linux/Documentation/serial/driver
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   2                        Low Level Serial API
   3                        --------------------
   4
   5
   6This document is meant as a brief overview of some aspects of the new serial
   7driver.  It is not complete, any questions you have should be directed to
   8<rmk@arm.linux.org.uk>
   9
  10The reference implementation is contained within amba_pl011.c.
  11
  12
  13
  14Low Level Serial Hardware Driver
  15--------------------------------
  16
  17The low level serial hardware driver is responsible for supplying port
  18information (defined by uart_port) and a set of control methods (defined
  19by uart_ops) to the core serial driver.  The low level driver is also
  20responsible for handling interrupts for the port, and providing any
  21console support.
  22
  23
  24Console Support
  25---------------
  26
  27The serial core provides a few helper functions.  This includes identifing
  28the correct port structure (via uart_get_console) and decoding command line
  29arguments (uart_parse_options).
  30
  31There is also a helper function (uart_write_console) which performs a
  32character by character write, translating newlines to CRLF sequences.
  33Driver writers are recommended to use this function rather than implementing
  34their own version.
  35
  36
  37Locking
  38-------
  39
  40It is the responsibility of the low level hardware driver to perform the
  41necessary locking using port->lock.  There are some exceptions (which
  42are described in the uart_ops listing below.)
  43
  44There are three locks.  A per-port spinlock, a per-port tmpbuf semaphore,
  45and an overall semaphore.
  46
  47From the core driver perspective, the port->lock locks the following
  48data:
  49
  50        port->mctrl
  51        port->icount
  52        info->xmit.head (circ->head)
  53        info->xmit.tail (circ->tail)
  54
  55The low level driver is free to use this lock to provide any additional
  56locking.
  57
  58The core driver uses the info->tmpbuf_sem lock to prevent multi-threaded
  59access to the info->tmpbuf bouncebuffer used for port writes.
  60
  61The port_sem semaphore is used to protect against ports being added/
  62removed or reconfigured at inappropriate times.
  63
  64
  65uart_ops
  66--------
  67
  68The uart_ops structure is the main interface between serial_core and the
  69hardware specific driver.  It contains all the methods to control the
  70hardware.
  71
  72  tx_empty(port)
  73        This function tests whether the transmitter fifo and shifter
  74        for the port described by 'port' is empty.  If it is empty,
  75        this function should return TIOCSER_TEMT, otherwise return 0.
  76        If the port does not support this operation, then it should
  77        return TIOCSER_TEMT.
  78
  79        Locking: none.
  80        Interrupts: caller dependent.
  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                - TIOCM_RTS     RTS signal.
  88                - TIOCM_DTR     DTR signal.
  89                - TIOCM_OUT1    OUT1 signal.
  90                - TIOCM_OUT2    OUT2 signal.
  91                - TIOCM_LOOP    Set the port into loopback mode.
  92        If the appropriate bit is set, the signal should be driven
  93        active.  If the bit is clear, the signal should be driven
  94        inactive.
  95
  96        Locking: port->lock taken.
  97        Interrupts: locally disabled.
  98        This call must not sleep
  99
 100  get_mctrl(port)
 101        Returns the current state of modem control inputs.  The state
 102        of the outputs should not be returned, since the core keeps
 103        track of their state.  The state information should include:
 104                - TIOCM_CAR     state of DCD signal
 105                - TIOCM_CTS     state of CTS signal
 106                - TIOCM_DSR     state of DSR signal
 107                - TIOCM_RI      state of RI signal
 108        The bit is set if the signal is currently driven active.  If
 109        the port does not support CTS, DCD or DSR, the driver should
 110        indicate that the signal is permanently active.  If RI is
 111        not available, the signal should not be indicated as active.
 112
 113        Locking: port->lock taken.
 114        Interrupts: locally disabled.
 115        This call must not sleep
 116
 117  stop_tx(port)
 118        Stop transmitting characters.  This might be due to the CTS
 119        line becoming inactive or the tty layer indicating we want
 120        to stop transmission due to an XOFF character.
 121
 122        The driver should stop transmitting characters as soon as
 123        possible.
 124
 125        Locking: port->lock taken.
 126        Interrupts: locally disabled.
 127        This call must not sleep
 128
 129  start_tx(port)
 130        Start transmitting characters.
 131
 132        Locking: port->lock taken.
 133        Interrupts: locally disabled.
 134        This call must not sleep
 135
 136  stop_rx(port)
 137        Stop receiving characters; the port is in the process of
 138        being closed.
