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