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 three locks.  A per-port spinlock, a per-port tmpbuf semaphore,
  40 and an overall semaphore.
  41
  42 From the core driver perspective, the port->lock locks the following
  43 data:
  44
  45         port->mctrl
  46         port->icount
  47         info->xmit.head (circ->head)
  48         info->xmit.tail (circ->tail)
  49
  50 The low level driver is free to use this lock to provide any additional
  51 locking.
  52
  53 The core driver uses the info->tmpbuf_sem lock to prevent multi-threaded
  54 access to the info->tmpbuf bouncebuffer used for port writes.
  55
  56 The port_sem semaphore is used to protect against ports being added/
  57 removed or reconfigured at inappropriate times. Since v2.6.27, this
  58 semaphore has been the 'mutex' member of the tty_port struct, and
  59 commonly referred to as the port mutex (or port->mutex).
  60
  61
  62 uart_ops
  63 --------
  64
  65 The uart_ops structure is the main interface between serial_core and the
  66 hardware specific driver.  It contains all the methods to control the
  67 hardware.
  68
  69   tx_empty(port)
  70         This function tests whether the transmitter fifo and shifter
  71         for the port described by 'port' is empty.  If it is empty,
  72         this function should return TIOCSER_TEMT, otherwise return 0.
  73         If the port does not support this operation, then it should
  74         return TIOCSER_TEMT.
  75
  76         Locking: none.
  77         Interrupts: caller dependent.
  78         This call must not sleep
  79
  80   set_mctrl(port, mctrl)
  81         This function sets the modem control lines for port described
  82         by 'port' to the state described by mctrl.  The relevant bits
  83         of mctrl are:
  84                 - TIOCM_RTS     RTS signal.
  85                 - TIOCM_DTR     DTR signal.
  86                 - TIOCM_OUT1    OUT1 signal.
  87                 - TIOCM_OUT2    OUT2 signal.
  88                 - TIOCM_LOOP    Set the port into loopback mode.
  89         If the appropriate bit is set, the signal should be driven
  90         active.  If the bit is clear, the signal should be driven
  91         inactive.
  92
  93         Locking: port->lock taken.
  94         Interrupts: locally disabled.
  95         This call must not sleep
  96
  97   get_mctrl(port)
  98         Returns the current state of modem control inputs.  The state
  99         of the outputs should not be returned, since the core keeps
 100         track of their state.  The state information should include:
 101                 - TIOCM_CAR     state of DCD signal
 102                 - TIOCM_CTS     state of CTS signal
 103                 - TIOCM_DSR     state of DSR signal
 104                 - TIOCM_RI      state of RI signal
 105         The bit is set if the signal is currently driven active.  If
 106         the port does not support CTS, DCD or DSR, the driver should
 107         indicate that the signal is permanently active.  If RI is
 108         not available, the signal should not be indicated as active.
 109
 110         Locking: port->lock taken.
 111         Interrupts: locally disabled.
 112         This call must not sleep
 113
 114   stop_tx(port)
 115         Stop transmitting characters.  This might be due to the CTS
 116         line becoming inactive or the tty layer indicating we want
 117         to stop transmission due to an XOFF character.
 118
 119         The driver should stop transmitting characters as soon as
 120         possible.
 121
 122         Locking: port->lock taken.
 123         Interrupts: locally disabled.
 124         This call must not sleep
 125
 126   start_tx(port)
 127         Start transmitting characters.
 128
 129         Locking: port->lock taken.
 130         Interrupts: locally disabled.
 131         This call must not sleep
 132
 133   send_xchar(port,ch)
 134         Transmit a high priority character, even if the port is stopped.
 135         This is used to implement XON/XOFF flow control and tcflow().  If
 136         the serial driver does not implement this function, the tty core
 137         will append the character to the circular buffer and then call
 138         start_tx() / stop_tx() to flush the data out.
 139
 140         Do not transmit if ch == '\0' (__DISABLED_CHAR).
 141
 142         Locking: none.
 143         Interrupts: caller dependent.
 144
 145   stop_rx(port)
 146         Stop receiving characters; the port is in the process of
 147         being closed.
 148
 149         Locking: port->lock taken.
 150         Interrupts: locally disabled.
 151         This call must not sleep
 152
 153   enable_ms(port)
 154         Enable the modem status interrupts.
 155
 156         This method may be called multiple times.  Modem status
 157         interrupts should be disabled when the shutdown method is
 158         called.
 159
 160         Locking: port->lock taken.
 161         Interrupts: locally disabled.
 162         This call must not sleep
 163
 164   break_ctl(port,ctl)
 165         Control the transmission of a break signal.  If ctl is
 166         nonzero, the break signal should be transmitted.  The signal
 167         should be terminated when another call is made with a zero
 168         ctl.
