Documentation/serial/driver

   1
   2                         Low Level Serial API
   3                         --------------------
   4
   5
   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
   8 <rmk@arm.linux.org.uk>
   9
  10 The reference implementation is contained within amba_pl011.c.
  11
  12
  13
  14 Low Level Serial Hardware Driver
  15 --------------------------------
  16
  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
  21 console support.
  22
  23
  24 Console Support
  25 ---------------
  26
  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).
  30
  31 There is also a helper function (uart_console_write) 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
  34 their own version.
  35
  36
  37 Locking
  38 -------
  39
  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.)
  43
  44 There are two locks.  A per-port spinlock, and an overall semaphore.
  45
  46 From the core driver perspective, the port->lock locks the following
  47 data:
  48
  49         port->mctrl
  50         port->icount
  51         port->state->xmit.head (circ_buf->head)
  52         port->state->xmit.tail (circ_buf->tail)
  53
  54 The low level driver is free to use this lock to provide any additional
  55 locking.
  56
  57 The port_sem semaphore is used to protect against ports being added/
  58 removed or reconfigured at inappropriate times. Since v2.6.27, this
  59 semaphore has been the 'mutex' member of the tty_port struct, and
  60 commonly referred to as the port mutex.
  61
  62
  63 uart_ops
  64 --------
  65
  66 The uart_ops structure is the main interface between serial_core and the
  67 hardware specific driver.  It contains all the methods to control the
  68 hardware.
  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                 - TIOCM_LOOP    Set the port into loopback mode.
  90         If the appropriate bit is set, the signal should be driven
  91         active.  If the bit is clear, the signal should be driven
  92         inactive.
  93
  94         Locking: port->lock taken.
  95         Interrupts: locally disabled.
  96         This call must not sleep
  97
  98   get_mctrl(port)
  99         Returns the current state of modem control inputs.  The state
 100         of the outputs should not be returned, since the core keeps
 101         track of their state.  The state information should include:
 102                 - TIOCM_CAR     state of DCD signal
 103                 - TIOCM_CTS     state of CTS signal
 104                 - TIOCM_DSR     state of DSR signal
 105                 - TIOCM_RI      state of RI signal
 106         The bit is set if the signal is currently driven active.  If
 107         the port does not support CTS, DCD or DSR, the driver should
 108         indicate that the signal is permanently active.  If RI is
 109         not available, the signal should not be indicated as active.
 110
 111         Locking: port->lock taken.
 112         Interrupts: locally disabled.
 113         This call must not sleep
 114
 115   stop_tx(port)
 116         Stop transmitting characters.  This might be due to the CTS
 117         line becoming inactive or the tty layer indicating we want
 118         to stop transmission due to an XOFF character.
 119
 120         The driver should stop transmitting characters as soon as
 121         possible.
 122
 123         Locking: port->lock taken.
 124         Interrupts: locally disabled.
 125         This call must not sleep
 126
 127   start_tx(port)
 128         Start transmitting characters.
 129
 130         Locking: port->lock taken.
 131         Interrupts: locally disabled.
 132         This call must not sleep
 133
 134   throttle(port)
 135         Notify the serial driver that input buffers for the line discipline are
 136         close to full, and it should somehow signal that no more characters
 137         should be sent to the serial port.
 138         This will be called only if hardware assisted flow control is enabled.
 139
 140         Locking: serialized with .unthrottle() and termios modification by the
 141                  tty layer.
 142
 143   unthrottle(port)
 144         Notify the serial driver that characters can now be sent to the serial
 145         port without fear of overrunning the input buffers of the line
 146         disciplines.
 147         This will be called only if hardware assisted flow control is enabled.
 148
 149         Locking: serialized with .throttle() and termios modification by the
 150                  tty layer.
 151
 152   send_xchar(port,ch)
 153         Transmit a high priority character, even if the port is stopped.
 154         This is used to implement XON/XOFF flow control and tcflow().  If
 155         the serial driver does not implement this function, the tty core
 156         will append the character to the circular buffer and then call
 157         start_tx() / stop_tx() to flush the data out.
