Root/
| 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 | 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 | |
| 310 | Other functions |
| 311 | --------------- |
| 312 | |
| 313 | uart_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 | |
| 320 | uart_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 | |
| 335 | uart_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 | |
| 345 | uart_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 | |
| 352 | uart_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 | |
| 359 | uart_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 | |
| 369 | uart_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 | |
| 377 | uart_suspend_port() |
| 378 | |
| 379 | uart_resume_port() |
| 380 | |
| 381 | uart_add_one_port() |
| 382 | |
| 383 | uart_remove_one_port() |
| 384 | |
| 385 | Other notes |
| 386 | ----------- |
| 387 | |
| 388 | It is intended some day to drop the 'unused' entries from uart_port, and |
| 389 | allow low level drivers to register their own individual uart_port's with |
| 390 | the core. This will allow drivers to use uart_port as a pointer to a |
| 391 | structure containing both the uart_port entry with their own extensions, |
| 392 | thus: |
| 393 | |
| 394 | struct my_port { |
| 395 | struct uart_port port; |
| 396 | int my_stuff; |
| 397 | }; |
| 398 |
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