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_write_console) 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 three locks.  A per-port spinlock, a per-port tmpbuf semaphore,
  45 and an overall semaphore.
  46
  47 From the core driver perspective, the port->lock locks the following
  48 data:
  49
  50         port->mctrl
  51         port->icount
  52         info->xmit.head (circ->head)
  53         info->xmit.tail (circ->tail)
  54
  55 The low level driver is free to use this lock to provide any additional
  56 locking.
  57
  58 The core driver uses the info->tmpbuf_sem lock to prevent multi-threaded
  59 access to the info->tmpbuf bouncebuffer used for port writes.
  60
  61 The port_sem semaphore is used to protect against ports being added/
  62 removed or reconfigured at inappropriate times. Since v2.6.27, this
  63 semaphore has been the 'mutex' member of the tty_port struct, and
  64 commonly referred to as the port mutex (or port->mutex).
  65
  66
  67 uart_ops
  68 --------
  69
  70 The uart_ops structure is the main interface between serial_core and the
  71 hardware specific driver.  It contains all the methods to control the
  72 hardware.
  73
  74   tx_empty(port)
  75         This function tests whether the transmitter fifo and shifter
  76         for the port described by 'port' is empty.  If it is empty,
  77         this function should return TIOCSER_TEMT, otherwise return 0.
  78         If the port does not support this operation, then it should
  79         return TIOCSER_TEMT.
  80
  81         Locking: none.
  82         Interrupts: caller dependent.
  83         This call must not sleep
  84
  85   set_mctrl(port, mctrl)
  86         This function sets the modem control lines for port described
  87         by 'port' to the state described by mctrl.  The relevant bits
  88         of mctrl are:
  89                 - TIOCM_RTS     RTS signal.
  90                 - TIOCM_DTR     DTR signal.
  91                 - TIOCM_OUT1    OUT1 signal.
  92                 - TIOCM_OUT2    OUT2 signal.
  93                 - TIOCM_LOOP    Set the port into loopback mode.
  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         Interrupts: locally disabled.
 100         This call must not sleep
 101
 102   get_mctrl(port)
 103         Returns the current state of modem control inputs.  The state
 104         of the outputs should not be returned, since the core keeps
 105         track of their state.  The state information should include:
 106                 - TIOCM_CAR     state of DCD signal
 107                 - TIOCM_CTS     state of CTS signal
 108                 - TIOCM_DSR     state of DSR signal
 109                 - TIOCM_RI      state of RI signal
 110         The bit is set if the signal is currently driven active.  If
 111         the port does not support CTS, DCD or DSR, the driver should
 112         indicate that the signal is permanently active.  If RI is
 113         not available, the signal should not be indicated as active.
 114
 115         Locking: port->lock taken.
 116         Interrupts: locally disabled.
 117         This call must not sleep
 118
 119   stop_tx(port)
 120         Stop transmitting characters.  This might be due to the CTS
 121         line becoming inactive or the tty layer indicating we want
 122         to stop transmission due to an XOFF character.
 123
 124         The driver should stop transmitting characters as soon as
 125         possible.
 126
 127         Locking: port->lock taken.
 128         Interrupts: locally disabled.
 129         This call must not sleep
 130
 131   start_tx(port)
 132         Start transmitting characters.
 133
 134         Locking: port->lock taken.
 135         Interrupts: locally disabled.
 136         This call must not sleep
 137
 138   send_xchar(port,ch)
 139         Transmit a high priority character, even if the port is stopped.
 140         This is used to implement XON/XOFF flow control and tcflow().  If
 141         the serial driver does not implement this function, the tty core
 142         will append the character to the circular buffer and then call
 143         start_tx() / stop_tx() to flush the data out.
 144
 145         Do not transmit if ch == '\0' (__DISABLED_CHAR).
 146
 147         Locking: none.
 148         Interrupts: caller dependent.
 149
 150   stop_rx(port)
 151         Stop receiving characters; the port is in the process of
 152         being closed.
 153
 154         Locking: port->lock taken.
 155         Interrupts: locally disabled.
 156         This call must not sleep
 157
 158   enable_ms(port)
 159         Enable the modem status interrupts.
 160
 161         This method may be called multiple times.  Modem status
 162         interrupts should be disabled when the shutdown method is
 163         called.
 164
 165         Locking: port->lock taken.
 166         Interrupts: locally disabled.
 167         This call must not sleep
 168
 169   break_ctl(port,ctl)
 170         Control the transmission of a break signal.  If ctl is
 171         nonzero, the break signal should be transmitted.  The signal
 172         should be terminated when another call is made with a zero
 173         ctl.