 158
 159         Do not transmit if ch == '\0' (__DISABLED_CHAR).
 160
 161         Locking: none.
 162         Interrupts: caller dependent.
 163
 164   stop_rx(port)
 165         Stop receiving characters; the port is in the process of
 166         being closed.
 167
 168         Locking: port->lock taken.
 169         Interrupts: locally disabled.
 170         This call must not sleep
 171
 172   enable_ms(port)
 173         Enable the modem status interrupts.
 174
 175         This method may be called multiple times.  Modem status
 176         interrupts should be disabled when the shutdown method is
 177         called.
 178
 179         Locking: port->lock taken.
 180         Interrupts: locally disabled.
 181         This call must not sleep
 182
 183   break_ctl(port,ctl)
 184         Control the transmission of a break signal.  If ctl is
 185         nonzero, the break signal should be transmitted.  The signal
 186         should be terminated when another call is made with a zero
 187         ctl.
 188
 189         Locking: caller holds tty_port->mutex
 190
 191   startup(port)
 192         Grab any interrupt resources and initialise any low level driver
 193         state.  Enable the port for reception.  It should not activate
 194         RTS nor DTR; this will be done via a separate call to set_mctrl.
 195
 196         This method will only be called when the port is initially opened.
 197
 198         Locking: port_sem taken.
 199         Interrupts: globally disabled.
 200
 201   shutdown(port)
 202         Disable the port, disable any break condition that may be in
 203         effect, and free any interrupt resources.  It should not disable
 204         RTS nor DTR; this will have already been done via a separate
 205         call to set_mctrl.
 206
 207         Drivers must not access port->state once this call has completed.
 208
 209         This method will only be called when there are no more users of
 210         this port.
 211
 212         Locking: port_sem taken.
 213         Interrupts: caller dependent.
 214
 215   flush_buffer(port)
 216         Flush any write buffers, reset any DMA state and stop any
 217         ongoing DMA transfers.
 218
 219         This will be called whenever the port->state->xmit circular
 220         buffer is cleared.
 221
 222         Locking: port->lock taken.
 223         Interrupts: locally disabled.
 224         This call must not sleep
 225
 226   set_termios(port,termios,oldtermios)
 227         Change the port parameters, including word length, parity, stop
 228         bits.  Update read_status_mask and ignore_status_mask to indicate
 229         the types of events we are interested in receiving.  Relevant
 230         termios->c_cflag bits are:
 231                 CSIZE   - word size
 232                 CSTOPB  - 2 stop bits
 233                 PARENB  - parity enable
 234                 PARODD  - odd parity (when PARENB is in force)
 235                 CREAD   - enable reception of characters (if not set,
 236                           still receive characters from the port, but
 237                           throw them away.
 238                 CRTSCTS - if set, enable CTS status change reporting
 239                 CLOCAL  - if not set, enable modem status change
 240                           reporting.
 241         Relevant termios->c_iflag bits are:
 242                 INPCK   - enable frame and parity error events to be
 243                           passed to the TTY layer.
 244                 BRKINT
 245                 PARMRK  - both of these enable break events to be
 246                           passed to the TTY layer.
 247
 248                 IGNPAR  - ignore parity and framing errors
 249                 IGNBRK  - ignore break errors,  If IGNPAR is also
 250                           set, ignore overrun errors as well.
 251         The interaction of the iflag bits is as follows (parity error
 252         given as an example):
 253         Parity error    INPCK   IGNPAR
 254         n/a             0       n/a     character received, marked as
 255                                         TTY_NORMAL
 256         None            1       n/a     character received, marked as
 257                                         TTY_NORMAL
 258         Yes             1       0       character received, marked as
 259                                         TTY_PARITY
 260         Yes             1       1       character discarded
 261
 262         Other flags may be used (eg, xon/xoff characters) if your
 263         hardware supports hardware "soft" flow control.
