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1
2 Low Level Serial API
3 --------------------
4
5
6 $Id: driver,v 1.10 2002/07/22 15:27:30 rmk Exp $
7
8
9This document is meant as a brief overview of some aspects of the new serial
10driver. It is not complete, any questions you have should be directed to
11<rmk@arm.linux.org.uk>
12
13The reference implementation is contained within serial_amba.c.
14
15
16
17Low Level Serial Hardware Driver
18--------------------------------
19
20The low level serial hardware driver is responsible for supplying port
21information (defined by uart_port) and a set of control methods (defined
22by uart_ops) to the core serial driver. The low level driver is also
23responsible for handling interrupts for the port, and providing any
24console support.
25
26
27Console Support
28---------------
29
30The serial core provides a few helper functions. This includes identifing
31the correct port structure (via uart_get_console) and decoding command line
32arguments (uart_parse_options).
33
34
35Locking
36-------
37
38It is the responsibility of the low level hardware driver to perform the
39necessary locking using port->lock. There are some exceptions (which
40are described in the uart_ops listing below.)
41
42There are three locks. A per-port spinlock, a per-port tmpbuf semaphore,
43and an overall semaphore.
44
45From the core driver perspective, the port->lock locks the following
46data:
47
48 port->mctrl
49 port->icount
50 info->xmit.head (circ->head)
51 info->xmit.tail (circ->tail)
52
53The low level driver is free to use this lock to provide any additional
54locking.
55
56The core driver uses the info->tmpbuf_sem lock to prevent multi-threaded
57access to the info->tmpbuf bouncebuffer used for port writes.
58
59The port_sem semaphore is used to protect against ports being added/
60removed or reconfigured at inappropriate times.
61
62
63uart_ops
64--------
65
66The uart_ops structure is the main interface between serial_core and the
67hardware specific driver. It contains all the methods to control the
68hardware.
69
70 tx_empty(port)
71 This function tests whether the transmitter fifo and shifter
72 for the port described by 'port' is empty. If it is empty,
73 this function should return TIOCSER_TEMT, otherwise return 0.
74 If the port does not support this operation, then it should
75 return TIOCSER_TEMT.
76
77 Locking: none.
78 Interrupts: caller dependent.
79 This call must not sleep
80
81 set_mctrl(port, mctrl)
82 This function sets the modem control lines for port described
83 by 'port' to the state described by mctrl. The relevant bits
84 of mctrl are:
85 - TIOCM_RTS RTS signal.
86 - TIOCM_DTR DTR signal.
87 - TIOCM_OUT1 OUT1 signal.
88 - TIOCM_OUT2 OUT2 signal.
89 If the appropriate bit is set, the signal should be driven
90 active. If the bit is clear, the signal should be driven
91 inactive.
92
93 Locking: port->lock taken.
94 Interrupts: locally disabled.
95 This call must not sleep
96
97 get_mctrl(port)
98 Returns the current state of modem control inputs. The state
99 of the outputs should not be returned, since the core keeps
100 track of their state. The state information should include:
101 - TIOCM_DCD state of DCD signal
102 - TIOCM_CTS state of CTS signal
103 - TIOCM_DSR state of DSR signal
104 - TIOCM_RI state of RI signal
105 The bit is set if the signal is currently driven active. If
106 the port does not support CTS, DCD or DSR, the driver should
107 indicate that the signal is permanently active. If RI is
108 not available, the signal should not be indicated as active.
109
110 Locking: port->lock taken.
111 Interrupts: locally disabled.
112 This call must not sleep
113
114 stop_tx(port)
115 Stop transmitting characters. This might be due to the CTS
116 line becoming inactive or the tty layer indicating we want
117 to stop transmission due to an XOFF character.
118
119 The driver should stop transmitting characters as soon as
120 possible.
121
122 Locking: port->lock taken.
123 Interrupts: locally disabled.
124 This call must not sleep
125
126 start_tx(port)
127 Start transmitting characters.
128
129 Locking: port->lock taken.
130 Interrupts: locally disabled.
131 This call must not sleep
132
133 stop_rx(port)
134 Stop receiving characters; the port is in the process of
135 being closed.
136
137 Locking: port->lock taken.
138 Interrupts: locally disabled.
139 This call must not sleep
140
141 enable_ms(port)
142 Enable the modem status interrupts.
143
144 Locking: port->lock taken.
145 Interrupts: locally disabled.
146 This call must not sleep
147
148 break_ctl(port,ctl)
149 Control the transmission of a break signal. If ctl is
150 nonzero, the break signal should be transmitted. The signal
151 should be terminated when another call is made with a zero
152 ctl.
153
154 Locking: none.
155 Interrupts: caller dependent.
156 This call must not sleep
157
158 startup(port)
159 Grab any interrupt resources and initialise any low level driver
160 state. Enable the port for reception. It should not activate
161 RTS nor DTR; this will be done via a separate call to set_mctrl.
162
163 Locking: port_sem taken.
164 Interrupts: globally disabled.
165
166 shutdown(port)
167 Disable the port, disable any break condition that may be in
168 effect, and free any interrupt resources. It should not disable
169 RTS nor DTR; this will have already been done via a separate
170 call to set_mctrl.
171
172 Locking: port_sem taken.
173 Interrupts: caller dependent.
174
175 set_termios(port,termios,oldtermios)
176 Change the port parameters, including word length, parity, stop
177 bits. Update read_status_mask and ignore_status_mask to indicate
178 the types of events we are interested in receiving. Relevant
179 termios->c_cflag bits are:
180 CSIZE - word size
181 CSTOPB - 2 stop bits
182 PARENB - parity enable
183 PARODD - odd parity (when PARENB is in force)
184 CREAD - enable reception of characters (if not set,
185 still receive characters from the port, but
186 throw them away.