 174
 175         Locking: none.
 176         Interrupts: caller dependent.
 177         This call must not sleep
 178
 179   startup(port)
 180         Grab any interrupt resources and initialise any low level driver
 181         state.  Enable the port for reception.  It should not activate
 182         RTS nor DTR; this will be done via a separate call to set_mctrl.
 183
 184         This method will only be called when the port is initially opened.
 185
 186         Locking: port_sem taken.
 187         Interrupts: globally disabled.
 188
 189   shutdown(port)
 190         Disable the port, disable any break condition that may be in
 191         effect, and free any interrupt resources.  It should not disable
 192         RTS nor DTR; this will have already been done via a separate
 193         call to set_mctrl.
 194
 195         Drivers must not access port->info once this call has completed.
 196
 197         This method will only be called when there are no more users of
 198         this port.
 199
 200         Locking: port_sem taken.
 201         Interrupts: caller dependent.
 202
 203   flush_buffer(port)
 204         Flush any write buffers, reset any DMA state and stop any
 205         ongoing DMA transfers.
 206
 207         This will be called whenever the port->info->xmit circular
 208         buffer is cleared.
 209
 210         Locking: port->lock taken.
 211         Interrupts: locally disabled.
 212         This call must not sleep
 213
 214   set_termios(port,termios,oldtermios)
 215         Change the port parameters, including word length, parity, stop
 216         bits.  Update read_status_mask and ignore_status_mask to indicate
 217         the types of events we are interested in receiving.  Relevant
 218         termios->c_cflag bits are:
 219                 CSIZE   - word size
 220                 CSTOPB  - 2 stop bits
 221                 PARENB  - parity enable
 222                 PARODD  - odd parity (when PARENB is in force)
 223                 CREAD   - enable reception of characters (if not set,
 224                           still receive characters from the port, but
 225                           throw them away.
 226                 CRTSCTS - if set, enable CTS status change reporting
 227                 CLOCAL  - if not set, enable modem status change
 228                           reporting.
 229         Relevant termios->c_iflag bits are:
 230                 INPCK   - enable frame and parity error events to be
 231                           passed to the TTY layer.
 232                 BRKINT
 233                 PARMRK  - both of these enable break events to be
 234                           passed to the TTY layer.
 235
 236                 IGNPAR  - ignore parity and framing errors
 237                 IGNBRK  - ignore break errors,  If IGNPAR is also
 238                           set, ignore overrun errors as well.
 239         The interaction of the iflag bits is as follows (parity error
 240         given as an example):
 241         Parity error    INPCK   IGNPAR
 242         n/a             0       n/a     character received, marked as
 243                                         TTY_NORMAL
 244         None            1       n/a     character received, marked as
 245                                         TTY_NORMAL
 246         Yes             1       0       character received, marked as
 247                                         TTY_PARITY
 248         Yes             1       1       character discarded
 249
 250         Other flags may be used (eg, xon/xoff characters) if your
 251         hardware supports hardware "soft" flow control.
 252
 253         Locking: caller holds port->mutex
 254         Interrupts: caller dependent.
 255         This call must not sleep
 256
 257   pm(port,state,oldstate)
 258         Perform any power management related activities on the specified
 259         port.  State indicates the new state (defined by
 260         enum uart_pm_state), oldstate indicates the previous state.
 261
 262         This function should not be used to grab any resources.
 263
 264         This will be called when the port is initially opened and finally
 265         closed, except when the port is also the system console.  This
 266         will occur even if CONFIG_PM is not set.
 267
 268         Locking: none.
 269         Interrupts: caller dependent.
 270
 271   type(port)
 272         Return a pointer to a string constant describing the specified
 273         port, or return NULL, in which case the string 'unknown' is
 274         substituted.
 275
 276         Locking: none.
 277         Interrupts: caller dependent.
 278
 279   release_port(port)
 280         Release any memory and IO region resources currently in use by
 281         the port.
 282
 283         Locking: none.
 284         Interrupts: caller dependent.
 285
 286   request_port(port)
 287         Request any memory and IO region resources required by the port.
 288         If any fail, no resources should be registered when this function
 289         returns, and it should return -EBUSY on failure.
 290
 291         Locking: none.
 292         Interrupts: caller dependent.
 293
 294   config_port(port,type)
 295         Perform any autoconfiguration steps required for the port.  `type`
 296         contains a bit mask of the required configuration.  UART_CONFIG_TYPE
 297         indicates that the port requires detection and identification.
 298         port->type should be set to the type found, or PORT_UNKNOWN if
 299         no port was detected.
 300
 301         UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal,
 302         which should be probed using standard kernel autoprobing techniques.
 303         This is not necessary on platforms where ports have interrupts
 304         internally hard wired (eg, system on a chip implementations).
 305
 306         Locking: none.
 307         Interrupts: caller dependent.
 308
 309   verify_port(port,serinfo)
 310         Verify the new serial port information contained within serinfo is
 311         suitable for this port type.