 264
 265         Locking: caller holds tty_port->mutex
 266         Interrupts: caller dependent.
 267         This call must not sleep
 268
 269   set_ldisc(port,termios)
 270         Notifier for discipline change. See Documentation/serial/tty.txt.
 271
 272         Locking: caller holds tty_port->mutex
 273
 274   pm(port,state,oldstate)
 275         Perform any power management related activities on the specified
 276         port.  State indicates the new state (defined by
 277         enum uart_pm_state), oldstate indicates the previous state.
 278
 279         This function should not be used to grab any resources.
 280
 281         This will be called when the port is initially opened and finally
 282         closed, except when the port is also the system console.  This
 283         will occur even if CONFIG_PM is not set.
 284
 285         Locking: none.
 286         Interrupts: caller dependent.
 287
 288   type(port)
 289         Return a pointer to a string constant describing the specified
 290         port, or return NULL, in which case the string 'unknown' is
 291         substituted.
 292
 293         Locking: none.
 294         Interrupts: caller dependent.
 295
 296   release_port(port)
 297         Release any memory and IO region resources currently in use by
 298         the port.
 299
 300         Locking: none.
 301         Interrupts: caller dependent.
 302
 303   request_port(port)
 304         Request any memory and IO region resources required by the port.
 305         If any fail, no resources should be registered when this function
 306         returns, and it should return -EBUSY on failure.
 307
 308         Locking: none.
 309         Interrupts: caller dependent.
 310
 311   config_port(port,type)
 312         Perform any autoconfiguration steps required for the port.  `type`
 313         contains a bit mask of the required configuration.  UART_CONFIG_TYPE
 314         indicates that the port requires detection and identification.
 315         port->type should be set to the type found, or PORT_UNKNOWN if
 316         no port was detected.
 317
 318         UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal,
 319         which should be probed using standard kernel autoprobing techniques.
 320         This is not necessary on platforms where ports have interrupts
 321         internally hard wired (eg, system on a chip implementations).
 322
 323         Locking: none.
 324         Interrupts: caller dependent.
 325
 326   verify_port(port,serinfo)
 327         Verify the new serial port information contained within serinfo is
 328         suitable for this port type.
 329
 330         Locking: none.
 331         Interrupts: caller dependent.
 332
 333   ioctl(port,cmd,arg)
 334         Perform any port specific IOCTLs.  IOCTL commands must be defined
 335         using the standard numbering system found in <asm/ioctl.h>
 336
 337         Locking: none.
 338         Interrupts: caller dependent.
 339
 340   poll_init(port)
 341         Called by kgdb to perform the minimal hardware initialization needed
 342         to support poll_put_char() and poll_get_char().  Unlike ->startup()
 343         this should not request interrupts.
 344
 345         Locking: tty_mutex and tty_port->mutex taken.
 346         Interrupts: n/a.
 347
 348   poll_put_char(port,ch)
 349         Called by kgdb to write a single character directly to the serial
 350         port.  It can and should block until there is space in the TX FIFO.
 351
 352         Locking: none.
 353         Interrupts: caller dependent.
 354         This call must not sleep
 355
 356   poll_get_char(port)
 357         Called by kgdb to read a single character directly from the serial
 358         port.  If data is available, it should be returned; otherwise
 359         the function should return NO_POLL_CHAR immediately.
 360
 361         Locking: none.
 362         Interrupts: caller dependent.
 363         This call must not sleep
 364
 365 Other functions
 366 ---------------
 367
 368 uart_update_timeout(port,cflag,baud)
 369         Update the FIFO drain timeout, port->timeout, according to the
 370         number of bits, parity, stop bits and baud rate.
