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