 139
 140        Locking: port->lock taken.
 141        Interrupts: locally disabled.
 142        This call must not sleep
 143
 144  enable_ms(port)
 145        Enable the modem status interrupts.
 146
 147        This method may be called multiple times.  Modem status
 148        interrupts should be disabled when the shutdown method is
 149        called.
 150
 151        Locking: port->lock taken.
 152        Interrupts: locally disabled.
 153        This call must not sleep
 154
 155  break_ctl(port,ctl)
 156        Control the transmission of a break signal.  If ctl is
 157        nonzero, the break signal should be transmitted.  The signal
 158        should be terminated when another call is made with a zero
 159        ctl.
 160
 161        Locking: none.
 162        Interrupts: caller dependent.
 163        This call must not sleep
 164
 165  startup(port)
 166        Grab any interrupt resources and initialise any low level driver
 167        state.  Enable the port for reception.  It should not activate
 168        RTS nor DTR; this will be done via a separate call to set_mctrl.
 169
 170        This method will only be called when the port is initially opened.
 171
 172        Locking: port_sem taken.
 173        Interrupts: globally disabled.
 174
 175  shutdown(port)
 176        Disable the port, disable any break condition that may be in
 177        effect, and free any interrupt resources.  It should not disable
 178        RTS nor DTR; this will have already been done via a separate
 179        call to set_mctrl.
 180
 181        Drivers must not access port->info once this call has completed.
 182
 183        This method will only be called when there are no more users of
 184        this port.
 185
 186        Locking: port_sem taken.
 187        Interrupts: caller dependent.
 188
 189  flush_buffer(port)
 190        Flush any write buffers, reset any DMA state and stop any
 191        ongoing DMA transfers.
 192
 193        This will be called whenever the port->info->xmit circular
 194        buffer is cleared.
 195
 196        Locking: port->lock taken.
 197        Interrupts: locally disabled.
 198        This call must not sleep
 199
 200  set_termios(port,termios,oldtermios)
 201        Change the port parameters, including word length, parity, stop
 202        bits.  Update read_status_mask and ignore_status_mask to indicate
 203        the types of events we are interested in receiving.  Relevant
 204        termios->c_cflag bits are:
 205                CSIZE   - word size
 206                CSTOPB  - 2 stop bits
 207                PARENB  - parity enable
 208                PARODD  - odd parity (when PARENB is in force)
 209                CREAD   - enable reception of characters (if not set,
 210                          still receive characters from the port, but
 211                          throw them away.
 212                CRTSCTS - if set, enable CTS status change reporting
 213                CLOCAL  - if not set, enable modem status change
 214                          reporting.
 215        Relevant termios->c_iflag bits are:
 216                INPCK   - enable frame and parity error events to be
 217                          passed to the TTY layer.
 218                BRKINT
 219                PARMRK  - both of these enable break events to be
 220                          passed to the TTY layer.
 221
 222                IGNPAR  - ignore parity and framing errors
 223                IGNBRK  - ignore break errors,  If IGNPAR is also
 224                          set, ignore overrun errors as well.
 225        The interaction of the iflag bits is as follows (parity error
 226        given as an example):
 227        Parity error    INPCK   IGNPAR
 228        n/a             0       n/a     character received, marked as
 229                                        TTY_NORMAL
 230        None            1       n/a     character received, marked as
 231                                        TTY_NORMAL
 232        Yes             1       0       character received, marked as
 233                                        TTY_PARITY
 234        Yes             1       1       character discarded
 235
 236        Other flags may be used (eg, xon/xoff characters) if your
 237        hardware supports hardware "soft" flow control.
 238
 239        Locking: none.
 240        Interrupts: caller dependent.
 241        This call must not sleep
 242
 243  pm(port,state,oldstate)
 244        Perform any power management related activities on the specified
 245        port.  State indicates the new state (defined by ACPI D0-D3),
 246        oldstate indicates the previous state.  Essentially, D0 means
 247        fully on, D3 means powered down.
 248
 249        This function should not be used to grab any resources.
 250
 251        This will be called when the port is initially opened and finally
 252        closed, except when the port is also the system console.  This
 253        will occur even if CONFIG_PM is not set.
 254
 255        Locking: none.
 256        Interrupts: caller dependent.
 257
 258  type(port)
 259        Return a pointer to a string constant describing the specified
 260        port, or return NULL, in which case the string 'unknown' is
 261        substituted.