 312
 313         Locking: none.
 314         Interrupts: caller dependent.
 315
 316   ioctl(port,cmd,arg)
 317         Perform any port specific IOCTLs.  IOCTL commands must be defined
 318         using the standard numbering system found in <asm/ioctl.h>
 319
 320         Locking: none.
 321         Interrupts: caller dependent.
 322
 323   poll_init(port)
 324         Called by kgdb to perform the minimal hardware initialization needed
 325         to support poll_put_char() and poll_get_char().  Unlike ->startup()
 326         this should not request interrupts.
 327
 328         Locking: tty_mutex and tty_port->mutex taken.
 329         Interrupts: n/a.
 330
 331   poll_put_char(port,ch)
 332         Called by kgdb to write a single character directly to the serial
 333         port.  It can and should block until there is space in the TX FIFO.
 334
 335         Locking: none.
 336         Interrupts: caller dependent.
 337         This call must not sleep
 338
 339   poll_get_char(port)
 340         Called by kgdb to read a single character directly from the serial
 341         port.  If data is available, it should be returned; otherwise
 342         the function should return NO_POLL_CHAR immediately.
 343
 344         Locking: none.
 345         Interrupts: caller dependent.
 346         This call must not sleep
 347
 348 Other functions
 349 ---------------
 350
 351 uart_update_timeout(port,cflag,baud)
 352         Update the FIFO drain timeout, port->timeout, according to the
 353         number of bits, parity, stop bits and baud rate.
 354
 355         Locking: caller is expected to take port->lock
 356         Interrupts: n/a
 357
 358 uart_get_baud_rate(port,termios,old,min,max)
 359         Return the numeric baud rate for the specified termios, taking
 360         account of the special 38400 baud "kludge".  The B0 baud rate
 361         is mapped to 9600 baud.
 362
 363         If the baud rate is not within min..max, then if old is non-NULL,
 364         the original baud rate will be tried.  If that exceeds the
 365         min..max constraint, 9600 baud will be returned.  termios will
 366         be updated to the baud rate in use.
 367
 368         Note: min..max must always allow 9600 baud to be selected.
 369
 370         Locking: caller dependent.
 371         Interrupts: n/a
 372
 373 uart_get_divisor(port,baud)
 374         Return the divsor (baud_base / baud) for the specified baud
 375         rate, appropriately rounded.
 376
 377         If 38400 baud and custom divisor is selected, return the
 378         custom divisor instead.
 379
 380         Locking: caller dependent.
 381         Interrupts: n/a
 382
 383 uart_match_port(port1,port2)
 384         This utility function can be used to determine whether two
 385         uart_port structures describe the same port.
 386
 387         Locking: n/a
 388         Interrupts: n/a
 389
 390 uart_write_wakeup(port)
 391         A driver is expected to call this function when the number of
 392         characters in the transmit buffer have dropped below a threshold.
 393
 394         Locking: port->lock should be held.
 395         Interrupts: n/a
 396
 397 uart_register_driver(drv)
 398         Register a uart driver with the core driver.  We in turn register
 399         with the tty layer, and initialise the core driver per-port state.
 400
 401         drv->port should be NULL, and the per-port structures should be
 402         registered using uart_add_one_port after this call has succeeded.
 403
 404         Locking: none
 405         Interrupts: enabled
 406
 407 uart_unregister_driver()
 408         Remove all references to a driver from the core driver.  The low
 409         level driver must have removed all its ports via the
 410         uart_remove_one_port() if it registered them with uart_add_one_port().
 411
 412         Locking: none
 413         Interrupts: enabled
 414
 415 uart_suspend_port()
 416
 417 uart_resume_port()
 418
 419 uart_add_one_port()
 420
 421 uart_remove_one_port()
 422
 423 Other notes
 424 -----------
 425
 426 It is intended some day to drop the 'unused' entries from uart_port, and
 427 allow low level drivers to register their own individual uart_port's with
 428 the core.  This will allow drivers to use uart_port as a pointer to a
 429 structure containing both the uart_port entry with their own extensions,
 430 thus:
 431
 432         struct my_port {
 433                 struct uart_port        port;
 434                 int                     my_stuff;
 435         };
 436
 437 Modem control lines via GPIO
 438 ----------------------------
 439
 440 Some helpers are provided in order to set/get modem control lines via GPIO.
 441
 442 mctrl_gpio_init(dev, idx):
 443         This will get the {cts,rts,...}-gpios from device tree if they are
 444         present and request them, set direction etc, and return an
 445         allocated structure. devm_* functions are used, so there's no need
 446         to call mctrl_gpio_free().
 447
 448 mctrl_gpio_free(dev, gpios):
 449         This will free the requested gpios in mctrl_gpio_init().
 450         As devm_* function are used, there's generally no need to call
 451         this function.
 452
 453 mctrl_gpio_to_gpiod(gpios, gidx)
 454         This returns the gpio structure associated to the modem line index.
 455
 456 mctrl_gpio_set(gpios, mctrl):
 457         This will sets the gpios according to the mctrl state.
 458
 459 mctrl_gpio_get(gpios, mctrl):
 460         This will update mctrl with the gpios values.