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
  32 Locking
  33 -------
  34
  35 It is the responsibility of the low level hardware driver to perform the
  36 necessary locking using port->lock.  There are some exceptions (which
  37 are described in the uart_ops listing below.)
  38
  39 There are two locks.  A per-port spinlock, and an overall semaphore.
  40
  41 From the core driver perspective, the port->lock locks the following
  42 data:
  43
  44         port->mctrl
  45         port->icount
  46         info->xmit.head (circ->head)
  47         info->xmit.tail (circ->tail)
  48
  49 The low level driver is free to use this lock to provide any additional
  50 locking.
  51
  52 The port_sem semaphore is used to protect against ports being added/
  53 removed or reconfigured at inappropriate times. Since v2.6.27, this
  54 semaphore has been the 'mutex' member of the tty_port struct, and
  55 commonly referred to as the port mutex (or port->mutex).
  56
  57
  58 uart_ops
  59 --------
  60
  61 The uart_ops structure is the main interface between serial_core and the
  62 hardware specific driver.  It contains all the methods to control the
  63 hardware.
  64
  65   tx_empty(port)
  66         This function tests whether the transmitter fifo and shifter
  67         for the port described by 'port' is empty.  If it is empty,
  68         this function should return TIOCSER_TEMT, otherwise return 0.
  69         If the port does not support this operation, then it should
  70         return TIOCSER_TEMT.
  71
  72         Locking: none.
  73         Interrupts: caller dependent.
  74         This call must not sleep
  75
  76   set_mctrl(port, mctrl)
  77         This function sets the modem control lines for port described
  78         by 'port' to the state described by mctrl.  The relevant bits
  79         of mctrl are:
  80                 - TIOCM_RTS     RTS signal.
  81                 - TIOCM_DTR     DTR signal.
  82                 - TIOCM_OUT1    OUT1 signal.
  83                 - TIOCM_OUT2    OUT2 signal.
  84                 - TIOCM_LOOP    Set the port into loopback mode.
  85         If the appropriate bit is set, the signal should be driven
  86         active.  If the bit is clear, the signal should be driven
  87         inactive.
  88
  89         Locking: port->lock taken.
  90         Interrupts: locally disabled.
  91         This call must not sleep
  92
  93   get_mctrl(port)
  94         Returns the current state of modem control inputs.  The state
  95         of the outputs should not be returned, since the core keeps
  96         track of their state.  The state information should include:
  97                 - TIOCM_CAR     state of DCD signal
  98                 - TIOCM_CTS     state of CTS signal
  99                 - TIOCM_DSR     state of DSR signal
 100                 - TIOCM_RI      state of RI signal
 101         The bit is set if the signal is currently driven active.  If
 102         the port does not support CTS, DCD or DSR, the driver should
 103         indicate that the signal is permanently active.  If RI is
 104         not available, the signal should not be indicated as active.
 105
 106         Locking: port->lock taken.
 107         Interrupts: locally disabled.
 108         This call must not sleep
 109
 110   stop_tx(port)
 111         Stop transmitting characters.  This might be due to the CTS
 112         line becoming inactive or the tty layer indicating we want
 113         to stop transmission due to an XOFF character.
 114
 115         The driver should stop transmitting characters as soon as
 116         possible.
 117
 118         Locking: port->lock taken.
 119         Interrupts: locally disabled.
 120         This call must not sleep
 121
 122   start_tx(port)
 123         Start transmitting characters.
 124
 125         Locking: port->lock taken.
 126         Interrupts: locally disabled.
 127         This call must not sleep
 128
 129   throttle(port)
 130         Notify the serial driver that input buffers for the line discipline are
 131         close to full, and it should somehow signal that no more characters
 132         should be sent to the serial port.
 133
 134         Locking: none.
 135
 136   send_xchar(port,ch)
 137         Transmit a high priority character, even if the port is stopped.
 138         This is used to implement XON/XOFF flow control and tcflow().  If
 139         the serial driver does not implement this function, the tty core
 140         will append the character to the circular buffer and then call
 141         start_tx() / stop_tx() to flush the data out.
 142
 143         Do not transmit if ch == '\0' (__DISABLED_CHAR).
 144
 145         Locking: none.
 146         Interrupts: caller dependent.
 147
 148   stop_rx(port)
 149         Stop receiving characters; the port is in the process of
 150         being closed.
 151
 152         Locking: port->lock taken.
 153         Interrupts: locally disabled.
 154         This call must not sleep
 155
 156   enable_ms(port)
 157         Enable the modem status interrupts.
 158
 159         This method may be called multiple times.  Modem status
 160         interrupts should be disabled when the shutdown method is
 161         called.
 162
 163         Locking: port->lock taken.
 164         Interrupts: locally disabled.
 165         This call must not sleep
 166
 167   break_ctl(port,ctl)
 168         Control the transmission of a break signal.  If ctl is
 169         nonzero, the break signal should be transmitted.  The signal
 170         should be terminated when another call is made with a zero
 171         ctl.