 169
 170         Locking: none.
 171         Interrupts: caller dependent.
 172         This call must not sleep
 173
 174   startup(port)
 175         Grab any interrupt resources and initialise any low level driver
 176         state.  Enable the port for reception.  It should not activate
 177         RTS nor DTR; this will be done via a separate call to set_mctrl.
 178
 179         This method will only be called when the port is initially opened.
 180
 181         Locking: port_sem taken.
 182         Interrupts: globally disabled.
 183
 184   shutdown(port)
 185         Disable the port, disable any break condition that may be in
 186         effect, and free any interrupt resources.  It should not disable
 187         RTS nor DTR; this will have already been done via a separate
 188         call to set_mctrl.
 189
 190         Drivers must not access port->info once this call has completed.
 191
 192         This method will only be called when there are no more users of
 193         this port.
 194
 195         Locking: port_sem taken.
 196         Interrupts: caller dependent.
 197
 198   flush_buffer(port)
 199         Flush any write buffers, reset any DMA state and stop any
 200         ongoing DMA transfers.
 201
 202         This will be called whenever the port->info->xmit circular
 203         buffer is cleared.
 204
 205         Locking: port->lock taken.
 206         Interrupts: locally disabled.
 207         This call must not sleep
 208
 209   set_termios(port,termios,oldtermios)
 210         Change the port parameters, including word length, parity, stop
 211         bits.  Update read_status_mask and ignore_status_mask to indicate
 212         the types of events we are interested in receiving.  Relevant
 213         termios->c_cflag bits are:
 214                 CSIZE   - word size
 215                 CSTOPB  - 2 stop bits
 216                 PARENB  - parity enable
 217                 PARODD  - odd parity (when PARENB is in force)
 218                 CREAD   - enable reception of characters (if not set,
 219                           still receive characters from the port, but
 220                           throw them away.
 221                 CRTSCTS - if set, enable CTS status change reporting
 222                 CLOCAL  - if not set, enable modem status change
 223                           reporting.
 224         Relevant termios->c_iflag bits are:
 225                 INPCK   - enable frame and parity error events to be
 226                           passed to the TTY layer.
 227                 BRKINT
 228                 PARMRK  - both of these enable break events to be
 229                           passed to the TTY layer.
 230
 231                 IGNPAR  - ignore parity and framing errors
 232                 IGNBRK  - ignore break errors,  If IGNPAR is also
 233                           set, ignore overrun errors as well.
 234         The interaction of the iflag bits is as follows (parity error
 235         given as an example):
 236         Parity error    INPCK   IGNPAR
 237         n/a             0       n/a     character received, marked as
 238                                         TTY_NORMAL
 239         None            1       n/a     character received, marked as
 240                                         TTY_NORMAL
 241         Yes             1       0       character received, marked as
 242                                         TTY_PARITY
 243         Yes             1       1       character discarded
 244
 245         Other flags may be used (eg, xon/xoff characters) if your
 246         hardware supports hardware "soft" flow control.
 247
 248         Locking: caller holds port->mutex
 249         Interrupts: caller dependent.
 250         This call must not sleep
 251
 252   pm(port,state,oldstate)
 253         Perform any power management related activities on the specified
 254         port.  State indicates the new state (defined by
 255         enum uart_pm_state), oldstate indicates the previous state.
 256
 257         This function should not be used to grab any resources.
 258
 259         This will be called when the port is initially opened and finally
 260         closed, except when the port is also the system console.  This
 261         will occur even if CONFIG_PM is not set.
 262
 263         Locking: none.
 264         Interrupts: caller dependent.
 265
 266   type(port)
 267         Return a pointer to a string constant describing the specified
 268         port, or return NULL, in which case the string 'unknown' is
 269         substituted.
 270
 271         Locking: none.
 272         Interrupts: caller dependent.
 273
 274   release_port(port)
 275         Release any memory and IO region resources currently in use by
 276         the port.
 277
 278         Locking: none.
 279         Interrupts: caller dependent.
 280
 281   request_port(port)
 282         Request any memory and IO region resources required by the port.
 283         If any fail, no resources should be registered when this function
 284         returns, and it should return -EBUSY on failure.
 285
 286         Locking: none.
 287         Interrupts: caller dependent.
 288
 289   config_port(port,type)
 290         Perform any autoconfiguration steps required for the port.  `type`
 291         contains a bit mask of the required configuration.  UART_CONFIG_TYPE
 292         indicates that the port requires detection and identification.
 293         port->type should be set to the type found, or PORT_UNKNOWN if
 294         no port was detected.
 295
 296         UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal,
 297         which should be probed using standard kernel autoprobing techniques.
 298         This is not necessary on platforms where ports have interrupts
 299         internally hard wired (eg, system on a chip implementations).
 300
 301         Locking: none.
 302         Interrupts: caller dependent.
 303
 304   verify_port(port,serinfo)
 305         Verify the new serial port information contained within serinfo is
 306         suitable for this port type.
 307
 308         Locking: none.
 309         Interrupts: caller dependent.
 310
 311   ioctl(port,cmd,arg)
 312         Perform any port specific IOCTLs.  IOCTL commands must be defined
 313         using the standard numbering system found in <asm/ioctl.h>
 314
 315         Locking: none.
 316         Interrupts: caller dependent.
 317
 318   poll_init(port)
 319         Called by kgdb to perform the minimal hardware initialization needed
 320         to support poll_put_char() and poll_get_char().  Unlike ->startup()
 321         this should not request interrupts.
