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.
  63
  64
  65 uart_ops
  66 --------
  67
  68 The uart_ops structure is the main interface between serial_core and the
  69 hardware specific driver.  It contains all the methods to control the
  70 hardware.
  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_DCD     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   set_termios(port,termios,oldtermios)
 190         Change the port parameters, including word length, parity, stop
 191         bits.  Update read_status_mask and ignore_status_mask to indicate
 192         the types of events we are interested in receiving.  Relevant
 193         termios->c_cflag bits are:
 194                 CSIZE   - word size
 195                 CSTOPB  - 2 stop bits
 196                 PARENB  - parity enable
 197                 PARODD  - odd parity (when PARENB is in force)
 198                 CREAD   - enable reception of characters (if not set,
 199                           still receive characters from the port, but
 200                           throw them away.
 201                 CRTSCTS - if set, enable CTS status change reporting
 202                 CLOCAL  - if not set, enable modem status change
 203                           reporting.
 204         Relevant termios->c_iflag bits are:
 205                 INPCK   - enable frame and parity error events to be
 206                           passed to the TTY layer.
 207                 BRKINT
 208                 PARMRK  - both of these enable break events to be
 209                           passed to the TTY layer.
 210
 211                 IGNPAR  - ignore parity and framing errors
 212                 IGNBRK  - ignore break errors,  If IGNPAR is also
 213                           set, ignore overrun errors as well.
 214         The interaction of the iflag bits is as follows (parity error
 215         given as an example):
 216         Parity error    INPCK   IGNPAR
 217         n/a             0       n/a     character received, marked as
 218                                         TTY_NORMAL
 219         None            1       n/a     character received, marked as
 220                                         TTY_NORMAL
 221         Yes             1       0       character received, marked as
 222                                         TTY_PARITY
 223         Yes             1       1       character discarded
 224
 225         Other flags may be used (eg, xon/xoff characters) if your
 226         hardware supports hardware "soft" flow control.
 227
 228         Locking: none.
 229         Interrupts: caller dependent.
 230         This call must not sleep
 231
 232   pm(port,state,oldstate)
 233         Perform any power management related activities on the specified
 234         port.  State indicates the new state (defined by ACPI D0-D3),
 235         oldstate indicates the previous state.  Essentially, D0 means
 236         fully on, D3 means powered down.
 237
 238         This function should not be used to grab any resources.
 239
 240         This will be called when the port is initially opened and finally
 241         closed, except when the port is also the system console.  This
 242         will occur even if CONFIG_PM is not set.
 243
 244         Locking: none.
 245         Interrupts: caller dependent.
 246
 247   type(port)
 248         Return a pointer to a string constant describing the specified
 249         port, or return NULL, in which case the string 'unknown' is
 250         substituted.
 251
 252         Locking: none.
 253         Interrupts: caller dependent.
 254
 255   release_port(port)
 256         Release any memory and IO region resources currently in use by
 257         the port.
 258
 259         Locking: none.
 260         Interrupts: caller dependent.
 261
 262   request_port(port)
 263         Request any memory and IO region resources required by the port.
 264         If any fail, no resources should be registered when this function
 265         returns, and it should return -EBUSY on failure.
 266
 267         Locking: none.
 268         Interrupts: caller dependent.
 269
 270   config_port(port,type)
 271         Perform any autoconfiguration steps required for the port.  `type`
 272         contains a bit mask of the required configuration.  UART_CONFIG_TYPE
 273         indicates that the port requires detection and identification.
 274         port->type should be set to the type found, or PORT_UNKNOWN if
 275         no port was detected.
 276
 277         UART_CONFIG_IRQ indicates autoconfiguration of the interrupt signal,
 278         which should be probed using standard kernel autoprobing techniques.
 279         This is not necessary on platforms where ports have interrupts
 280         internally hard wired (eg, system on a chip implementations).
 281
 282         Locking: none.
 283         Interrupts: caller dependent.
 284
 285   verify_port(port,serinfo)
 286         Verify the new serial port information contained within serinfo is
 287         suitable for this port type.
 288
 289         Locking: none.
 290         Interrupts: caller dependent.
 291
 292   ioctl(port,cmd,arg)
 293         Perform any port specific IOCTLs.  IOCTL commands must be defined
 294         using the standard numbering system found in <asm/ioctl.h>
 295
 296         Locking: none.
 297         Interrupts: caller dependent.
 298
 299 Other functions
 300 ---------------
 301
 302 uart_update_timeout(port,cflag,baud)
 303         Update the FIFO drain timeout, port->timeout, according to the
 304         number of bits, parity, stop bits and baud rate.
 305
 306         Locking: caller is expected to take port->lock
 307         Interrupts: n/a
 308
 309 uart_get_baud_rate(port,termios,old,min,max)
 310         Return the numeric baud rate for the specified termios, taking
 311         account of the special 38400 baud "kludge".  The B0 baud rate
 312         is mapped to 9600 baud.
 313
 314         If the baud rate is not within min..max, then if old is non-NULL,
 315         the original baud rate will be tried.  If that exceeds the
 316         min..max constraint, 9600 baud will be returned.  termios will
 317         be updated to the baud rate in use.
 318
 319         Note: min..max must always allow 9600 baud to be selected.
 320
 321         Locking: caller dependent.
 322         Interrupts: n/a
 323
 324 uart_get_divisor(port,baud)
 325         Return the divsor (baud_base / baud) for the specified baud
 326         rate, appropriately rounded.
 327
 328         If 38400 baud and custom divisor is selected, return the
 329         custom divisor instead.
 330
 331         Locking: caller dependent.
 332         Interrupts: n/a
 333
 334 uart_match_port(port1,port2)
 335         This utility function can be used to determine whether two
 336         uart_port structures describe the same port.
 337
 338         Locking: n/a
 339         Interrupts: n/a
 340
 341 uart_write_wakeup(port)
 342         A driver is expected to call this function when the number of
 343         characters in the transmit buffer have dropped below a threshold.
 344
 345         Locking: port->lock should be held.
 346         Interrupts: n/a
 347
 348 uart_register_driver(drv)
 349         Register a uart driver with the core driver.  We in turn register
 350         with the tty layer, and initialise the core driver per-port state.
 351
 352         drv->port should be NULL, and the per-port structures should be
 353         registered using uart_add_one_port after this call has succeeded.
 354
 355         Locking: none
 356         Interrupts: enabled
 357
 358 uart_unregister_driver()
 359         Remove all references to a driver from the core driver.  The low
 360         level driver must have removed all its ports via the
 361         uart_remove_one_port() if it registered them with uart_add_one_port().
 362
 363         Locking: none
 364         Interrupts: enabled
 365
 366 uart_suspend_port()
 367
 368 uart_resume_port()
 369
 370 uart_add_one_port()
 371
 372 uart_remove_one_port()
 373
 374 Other notes
 375 -----------
 376
 377 It is intended some day to drop the 'unused' entries from uart_port, and
 378 allow low level drivers to register their own individual uart_port's with
 379 the core.  This will allow drivers to use uart_port as a pointer to a
 380 structure containing both the uart_port entry with their own extensions,
 381 thus:
 382
 383         struct my_port {
 384                 struct uart_port        port;
 385                 int                     my_stuff;
 386         };