 172
 173         Locking: none.
 174         Interrupts: caller dependent.
 175         This call must not sleep
 176
 177   startup(port)
 178         Grab any interrupt resources and initialise any low level driver
 179         state.  Enable the port for reception.  It should not activate
 180         RTS nor DTR; this will be done via a separate call to set_mctrl.
 181
 182         This method will only be called when the port is initially opened.
 183
 184         Locking: port_sem taken.
 185         Interrupts: globally disabled.
 186
 187   shutdown(port)
 188         Disable the port, disable any break condition that may be in
 189         effect, and free any interrupt resources.  It should not disable
 190         RTS nor DTR; this will have already been done via a separate
 191         call to set_mctrl.
 192
 193         Drivers must not access port->info once this call has completed.
 194
 195         This method will only be called when there are no more users of
 196         this port.
 197
 198         Locking: port_sem taken.
 199         Interrupts: caller dependent.
 200
 201   flush_buffer(port)
 202         Flush any write buffers, reset any DMA state and stop any
 203         ongoing DMA transfers.
 204
 205         This will be called whenever the port->info->xmit circular
 206         buffer is cleared.
 207
 208         Locking: port->lock taken.
 209         Interrupts: locally disabled.
 210         This call must not sleep
 211
 212   set_termios(port,termios,oldtermios)
 213         Change the port parameters, including word length, parity, stop
 214         bits.  Update read_status_mask and ignore_status_mask to indicate
 215         the types of events we are interested in receiving.  Relevant
 216         termios->c_cflag bits are:
 217                 CSIZE   - word size
 218                 CSTOPB  - 2 stop bits
 219                 PARENB  - parity enable
 220                 PARODD  - odd parity (when PARENB is in force)
 221                 CREAD   - enable reception of characters (if not set,
 222                           still receive characters from the port, but
 223                           throw them away.
 224                 CRTSCTS - if set, enable CTS status change reporting
 225                 CLOCAL  - if not set, enable modem status change
 226                           reporting.
 227         Relevant termios->c_iflag bits are:
 228                 INPCK   - enable frame and parity error events to be
 229                           passed to the TTY layer.
 230                 BRKINT
 231                 PARMRK  - both of these enable break events to be
 232                           passed to the TTY layer.
 233
 234                 IGNPAR  - ignore parity and framing errors
 235                 IGNBRK  - ignore break errors,  If IGNPAR is also
 236                           set, ignore overrun errors as well.
 237         The interaction of the iflag bits is as follows (parity error
 238         given as an example):
 239         Parity error    INPCK   IGNPAR
 240         n/a             0       n/a     character received, marked as
 241                                         TTY_NORMAL
 242         None            1       n/a     character received, marked as
 243                                         TTY_NORMAL
 244         Yes             1       0       character received, marked as
 245                                         TTY_PARITY
 246         Yes             1       1       character discarded
 247
 248         Other flags may be used (eg, xon/xoff characters) if your
 249         hardware supports hardware "soft" flow control.
 250
 251         Locking: caller holds port->mutex
 252         Interrupts: caller dependent.
 253         This call must not sleep
 254
 255   pm(port,state,oldstate)
 256         Perform any power management related activities on the specified
 257         port.  State indicates the new state (defined by
 258         enum uart_pm_state), oldstate indicates the previous state.
 259
 260         This function should not be used to grab any resources.
 261
 262         This will be called when the port is initially opened and finally
 263         closed, except when the port is also the system console.  This
 264         will occur even if CONFIG_PM is not set.
 265
 266         Locking: none.
 267         Interrupts: caller dependent.
 268
 269   type(port)
 270         Return a pointer to a string constant describing the specified
 271         port, or return NULL, in which case the string 'unknown' is
 272         substituted.
 273
 274         Locking: none.
 275         Interrupts: caller dependent.
 276
 277   release_port(port)
 278         Release any memory and IO region resources currently in use by
 279         the port.
 280
 281         Locking: none.
 282         Interrupts: caller dependent.
 283
 284   request_port(port)
 285         Request any memory and IO region resources required by the port.
 286         If any fail, no resources should be registered when this function
 287         returns, and it should return -EBUSY on failure.
 288
 289         Locking: none.
 290         Interrupts: caller dependent.
 291
 292   config_port(port,type)
 293         Perform any autoconfiguration steps required for the port.  `type`
 294         contains a bit mask of the required configuration.  UART_CONFIG_TYPE
 295         indicates that the port requires detection and identification.
 296         port->type should be set to the type found, or PORT_UNKNOWN if
 297         no port was detected.
 298
 299         UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal,
 300         which should be probed using standard kernel autoprobing techniques.
 301         This is not necessary on platforms where ports have interrupts
 302         internally hard wired (eg, system on a chip implementations).
 303
 304         Locking: none.
 305         Interrupts: caller dependent.
 306
 307   verify_port(port,serinfo)
 308         Verify the new serial port information contained within serinfo is
 309         suitable for this port type.
