Documentation/serial/driver.rst

   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
  79         Interrupts: caller dependent.
  80
  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
  88                 - TIOCM_RTS     RTS signal.
  89                 - TIOCM_DTR     DTR signal.
  90                 - TIOCM_OUT1    OUT1 signal.
  91                 - TIOCM_OUT2    OUT2 signal.
  92                 - TIOCM_LOOP    Set the port into loopback mode.
  93
  94         If the appropriate bit is set, the signal should be driven
  95         active.  If the bit is clear, the signal should be driven
  96         inactive.
  97
  98         Locking: port->lock taken.
  99
 100         Interrupts: locally disabled.
 101
 102         This call must not sleep
 103
 104   get_mctrl(port)
 105         Returns the current state of modem control inputs.  The state
 106         of the outputs should not be returned, since the core keeps
 107         track of their state.  The state information should include:
 108
 109                 - TIOCM_CAR     state of DCD signal
 110                 - TIOCM_CTS     state of CTS signal
 111                 - TIOCM_DSR     state of DSR signal
 112                 - TIOCM_RI      state of RI signal
 113
 114         The bit is set if the signal is currently driven active.  If
 115         the port does not support CTS, DCD or DSR, the driver should
 116         indicate that the signal is permanently active.  If RI is
 117         not available, the signal should not be indicated as active.
 118
 119         Locking: port->lock taken.
 120
 121         Interrupts: locally disabled.
 122
 123         This call must not sleep
 124
 125   stop_tx(port)
 126         Stop transmitting characters.  This might be due to the CTS
 127         line becoming inactive or the tty layer indicating we want
 128         to stop transmission due to an XOFF character.
 129
 130         The driver should stop transmitting characters as soon as
 131         possible.
 132
 133         Locking: port->lock taken.
 134
 135         Interrupts: locally disabled.
 136
 137         This call must not sleep
 138
 139   start_tx(port)
 140         Start transmitting characters.
 141
 142         Locking: port->lock taken.
 143
 144         Interrupts: locally disabled.
 145
 146         This call must not sleep
 147
 148   throttle(port)
 149         Notify the serial driver that input buffers for the line discipline are
 150         close to full, and it should somehow signal that no more characters
 151         should be sent to the serial port.
 152         This will be called only if hardware assisted flow control is enabled.
 153
 154         Locking: serialized with .unthrottle() and termios modification by the
 155         tty layer.
 156
 157   unthrottle(port)
 158         Notify the serial driver that characters can now be sent to the serial
 159         port without fear of overrunning the input buffers of the line
 160         disciplines.
 161
 162         This will be called only if hardware assisted flow control is enabled.
 163
 164         Locking: serialized with .throttle() and termios modification by the
 165         tty layer.
 166
 167   send_xchar(port,ch)
 168         Transmit a high priority character, even if the port is stopped.
 169         This is used to implement XON/XOFF flow control and tcflow().  If
 170         the serial driver does not implement this function, the tty core
 171         will append the character to the circular buffer and then call
 172         start_tx() / stop_tx() to flush the data out.
 173
 174         Do not transmit if ch == '\0' (__DISABLED_CHAR).
 175
 176         Locking: none.
 177
 178         Interrupts: caller dependent.
 179
 180   stop_rx(port)
 181         Stop receiving characters; the port is in the process of
 182         being closed.
 183
 184         Locking: port->lock taken.
 185
 186         Interrupts: locally disabled.
 187
 188         This call must not sleep
 189
 190   enable_ms(port)
 191         Enable the modem status interrupts.
 192
 193         This method may be called multiple times.  Modem status
 194         interrupts should be disabled when the shutdown method is
 195         called.
 196
 197         Locking: port->lock taken.
 198
 199         Interrupts: locally disabled.
 200
 201         This call must not sleep
 202
 203   break_ctl(port,ctl)
 204         Control the transmission of a break signal.  If ctl is
 205         nonzero, the break signal should be transmitted.  The signal
 206         should be terminated when another call is made with a zero
 207         ctl.
 208
 209         Locking: caller holds tty_port->mutex
 210
 211   startup(port)
 212         Grab any interrupt resources and initialise any low level driver
 213         state.  Enable the port for reception.  It should not activate
 214         RTS nor DTR; this will be done via a separate call to set_mctrl.
 215
 216         This method will only be called when the port is initially opened.
 217
 218         Locking: port_sem taken.
 219
 220         Interrupts: globally disabled.
 221
 222   shutdown(port)
 223         Disable the port, disable any break condition that may be in
 224         effect, and free any interrupt resources.  It should not disable
 225         RTS nor DTR; this will have already been done via a separate
 226         call to set_mctrl.
 227
 228         Drivers must not access port->state once this call has completed.
 229
 230         This method will only be called when there are no more users of
 231         this port.
 232
 233         Locking: port_sem taken.
 234
 235         Interrupts: caller dependent.
 236
 237   flush_buffer(port)
 238         Flush any write buffers, reset any DMA state and stop any
 239         ongoing DMA transfers.
