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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_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.
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 stop_rx(port)
137 Stop receiving characters; the port is in the process of
138 being closed.
139
140 Locking: port->lock taken.
141 Interrupts: locally disabled.
142 This call must not sleep
143
144 enable_ms(port)
145 Enable the modem status interrupts.
146
147 This method may be called multiple times. Modem status
148 interrupts should be disabled when the shutdown method is
149 called.
150
151 Locking: port->lock taken.
152 Interrupts: locally disabled.
153 This call must not sleep
154
155 break_ctl(port,ctl)
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
159 ctl.
160
161 Locking: none.
162 Interrupts: caller dependent.
163 This call must not sleep
164
165 startup(port)
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.
169
170 This method will only be called when the port is initially opened.
171
172 Locking: port_sem taken.
173 Interrupts: globally disabled.
174
175 shutdown(port)
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
179 call to set_mctrl.
180
181 Drivers must not access port->info once this call has completed.
182
183 This method will only be called when there are no more users of
184 this port.
185
186 Locking: port_sem taken.
187 Interrupts: caller dependent.
188
189 set_termios(port,termios,oldtermios)
190 Change the port parameters, including word length, parity, stop
191 bits. Update read_status_mask and ignore_status_mask to indicate
192 the types of events we are interested in receiving. Relevant
193 termios->c_cflag bits are:
194 CSIZE - word size
195 CSTOPB - 2 stop bits
196 PARENB - parity enable
197 PARODD - odd parity (when PARENB is in force)
198 CREAD - enable reception of characters (if not set,
199 still receive characters from the port, but
200 throw them away.
201 CRTSCTS - if set, enable CTS status change reporting
202 CLOCAL - if not set, enable modem status change
203 reporting.
204 Relevant termios->c_iflag bits are:
205 INPCK - enable frame and parity error events to be
206 passed to the TTY layer.
207 BRKINT
208 PARMRK - both of these enable break events to be
209 passed to the TTY layer.
210
211 IGNPAR - ignore parity and framing errors
212 IGNBRK - ignore break errors, If IGNPAR is also
213 set, ignore overrun errors as well.
214 The interaction of the iflag bits is as follows (parity error
215 given as an example):
216 Parity error INPCK IGNPAR
217 n/a 0 n/a character received, marked as
218 TTY_NORMAL
219 None 1 n/a character received, marked as
220 TTY_NORMAL
221 Yes 1 0 character received, marked as
222 TTY_PARITY
223 Yes 1 1 character discarded
224
225 Other flags may be used (eg, xon/xoff characters) if your
226 hardware supports hardware "soft" flow control.
227
228 Locking: none.
229 Interrupts: caller dependent.
230 This call must not sleep
231
232 pm(port,state,oldstate)
233 Perform any power management related activities on the specified
234 port. State indicates the new state (defined by ACPI D0-D3),
235 oldstate indicates the previous state. Essentially, D0 means
236 fully on, D3 means powered down.
237
238 This function should not be used to grab any resources.
239
240 This will be called when the port is initially opened and finally
241 closed, except when the port is also the system console. This
242 will occur even if CONFIG_PM is not set.
243
244 Locking: none.
245 Interrupts: caller dependent.
246
247 type(port)
248 Return a pointer to a string constant describing the specified
249 port, or return NULL, in which case the string 'unknown' is
250 substituted.
251
252 Locking: none.
253 Interrupts: caller dependent.
254
255 release_port(port)
256 Release any memory and IO region resources currently in use by
257 the port.
258
259 Locking: none.
260 Interrupts: caller dependent.
261
262 request_port(port)
263 Request any memory and IO region resources required by the port.
264 If any fail, no resources should be registered when this function
265 returns, and it should return -EBUSY on failure.
266
267 Locking: none.
268 Interrupts: caller dependent.
269
270 config_port(port,type)
271 Perform any autoconfiguration steps required for the port. `type`
272 contains a bit mask of the required configuration. UART_CONFIG_TYPE
273 indicates that the port requires detection and identification.
274 port->type should be set to the type found, or PORT_UNKNOWN if
275 no port was detected.
276
277 UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal,
278 which should be probed using standard kernel autoprobing techniques.
279 This is not necessary on platforms where ports have interrupts
280 internally hard wired (eg, system on a chip implementations).
281
282 Locking: none.
283 Interrupts: caller dependent.
284
285 verify_port(port,serinfo)
286 Verify the new serial port information contained within serinfo is
287 suitable for this port type.
288
289 Locking: none.
290 Interrupts: caller dependent.
291
292 ioctl(port,cmd,arg)
293 Perform any port specific IOCTLs. IOCTL commands must be defined
294 using the standard numbering system found in <asm/ioctl.h>
295
296 Locking: none.
297 Interrupts: caller dependent.
298
299Other functions
300---------------
301
302uart_update_timeout(port,cflag,baud)
303 Update the FIFO drain timeout, port->timeout, according to the
304 number of bits, parity, stop bits and baud rate.
305
306 Locking: caller is expected to take port->lock
307 Interrupts: n/a
308
309uart_get_baud_rate(port,termios,old,min,max)
310 Return the numeric baud rate for the specified termios, taking
311 account of the special 38400 baud "kludge". The B0 baud rate
312 is mapped to 9600 baud.
313
314 If the baud rate is not within min..max, then if old is non-NULL,
315 the original baud rate will be tried. If that exceeds the
316 min..max constraint, 9600 baud will be returned. termios will
317 be updated to the baud rate in use.
318
319 Note: min..max must always allow 9600 baud to be selected.
320
321 Locking: caller dependent.
322 Interrupts: n/a
323
324uart_get_divisor(port,baud)
325 Return the divsor (baud_base / baud) for the specified baud
326 rate, appropriately rounded.
327
328 If 38400 baud and custom divisor is selected, return the
329 custom divisor instead.
330
331 Locking: caller dependent.
332 Interrupts: n/a
333
334uart_match_port(port1,port2)
335 This utility function can be used to determine whether two
336 uart_port structures describe the same port.
337
338 Locking: n/a
339 Interrupts: n/a
340
341uart_write_wakeup(port)
342 A driver is expected to call this function when the number of
343 characters in the transmit buffer have dropped below a threshold.
344
345 Locking: port->lock should be held.
346 Interrupts: n/a
347
348uart_register_driver(drv)
349 Register a uart driver with the core driver. We in turn register
350 with the tty layer, and initialise the core driver per-port state.
351
352 drv->port should be NULL, and the per-port structures should be
353 registered using uart_add_one_port after this call has succeeded.
354
355 Locking: none
356 Interrupts: enabled
357
358uart_unregister_driver()
359 Remove all references to a driver from the core driver. The low
360 level driver must have removed all its ports via the
361 uart_remove_one_port() if it registered them with uart_add_one_port().
362
363 Locking: none
364 Interrupts: enabled
365
366uart_suspend_port()
367
368uart_resume_port()
369
370uart_add_one_port()
371
372uart_remove_one_port()
373
374Other notes
375-----------
376
377It is intended some day to drop the 'unused' entries from uart_port, and
378allow low level drivers to register their own individual uart_port's with
379the core. This will allow drivers to use uart_port as a pointer to a
380structure containing both the uart_port entry with their own extensions,
381thus:
382
383 struct my_port {
384 struct uart_port port;
385 int my_stuff;
386 };