 310
 311         Locking: none.
 312         Interrupts: caller dependent.
 313
 314   ioctl(port,cmd,arg)
 315         Perform any port specific IOCTLs.  IOCTL commands must be defined
 316         using the standard numbering system found in <asm/ioctl.h>
 317
 318         Locking: none.
 319         Interrupts: caller dependent.
 320
 321   poll_init(port)
 322         Called by kgdb to perform the minimal hardware initialization needed
 323         to support poll_put_char() and poll_get_char().  Unlike ->startup()
 324         this should not request interrupts.
 325
 326         Locking: tty_mutex and tty_port->mutex taken.
 327         Interrupts: n/a.
 328
 329   poll_put_char(port,ch)
 330         Called by kgdb to write a single character directly to the serial
 331         port.  It can and should block until there is space in the TX FIFO.
 332
 333         Locking: none.
 334         Interrupts: caller dependent.
 335         This call must not sleep
 336
 337   poll_get_char(port)
 338         Called by kgdb to read a single character directly from the serial
 339         port.  If data is available, it should be returned; otherwise
 340         the function should return NO_POLL_CHAR immediately.
 341
 342         Locking: none.
 343         Interrupts: caller dependent.
 344         This call must not sleep
 345
 346 Other functions
 347 ---------------
 348
 349 uart_update_timeout(port,cflag,baud)
 350         Update the FIFO drain timeout, port->timeout, according to the
 351         number of bits, parity, stop bits and baud rate.
 352
 353         Locking: caller is expected to take port->lock
 354         Interrupts: n/a
 355
 356 uart_get_baud_rate(port,termios,old,min,max)
 357         Return the numeric baud rate for the specified termios, taking
 358         account of the special 38400 baud "kludge".  The B0 baud rate
 359         is mapped to 9600 baud.
 360
 361         If the baud rate is not within min..max, then if old is non-NULL,
 362         the original baud rate will be tried.  If that exceeds the
 363         min..max constraint, 9600 baud will be returned.  termios will
 364         be updated to the baud rate in use.
 365
 366         Note: min..max must always allow 9600 baud to be selected.
 367
 368         Locking: caller dependent.
 369         Interrupts: n/a
 370
 371 uart_get_divisor(port,baud)
 372         Return the divsor (baud_base / baud) for the specified baud
 373         rate, appropriately rounded.
 374
 375         If 38400 baud and custom divisor is selected, return the
 376         custom divisor instead.
 377
 378         Locking: caller dependent.
 379         Interrupts: n/a
 380
 381 uart_match_port(port1,port2)
 382         This utility function can be used to determine whether two
 383         uart_port structures describe the same port.
 384
 385         Locking: n/a
 386         Interrupts: n/a
 387
 388 uart_write_wakeup(port)
 389         A driver is expected to call this function when the number of
 390         characters in the transmit buffer have dropped below a threshold.
 391
 392         Locking: port->lock should be held.
 393         Interrupts: n/a
 394
 395 uart_register_driver(drv)
 396         Register a uart driver with the core driver.  We in turn register
 397         with the tty layer, and initialise the core driver per-port state.
 398
 399         drv->port should be NULL, and the per-port structures should be
 400         registered using uart_add_one_port after this call has succeeded.
 401
 402         Locking: none
 403         Interrupts: enabled
 404
 405 uart_unregister_driver()
 406         Remove all references to a driver from the core driver.  The low
 407         level driver must have removed all its ports via the
 408         uart_remove_one_port() if it registered them with uart_add_one_port().
 409
 410         Locking: none
 411         Interrupts: enabled
 412
 413 uart_suspend_port()
 414
 415 uart_resume_port()
 416
 417 uart_add_one_port()
 418
 419 uart_remove_one_port()
 420
 421 Other notes
 422 -----------
 423
 424 It is intended some day to drop the 'unused' entries from uart_port, and
 425 allow low level drivers to register their own individual uart_port's with
 426 the core.  This will allow drivers to use uart_port as a pointer to a
 427 structure containing both the uart_port entry with their own extensions,
 428 thus:
 429
 430         struct my_port {
 431                 struct uart_port        port;
 432                 int                     my_stuff;
 433         };
 434
 435 Modem control lines via GPIO
 436 ----------------------------
 437
 438 Some helpers are provided in order to set/get modem control lines via GPIO.
 439
 440 mctrl_gpio_init(port, idx):
 441         This will get the {cts,rts,...}-gpios from device tree if they are
 442         present and request them, set direction etc, and return an
 443         allocated structure. devm_* functions are used, so there's no need
 444         to call mctrl_gpio_free().
 445         As this sets up the irq handling make sure to not handle changes to the
 446         gpio input lines in your driver, too.
 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.
 461
 462 mctrl_gpio_enable_ms(gpios):
 463         Enables irqs and handling of changes to the ms lines.
 464
 465 mctrl_gpio_disable_ms(gpios):
 466         Disables irqs and handling of changes to the ms lines.