.
288 Interrupts: caller dependent.
289
290 config_port(port,type)
291 Perform any autoconfiguration steps required for the port. `type`
292 contains a bit mask of the required configuration. UART_CONFIG_TYPE
293 indicates that the port requires detection and identification.
294 port->type should be set to the type found, or PORT_UNKNOWN if
295 no port was detected.
296
297 UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal,
298 which should be probed using standard kernel autoprobing techniques.
299 This is not necessary on platforms where ports have interrupts
300 internally hard wired (eg, system on a chip implementations).
301
302 Locking: none.
303 Interrupts: caller dependent.
304
305 verify_port(port,serinfo)
306 Verify the new serial port information contained within serinfo is
307 suitable for this port type.
308
309 Locking: none.
310 Interrupts: caller dependent.
311
312 ioctl(port,cmd,arg)
313 Perform any port specific IOCTLs. IOCTL commands must be defined
314 using the standard numbering system found in <asm/ioctl.h>
315
316 Locking: none.
317 Interrupts: caller dependent.
318
319 poll_init(port)
320 Called by kgdb to perform the minimal hardware initialization needed
321 to support poll_put_char() and poll_get_char(). Unlike ->startup()
322 this should not request interrupts.
323
324 Locking: tty_mutex and tty_port->mutex taken.
325 Interrupts: n/a.
326
327 poll_put_char(port,ch)
328 Called by kgdb to write a single character directly to the serial
329 port. It can and should block until there is space in the TX FIFO.
330
331 Locking: none.
332 Interrupts: caller dependent.
333 This call must not sleep
334
335 poll_get_char(port)
336 Called by kgdb to read a single character directly from the serial
337 port. If data is available, it should be returned; otherwise
338 the function should return NO_POLL_CHAR immediately.
339
340 Locking: none.
341 Interrupts: caller dependent.
342 This call must not sleep
343
344Other functions
345---------------
346
347uart_update_timeout(port,cflag,baud)
348 Update the FIFO drain timeout, port->timeout, according to the
349 number of bits, parity, stop bits and baud rate.
350
351 Locking: caller is expected to take port->lock
352 Interrupts: n/a
353
354uart_get_baud_rate(port,termios,old,min,max)
355 Return the numeric baud rate for the specified termios, taking
356 account of the special 38400 baud "kludge". The B0 baud rate
357 is mapped to 9600 baud.
358
359 If the baud rate is not within min..max, then if old is non-NULL,
360 the original baud rate will be tried. If that exceeds the
361 min..max constraint, 9600 baud will be returned. termios will
362 be updated to the baud rate in use.
363
364 Note: min..max must always allow 9600 baud to be selected.
365
366 Locking: caller dependent.
367 Interrupts: n/a
368
369uart_get_divisor(port,baud)
370 Return the divsor (baud_base / baud) for the specified baud
371 rate, appropriately rounded.
372
373 If 38400 baud and custom divisor is selected, return the
374 custom divisor instead.
375
376 Locking: caller dependent.
377 Interrupts: n/a
378
379uart_match_port(port1,port2)
380 This utility function can be used to determine whether two
381 uart_port structures describe the same port.
382
383 Locking: n/a
384 Interrupts: n/a
385
386uart_write_wakeup(port)
387 A driver is expected to call this function when the number of
388 characters in the transmit buffer have dropped below a threshold.
389
390 Locking: port->lock should be held.
391 Interrupts: n/a
392
393uart_register_driver(drv)
394 Register a uart driver with the core driver. We in turn register
395 with the tty layer, and initialise the core driver per-port state.
396
397 drv->port should be NULL, and the per-port structures should be
398 registered using uart_add_one_port after this call has succeeded.
399
400 Locking: none
401 Interrupts: enabled
402
403uart_unregister_driver()
404 Remove all references to a driver from the core driver. The low
405 level driver must have removed all its ports via the
406 uart_remove_one_port() if it registered them with uart_add_one_port().
407
408 Locking: none
409 Interrupts: enabled
410
411uart_suspend_port()
412
413uart_resume_port()
414
415uart_add_one_port()
416
417uart_remove_one_port()
418
419Other notes
420-----------
421
422It is intended some day to drop the 'unused' entries from uart_port, and
423allow low level drivers to register their own individual uart_port's with
424the core. This will allow drivers to use uart_port as a pointer to a
425structure containing both the uart_port entry with their own extensions,
426thus:
427
428 struct my_port {
429 struct uart_port port;
430 int my_stuff;
431 };