Root/Documentation/serial/driver

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  flush_buffer(port)
190    Flush any write buffers, reset any DMA state and stop any
191    ongoing DMA transfers.
192
193    This will be called whenever the port->info->xmit circular
194    buffer is cleared.
195
196    Locking: port->lock taken.
197    Interrupts: locally disabled.
198    This call must not sleep
199
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:
205        CSIZE - word size
206        CSTOPB - 2 stop bits
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
211              throw them away.
212        CRTSCTS - if set, enable CTS status change reporting
213        CLOCAL - if not set, enable modem status change
214              reporting.
215    Relevant termios->c_iflag bits are:
216        INPCK - enable frame and parity error events to be
217              passed to the TTY layer.
218        BRKINT
219        PARMRK - both of these enable break events to be
220              passed to the TTY layer.
221
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
229                    TTY_NORMAL
230    None 1 n/a character received, marked as
231                    TTY_NORMAL
232    Yes 1 0 character received, marked as
233                    TTY_PARITY
234    Yes 1 1 character discarded
235
236    Other flags may be used (eg, xon/xoff characters) if your
237    hardware supports hardware "soft" flow control.
238
239    Locking: none.
240    Interrupts: caller dependent.
241    This call must not sleep
242
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.
248
249    This function should not be used to grab any resources.
250
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.
254
255    Locking: none.
256    Interrupts: caller dependent.
257
258  type(port)
259    Return a pointer to a string constant describing the specified
260    port, or return NULL, in which case the string 'unknown' is
261    substituted.
262
263    Locking: none.
264    Interrupts: caller dependent.
265
266  release_port(port)
267    Release any memory and IO region resources currently in use by
268    the port.
269
270    Locking: none.
271    Interrupts: caller dependent.
272
273  request_port(port)
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.
277
278    Locking: none.
279    Interrupts: caller dependent.
280
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.
287
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).
292
293    Locking: none.
294    Interrupts: caller dependent.
295
296  verify_port(port,serinfo)
297    Verify the new serial port information contained within serinfo is
298    suitable for this port type.
299
300    Locking: none.
301    Interrupts: caller dependent.
302
303  ioctl(port,cmd,arg)
304    Perform any port specific IOCTLs. IOCTL commands must be defined
305    using the standard numbering system found in <asm/ioctl.h>
306
307    Locking: none.
308    Interrupts: caller dependent.
309
310Other functions
311---------------
312
313uart_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.
316
317    Locking: caller is expected to take port->lock
318    Interrupts: n/a
319
320uart_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.
324
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.
329
330    Note: min..max must always allow 9600 baud to be selected.
331
332    Locking: caller dependent.
333    Interrupts: n/a
334
335uart_get_divisor(port,baud)
336    Return the divsor (baud_base / baud) for the specified baud
337    rate, appropriately rounded.
338
339    If 38400 baud and custom divisor is selected, return the
340    custom divisor instead.
341
342    Locking: caller dependent.
343    Interrupts: n/a
344
345uart_match_port(port1,port2)
346    This utility function can be used to determine whether two
347    uart_port structures describe the same port.
348
349    Locking: n/a
350    Interrupts: n/a
351
352uart_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.
355
356    Locking: port->lock should be held.
357    Interrupts: n/a
358
359uart_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.
362
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.
365
366    Locking: none
367    Interrupts: enabled
368
369uart_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().
373
374    Locking: none
375    Interrupts: enabled
376
377uart_suspend_port()
378
379uart_resume_port()
380
381uart_add_one_port()
382
383uart_remove_one_port()
384
385Other notes
386-----------
387
388It is intended some day to drop the 'unused' entries from uart_port, and
389allow low level drivers to register their own individual uart_port's with
390the core. This will allow drivers to use uart_port as a pointer to a
391structure containing both the uart_port entry with their own extensions,
392thus:
393
394    struct my_port {
395        struct uart_port port;
396        int my_stuff;
397    };
398

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