 262
 263        Locking: none.
 264        Interrupts: caller dependent.
 265
 266  release_port(port)
 267        Release any memory and IO region resources currently in use by
 268        the port.
 269
 270        Locking: none.
 271        Interrupts: caller dependent.
 272
 273  request_port(port)
 274        Request any memory and IO region resources required by the port.
 275        If any fail, no resources should be registered when this function
 276        returns, and it should return -EBUSY on failure.
 277
 278        Locking: none.
 279        Interrupts: caller dependent.
 280
 281  config_port(port,type)
 282        Perform any autoconfiguration steps required for the port.  `type`
 283        contains a bit mask of the required configuration.  UART_CONFIG_TYPE
 284        indicates that the port requires detection and identification.
 285        port->type should be set to the type found, or PORT_UNKNOWN if
 286        no port was detected.
 287
 288        UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal,
 289        which should be probed using standard kernel autoprobing techniques.
 290        This is not necessary on platforms where ports have interrupts
 291        internally hard wired (eg, system on a chip implementations).
 292
 293        Locking: none.
 294        Interrupts: caller dependent.
 295
 296  verify_port(port,serinfo)
 297        Verify the new serial port information contained within serinfo is
 298        suitable for this port type.
 299
 300        Locking: none.
 301        Interrupts: caller dependent.
 302
 303  ioctl(port,cmd,arg)
 304        Perform any port specific IOCTLs.  IOCTL commands must be defined
 305        using the standard numbering system found in <asm/ioctl.h>
 306
 307        Locking: none.
 308        Interrupts: caller dependent.
 309
 310Other functions
 311---------------
 312
 313uart_update_timeout(port,cflag,baud)
 314        Update the FIFO drain timeout, port->timeout, according to the
 315        number of bits, parity, stop bits and baud rate.
 316
 317        Locking: caller is expected to take port->lock
 318        Interrupts: n/a
 319
 320uart_get_baud_rate(port,termios,old,min,max)
 321        Return the numeric baud rate for the specified termios, taking
 322        account of the special 38400 baud "kludge".  The B0 baud rate
 323        is mapped to 9600 baud.
 324
 325        If the baud rate is not within min..max, then if old is non-NULL,
 326        the original baud rate will be tried.  If that exceeds the
 327        min..max constraint, 9600 baud will be returned.  termios will
 328        be updated to the baud rate in use.
 329
 330        Note: min..max must always allow 9600 baud to be selected.
 331
 332        Locking: caller dependent.
 333        Interrupts: n/a
 334
 335uart_get_divisor(port,baud)
 336        Return the divsor (baud_base / baud) for the specified baud
 337        rate, appropriately rounded.
 338
 339        If 38400 baud and custom divisor is selected, return the
 340        custom divisor instead.
 341
 342        Locking: caller dependent.
 343        Interrupts: n/a
 344
 345uart_match_port(port1,port2)
 346        This utility function can be used to determine whether two
 347        uart_port structures describe the same port.
 348
 349        Locking: n/a
 350        Interrupts: n/a
 351
 352uart_write_wakeup(port)
 353        A driver is expected to call this function when the number of
 354        characters in the transmit buffer have dropped below a threshold.
 355
 356        Locking: port->lock should be held.
 357        Interrupts: n/a
 358
 359uart_register_driver(drv)
 360        Register a uart driver with the core driver.  We in turn register
 361        with the tty layer, and initialise the core driver per-port state.
 362
 363        drv->port should be NULL, and the per-port structures should be
 364        registered using uart_add_one_port after this call has succeeded.
 365
 366        Locking: none
 367        Interrupts: enabled
 368
 369uart_unregister_driver()
 370        Remove all references to a driver from the core driver.  The low
 371        level driver must have removed all its ports via the
 372        uart_remove_one_port() if it registered them with uart_add_one_port().
 373
 374        Locking: none
 375        Interrupts: enabled
 376
 377uart_suspend_port()
 378
 379uart_resume_port()
 380
 381uart_add_one_port()
 382
 383uart_remove_one_port()
 384
 385Other notes
 386-----------
 387
 388It is intended some day to drop the 'unused' entries from uart_port, and
 389allow low level drivers to register their own individual uart_port's with
 390the core.  This will allow drivers to use uart_port as a pointer to a
 391structure containing both the uart_port entry with their own extensions,
 392thus:
 393
 394        struct my_port {
 395                struct uart_port        port;
 396                int                     my_stuff;
 397        };
 398
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