 322
 323         Locking: tty_mutex and tty_port->mutex taken.
 324         Interrupts: n/a.
 325
 326   poll_put_char(port,ch)
 327         Called by kgdb to write a single character directly to the serial
 328         port.  It can and should block until there is space in the TX FIFO.
 329
 330         Locking: none.
 331         Interrupts: caller dependent.
 332         This call must not sleep
 333
 334   poll_get_char(port)
 335         Called by kgdb to read a single character directly from the serial
 336         port.  If data is available, it should be returned; otherwise
 337         the function should return NO_POLL_CHAR immediately.
 338
 339         Locking: none.
 340         Interrupts: caller dependent.
 341         This call must not sleep
 342
 343 Other functions
 344 ---------------
 345
 346 uart_update_timeout(port,cflag,baud)
 347         Update the FIFO drain timeout, port->timeout, according to the
 348         number of bits, parity, stop bits and baud rate.
 349
 350         Locking: caller is expected to take port->lock
 351         Interrupts: n/a
 352
 353 uart_get_baud_rate(port,termios,old,min,max)
 354         Return the numeric baud rate for the specified termios, taking
 355         account of the special 38400 baud "kludge".  The B0 baud rate
 356         is mapped to 9600 baud.
 357
 358         If the baud rate is not within min..max, then if old is non-NULL,
 359         the original baud rate will be tried.  If that exceeds the
 360         min..max constraint, 9600 baud will be returned.  termios will
 361         be updated to the baud rate in use.
 362
 363         Note: min..max must always allow 9600 baud to be selected.
 364
 365         Locking: caller dependent.
 366         Interrupts: n/a
 367
 368 uart_get_divisor(port,baud)
 369         Return the divsor (baud_base / baud) for the specified baud
 370         rate, appropriately rounded.
 371
 372         If 38400 baud and custom divisor is selected, return the
 373         custom divisor instead.
 374
 375         Locking: caller dependent.
 376         Interrupts: n/a
 377
 378 uart_match_port(port1,port2)
 379         This utility function can be used to determine whether two
 380         uart_port structures describe the same port.
 381
 382         Locking: n/a
 383         Interrupts: n/a
 384
 385 uart_write_wakeup(port)
 386         A driver is expected to call this function when the number of
 387         characters in the transmit buffer have dropped below a threshold.
 388
 389         Locking: port->lock should be held.
 390         Interrupts: n/a
 391
 392 uart_register_driver(drv)
 393         Register a uart driver with the core driver.  We in turn register
 394         with the tty layer, and initialise the core driver per-port state.
 395
 396         drv->port should be NULL, and the per-port structures should be
 397         registered using uart_add_one_port after this call has succeeded.
 398
 399         Locking: none
 400         Interrupts: enabled
 401
 402 uart_unregister_driver()
 403         Remove all references to a driver from the core driver.  The low
 404         level driver must have removed all its ports via the
 405         uart_remove_one_port() if it registered them with uart_add_one_port().
 406
 407         Locking: none
 408         Interrupts: enabled
 409
 410 uart_suspend_port()
 411
 412 uart_resume_port()
 413
 414 uart_add_one_port()
 415
 416 uart_remove_one_port()
 417
 418 Other notes
 419 -----------
 420
 421 It is intended some day to drop the 'unused' entries from uart_port, and
 422 allow low level drivers to register their own individual uart_port's with
 423 the core.  This will allow drivers to use uart_port as a pointer to a
 424 structure containing both the uart_port entry with their own extensions,
 425 thus:
 426
 427         struct my_port {
 428                 struct uart_port        port;
 429                 int                     my_stuff;
 430         };
 431
 432 Modem control lines via GPIO
 433 ----------------------------
 434
 435 Some helpers are provided in order to set/get modem control lines via GPIO.
 436
 437 mctrl_gpio_init(port, idx):
 438         This will get the {cts,rts,...}-gpios from device tree if they are
 439         present and request them, set direction etc, and return an
 440         allocated structure. devm_* functions are used, so there's no need
 441         to call mctrl_gpio_free().
 442         As this sets up the irq handling make sure to not handle changes to the
 443         gpio input lines in your driver, too.
 444
 445 mctrl_gpio_free(dev, gpios):
 446         This will free the requested gpios in mctrl_gpio_init().
 447         As devm_* function are used, there's generally no need to call
 448         this function.
 449
 450 mctrl_gpio_to_gpiod(gpios, gidx)
 451         This returns the gpio structure associated to the modem line index.
 452
 453 mctrl_gpio_set(gpios, mctrl):
 454         This will sets the gpios according to the mctrl state.
 455
 456 mctrl_gpio_get(gpios, mctrl):
 457         This will update mctrl with the gpios values.
 458
 459 mctrl_gpio_enable_ms(gpios):
 460         Enables irqs and handling of changes to the ms lines.
 461
 462 mctrl_gpio_disable_ms(gpios):
 463         Disables irqs and handling of changes to the ms lines.