 240
 241         This will be called whenever the port->state->xmit circular
 242         buffer is cleared.
 243
 244         Locking: port->lock taken.
 245
 246         Interrupts: locally disabled.
 247
 248         This call must not sleep
 249
 250   set_termios(port,termios,oldtermios)
 251         Change the port parameters, including word length, parity, stop
 252         bits.  Update read_status_mask and ignore_status_mask to indicate
 253         the types of events we are interested in receiving.  Relevant
 254         termios->c_cflag bits are:
 255
 256                 CSIZE
 257                         - word size
 258                 CSTOPB
 259                         - 2 stop bits
 260                 PARENB
 261                         - parity enable
 262                 PARODD
 263                         - odd parity (when PARENB is in force)
 264                 CREAD
 265                         - enable reception of characters (if not set,
 266                           still receive characters from the port, but
 267                           throw them away.
 268                 CRTSCTS
 269                         - if set, enable CTS status change reporting
 270                 CLOCAL
 271                         - if not set, enable modem status change
 272                           reporting.
 273
 274         Relevant termios->c_iflag bits are:
 275
 276                 INPCK
 277                         - enable frame and parity error events to be
 278                           passed to the TTY layer.
 279                 BRKINT / PARMRK
 280                         - both of these enable break events to be
 281                           passed to the TTY layer.
 282
 283                 IGNPAR
 284                         - ignore parity and framing errors
 285                 IGNBRK
 286                         - ignore break errors,  If IGNPAR is also
 287                           set, ignore overrun errors as well.
 288
 289         The interaction of the iflag bits is as follows (parity error
 290         given as an example):
 291
 292         =============== ======= ======  =============================
 293         Parity error    INPCK   IGNPAR
 294         =============== ======= ======  =============================
 295         n/a             0       n/a     character received, marked as
 296                                         TTY_NORMAL
 297         None            1       n/a     character received, marked as
 298                                         TTY_NORMAL
 299         Yes             1       0       character received, marked as
 300                                         TTY_PARITY
 301         Yes             1       1       character discarded
 302         =============== ======= ======  =============================
 303
 304         Other flags may be used (eg, xon/xoff characters) if your
 305         hardware supports hardware "soft" flow control.
 306
 307         Locking: caller holds tty_port->mutex
 308
 309         Interrupts: caller dependent.
 310
 311         This call must not sleep
 312
 313   set_ldisc(port,termios)
 314         Notifier for discipline change. See Documentation/serial/tty.rst.
 315
 316         Locking: caller holds tty_port->mutex
 317
 318   pm(port,state,oldstate)
 319         Perform any power management related activities on the specified
 320         port.  State indicates the new state (defined by
 321         enum uart_pm_state), oldstate indicates the previous state.
 322
 323         This function should not be used to grab any resources.
 324
 325         This will be called when the port is initially opened and finally
 326         closed, except when the port is also the system console.  This
 327         will occur even if CONFIG_PM is not set.
 328
 329         Locking: none.
 330
 331         Interrupts: caller dependent.
 332
 333   type(port)
 334         Return a pointer to a string constant describing the specified
 335         port, or return NULL, in which case the string 'unknown' is
 336         substituted.
 337
 338         Locking: none.
 339
 340         Interrupts: caller dependent.
 341
 342   release_port(port)
 343         Release any memory and IO region resources currently in use by
 344         the port.
 345
 346         Locking: none.
 347
 348         Interrupts: caller dependent.
 349
 350   request_port(port)
 351         Request any memory and IO region resources required by the port.
 352         If any fail, no resources should be registered when this function
 353         returns, and it should return -EBUSY on failure.
 354
 355         Locking: none.
 356
 357         Interrupts: caller dependent.
 358
 359   config_port(port,type)
 360         Perform any autoconfiguration steps required for the port.  `type`
 361         contains a bit mask of the required configuration.  UART_CONFIG_TYPE
 362         indicates that the port requires detection and identification.
 363         port->type should be set to the type found, or PORT_UNKNOWN if
 364         no port was detected.
 365
 366         UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal,
 367         which should be probed using standard kernel autoprobing techniques.
 368         This is not necessary on platforms where ports have interrupts
 369         internally hard wired (eg, system on a chip implementations).
 370
 371         Locking: none.
 372
 373         Interrupts: caller dependent.
 374
 375   verify_port(port,serinfo)
 376         Verify the new serial port information contained within serinfo is
 377         suitable for this port type.
 378
 379         Locking: none.
 380
 381         Interrupts: caller dependent.
 382
 383   ioctl(port,cmd,arg)
 384         Perform any port specific IOCTLs.  IOCTL commands must be defined
 385         using the standard numbering system found in <asm/ioctl.h>
 386
 387         Locking: none.
 388
 389         Interrupts: caller dependent.
 390
 391   poll_init(port)
 392         Called by kgdb to perform the minimal hardware initialization needed
 393         to support poll_put_char() and poll_get_char().  Unlike ->startup()
 394         this should not request interrupts.