 371
 372         Locking: caller is expected to take port->lock
 373         Interrupts: n/a
 374
 375 uart_get_baud_rate(port,termios,old,min,max)
 376         Return the numeric baud rate for the specified termios, taking
 377         account of the special 38400 baud "kludge".  The B0 baud rate
 378         is mapped to 9600 baud.
 379
 380         If the baud rate is not within min..max, then if old is non-NULL,
 381         the original baud rate will be tried.  If that exceeds the
 382         min..max constraint, 9600 baud will be returned.  termios will
 383         be updated to the baud rate in use.
 384
 385         Note: min..max must always allow 9600 baud to be selected.
 386
 387         Locking: caller dependent.
 388         Interrupts: n/a
 389
 390 uart_get_divisor(port,baud)
 391         Return the divisor (baud_base / baud) for the specified baud
 392         rate, appropriately rounded.
 393
 394         If 38400 baud and custom divisor is selected, return the
 395         custom divisor instead.
 396
 397         Locking: caller dependent.
 398         Interrupts: n/a
 399
 400 uart_match_port(port1,port2)
 401         This utility function can be used to determine whether two
 402         uart_port structures describe the same port.
 403
 404         Locking: n/a
 405         Interrupts: n/a
 406
 407 uart_write_wakeup(port)
 408         A driver is expected to call this function when the number of
 409         characters in the transmit buffer have dropped below a threshold.
 410
 411         Locking: port->lock should be held.
 412         Interrupts: n/a
 413
 414 uart_register_driver(drv)
 415         Register a uart driver with the core driver.  We in turn register
 416         with the tty layer, and initialise the core driver per-port state.
 417
 418         drv->port should be NULL, and the per-port structures should be
 419         registered using uart_add_one_port after this call has succeeded.
 420
 421         Locking: none
 422         Interrupts: enabled
 423
 424 uart_unregister_driver()
 425         Remove all references to a driver from the core driver.  The low
 426         level driver must have removed all its ports via the
 427         uart_remove_one_port() if it registered them with uart_add_one_port().
 428
 429         Locking: none
 430         Interrupts: enabled
 431
 432 uart_suspend_port()
 433
 434 uart_resume_port()
 435
 436 uart_add_one_port()
 437
 438 uart_remove_one_port()
 439
 440 Other notes
 441 -----------
 442
 443 It is intended some day to drop the 'unused' entries from uart_port, and
 444 allow low level drivers to register their own individual uart_port's with
 445 the core.  This will allow drivers to use uart_port as a pointer to a
 446 structure containing both the uart_port entry with their own extensions,
 447 thus:
 448
 449         struct my_port {
 450                 struct uart_port        port;
 451                 int                     my_stuff;
 452         };
 453
 454 Modem control lines via GPIO
 455 ----------------------------
 456
 457 Some helpers are provided in order to set/get modem control lines via GPIO.
 458
 459 mctrl_gpio_init(port, idx):
 460         This will get the {cts,rts,...}-gpios from device tree if they are
 461         present and request them, set direction etc, and return an
 462         allocated structure. devm_* functions are used, so there's no need
 463         to call mctrl_gpio_free().
 464         As this sets up the irq handling make sure to not handle changes to the
 465         gpio input lines in your driver, too.
 466
 467 mctrl_gpio_free(dev, gpios):
 468         This will free the requested gpios in mctrl_gpio_init().
 469         As devm_* functions are used, there's generally no need to call
 470         this function.
 471
 472 mctrl_gpio_to_gpiod(gpios, gidx)
 473         This returns the gpio_desc structure associated to the modem line
 474         index.
 475
 476 mctrl_gpio_set(gpios, mctrl):
 477         This will sets the gpios according to the mctrl state.
 478
 479 mctrl_gpio_get(gpios, mctrl):
 480         This will update mctrl with the gpios values.
 481
 482 mctrl_gpio_enable_ms(gpios):
 483         Enables irqs and handling of changes to the ms lines.
 484
 485 mctrl_gpio_disable_ms(gpios):
 486         Disables irqs and handling of changes to the ms lines.