187 CRTSCTS - if set, enable CTS status change reporting
188 CLOCAL - if not set, enable modem status change
189 reporting.
190 Relevant termios->c_iflag bits are:
191 INPCK - enable frame and parity error events to be
192 passed to the TTY layer.
193 BRKINT
194 PARMRK - both of these enable break events to be
195 passed to the TTY layer.
196
197 IGNPAR - ignore parity and framing errors
198 IGNBRK - ignore break errors, If IGNPAR is also
199 set, ignore overrun errors as well.
200 The interaction of the iflag bits is as follows (parity error
201 given as an example):
202 Parity error INPCK IGNPAR
203 None n/a n/a character received
204 Yes n/a 0 character discarded
205 Yes 0 1 character received, marked as
206 TTY_NORMAL
207 Yes 1 1 character received, marked as
208 TTY_PARITY
209
210 Other flags may be used (eg, xon/xoff characters) if your
211 hardware supports hardware "soft" flow control.
212
213 Locking: none.
214 Interrupts: caller dependent.
215 This call must not sleep
216
217 pm(port,state,oldstate)
218 Perform any power management related activities on the specified
219 port. State indicates the new state (defined by ACPI D0-D3),
220 oldstate indicates the previous state. Essentially, D0 means
221 fully on, D3 means powered down.
222
223 This function should not be used to grab any resources.
224
225 This will be called when the port is initially opened and finally
226 closed, except when the port is also the system console. This
227 will occur even if CONFIG_PM is not set.
228
229 Locking: none.
230 Interrupts: caller dependent.
231
232 type(port)
233 Return a pointer to a string constant describing the specified
234 port, or return NULL, in which case the string 'unknown' is
235 substituted.
236
237 Locking: none.
238 Interrupts: caller dependent.
239
240 release_port(port)
241 Release any memory and IO region resources currently in use by
242 the port.
243
244 Locking: none.
245 Interrupts: caller dependent.
246
247 request_port(port)
248 Request any memory and IO region resources required by the port.
249 If any fail, no resources should be registered when this function
250 returns, and it should return -EBUSY on failure.
251
252 Locking: none.
253 Interrupts: caller dependent.
254
255 config_port(port,type)
256 Perform any autoconfiguration steps required for the port. `type`
257 contains a bit mask of the required configuration. UART_CONFIG_TYPE
258 indicates that the port requires detection and identification.
259 port->type should be set to the type found, or PORT_UNKNOWN if
260 no port was detected.
261
262 UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal,
263 which should be probed using standard kernel autoprobing techniques.
264 This is not necessary on platforms where ports have interrupts
265 internally hard wired (eg, system on a chip implementations).
266
267 Locking: none.
268 Interrupts: caller dependent.
269
270 verify_port(port,serinfo)
271 Verify the new serial port information contained within serinfo is
272 suitable for this port type.
273
274 Locking: none.
275 Interrupts: caller dependent.
276
277 ioctl(port,cmd,arg)
278 Perform any port specific IOCTLs. IOCTL commands must be defined
279 using the standard numbering system found in <asm/ioctl.h>
280
281 Locking: none.
282 Interrupts: caller dependent.
283
284Other functions
285---------------
286
287uart_update_timeout(port,cflag,baud)
288 Update the FIFO drain timeout, port->timeout, according to the
289 number of bits, parity, stop bits and baud rate.
290
291 Locking: caller is expected to take port->lock
292 Interrupts: n/a
293
294uart_get_baud_rate(port,termios,old,min,max)
295 Return the numeric baud rate for the specified termios, taking
296 account of the special 38400 baud "kludge". The B0 baud rate
297 is mapped to 9600 baud.
298
299 If the baud rate is not within min..max, then if old is non-NULL,
300 the original baud rate will be tried. If that exceeds the
301 min..max constraint, 9600 baud will be returned. termios will
302 be updated to the baud rate in use.
303
304 Note: min..max must always allow 9600 baud to be selected.
305
306 Locking: caller dependent.
307 Interrupts: n/a
308
309uart_get_divisor(port,baud)
310 Return the divsor (baud_base / baud) for the specified baud
311 rate, appropriately rounded.
312
313 If 38400 baud and custom divisor is selected, return the
314 custom divisor instead.
315
316 Locking: caller dependent.
317 Interrupts: n/a
318
319uart_match_port(port1,port2)
320 This utility function can be used to determine whether two
321 uart_port structures describe the same port.
322
323 Locking: n/a
324 Interrupts: n/a
325
326uart_write_wakeup(port)
327 A driver is expected to call this function when the number of
328 characters in the transmit buffer have dropped below a threshold.
329
330 Locking: port->lock should be held.
331 Interrupts: n/a
332
333uart_register_driver(drv)
334 Register a uart driver with the core driver. We in turn register
335 with the tty layer, and initialise the core driver per-port state.
336
337 drv->port should be NULL, and the per-port structures should be
338 registered using uart_add_one_port after this call has succeeded.
339
340 Locking: none
341 Interrupts: enabled
342
343uart_unregister_driver()
344 Remove all references to a driver from the core driver. The low
345 level driver must have removed all its ports via the
346 uart_remove_one_port() if it registered them with uart_add_one_port().
347
348 Locking: none
349 Interrupts: enabled
350
351uart_suspend_port()
352
353uart_resume_port()
354
355uart_add_one_port()
356
357uart_remove_one_port()
358
359Other notes
360-----------
361
362It is intended some day to drop the 'unused' entries from uart_port, and
363allow low level drivers to register their own individual uart_port's with
364the core. This will allow drivers to use uart_port as a pointer to a
365structure containing both the uart_port entry with their own extensions,
366thus:
367
368 struct my_port {
369 struct uart_port port;
370 int my_stuff;
371 };