 395
 396         Locking: tty_mutex and tty_port->mutex taken.
 397
 398         Interrupts: n/a.
 399
 400   poll_put_char(port,ch)
 401         Called by kgdb to write a single character directly to the serial
 402         port.  It can and should block until there is space in the TX FIFO.
 403
 404         Locking: none.
 405
 406         Interrupts: caller dependent.
 407
 408         This call must not sleep
 409
 410   poll_get_char(port)
 411         Called by kgdb to read a single character directly from the serial
 412         port.  If data is available, it should be returned; otherwise
 413         the function should return NO_POLL_CHAR immediately.
 414
 415         Locking: none.
 416
 417         Interrupts: caller dependent.
 418
 419         This call must not sleep
 420
 421 Other functions
 422 ---------------
 423
 424 uart_update_timeout(port,cflag,baud)
 425         Update the FIFO drain timeout, port->timeout, according to the
 426         number of bits, parity, stop bits and baud rate.
 427
 428         Locking: caller is expected to take port->lock
 429
 430         Interrupts: n/a
 431
 432 uart_get_baud_rate(port,termios,old,min,max)
 433         Return the numeric baud rate for the specified termios, taking
 434         account of the special 38400 baud "kludge".  The B0 baud rate
 435         is mapped to 9600 baud.
 436
 437         If the baud rate is not within min..max, then if old is non-NULL,
 438         the original baud rate will be tried.  If that exceeds the
 439         min..max constraint, 9600 baud will be returned.  termios will
 440         be updated to the baud rate in use.
 441
 442         Note: min..max must always allow 9600 baud to be selected.
 443
 444         Locking: caller dependent.
 445
 446         Interrupts: n/a
 447
 448 uart_get_divisor(port,baud)
 449         Return the divisor (baud_base / baud) for the specified baud
 450         rate, appropriately rounded.
 451
 452         If 38400 baud and custom divisor is selected, return the
 453         custom divisor instead.
 454
 455         Locking: caller dependent.
 456
 457         Interrupts: n/a
 458
 459 uart_match_port(port1,port2)
 460         This utility function can be used to determine whether two
 461         uart_port structures describe the same port.
 462
 463         Locking: n/a
 464
 465         Interrupts: n/a
 466
 467 uart_write_wakeup(port)
 468         A driver is expected to call this function when the number of
 469         characters in the transmit buffer have dropped below a threshold.
 470
 471         Locking: port->lock should be held.
 472
 473         Interrupts: n/a
 474
 475 uart_register_driver(drv)
 476         Register a uart driver with the core driver.  We in turn register
 477         with the tty layer, and initialise the core driver per-port state.
 478
 479         drv->port should be NULL, and the per-port structures should be
 480         registered using uart_add_one_port after this call has succeeded.
 481
 482         Locking: none
 483
 484         Interrupts: enabled
 485
 486 uart_unregister_driver()
 487         Remove all references to a driver from the core driver.  The low
 488         level driver must have removed all its ports via the
 489         uart_remove_one_port() if it registered them with uart_add_one_port().
 490
 491         Locking: none
 492
 493         Interrupts: enabled
 494
 495 **uart_suspend_port()**
 496
 497 **uart_resume_port()**
 498
 499 **uart_add_one_port()**
 500
 501 **uart_remove_one_port()**
 502
 503 Other notes
 504 -----------
 505
 506 It is intended some day to drop the 'unused' entries from uart_port, and
 507 allow low level drivers to register their own individual uart_port's with
 508 the core.  This will allow drivers to use uart_port as a pointer to a
 509 structure containing both the uart_port entry with their own extensions,
 510 thus::
 511
 512         struct my_port {
 513                 struct uart_port        port;
 514                 int                     my_stuff;
 515         };
 516
 517 Modem control lines via GPIO
 518 ----------------------------
 519
 520 Some helpers are provided in order to set/get modem control lines via GPIO.
 521
 522 mctrl_gpio_init(port, idx):
 523         This will get the {cts,rts,...}-gpios from device tree if they are
 524         present and request them, set direction etc, and return an
 525         allocated structure. `devm_*` functions are used, so there's no need
 526         to call mctrl_gpio_free().
 527         As this sets up the irq handling make sure to not handle changes to the
 528         gpio input lines in your driver, too.
 529
 530 mctrl_gpio_free(dev, gpios):
 531         This will free the requested gpios in mctrl_gpio_init().
 532         As `devm_*` functions are used, there's generally no need to call
 533         this function.
 534
 535 mctrl_gpio_to_gpiod(gpios, gidx)
 536         This returns the gpio_desc structure associated to the modem line
 537         index.
 538
 539 mctrl_gpio_set(gpios, mctrl):
 540         This will sets the gpios according to the mctrl state.
 541
 542 mctrl_gpio_get(gpios, mctrl):
 543         This will update mctrl with the gpios values.
 544
 545 mctrl_gpio_enable_ms(gpios):
 546         Enables irqs and handling of changes to the ms lines.
 547
 548 mctrl_gpio_disable_ms(gpios):
 549         Disables irqs and handling of changes to the ms lines.