Based on kernel version 4.9. Page generated on 2016-12-21 14:37 EST.
1 2 The Lockronomicon 3 4 Your guide to the ancient and twisted locking policies of the tty layer and 5 the warped logic behind them. Beware all ye who read on. 6 7 8 Line Discipline 9 --------------- 10 11 Line disciplines are registered with tty_register_ldisc() passing the 12 discipline number and the ldisc structure. At the point of registration the 13 discipline must be ready to use and it is possible it will get used before 14 the call returns success. If the call returns an error then it won't get 15 called. Do not re-use ldisc numbers as they are part of the userspace ABI 16 and writing over an existing ldisc will cause demons to eat your computer. 17 After the return the ldisc data has been copied so you may free your own 18 copy of the structure. You must not re-register over the top of the line 19 discipline even with the same data or your computer again will be eaten by 20 demons. 21 22 In order to remove a line discipline call tty_unregister_ldisc(). 23 In ancient times this always worked. In modern times the function will 24 return -EBUSY if the ldisc is currently in use. Since the ldisc referencing 25 code manages the module counts this should not usually be a concern. 26 27 Heed this warning: the reference count field of the registered copies of the 28 tty_ldisc structure in the ldisc table counts the number of lines using this 29 discipline. The reference count of the tty_ldisc structure within a tty 30 counts the number of active users of the ldisc at this instant. In effect it 31 counts the number of threads of execution within an ldisc method (plus those 32 about to enter and exit although this detail matters not). 33 34 Line Discipline Methods 35 ----------------------- 36 37 TTY side interfaces: 38 39 open() - Called when the line discipline is attached to 40 the terminal. No other call into the line 41 discipline for this tty will occur until it 42 completes successfully. Should initialize any 43 state needed by the ldisc, and set receive_room 44 in the tty_struct to the maximum amount of data 45 the line discipline is willing to accept from the 46 driver with a single call to receive_buf(). 47 Returning an error will prevent the ldisc from 48 being attached. Can sleep. 49 50 close() - This is called on a terminal when the line 51 discipline is being unplugged. At the point of 52 execution no further users will enter the 53 ldisc code for this tty. Can sleep. 54 55 hangup() - Called when the tty line is hung up. 56 The line discipline should cease I/O to the tty. 57 No further calls into the ldisc code will occur. 58 The return value is ignored. Can sleep. 59 60 read() - (optional) A process requests reading data from 61 the line. Multiple read calls may occur in parallel 62 and the ldisc must deal with serialization issues. 63 If not defined, the process will receive an EIO 64 error. May sleep. 65 66 write() - (optional) A process requests writing data to the 67 line. Multiple write calls are serialized by the 68 tty layer for the ldisc. If not defined, the 69 process will receive an EIO error. May sleep. 70 71 flush_buffer() - (optional) May be called at any point between 72 open and close, and instructs the line discipline 73 to empty its input buffer. 74 75 set_termios() - (optional) Called on termios structure changes. 76 The caller passes the old termios data and the 77 current data is in the tty. Called under the 78 termios semaphore so allowed to sleep. Serialized 79 against itself only. 80 81 poll() - (optional) Check the status for the poll/select 82 calls. Multiple poll calls may occur in parallel. 83 May sleep. 84 85 ioctl() - (optional) Called when an ioctl is handed to the 86 tty layer that might be for the ldisc. Multiple 87 ioctl calls may occur in parallel. May sleep. 88 89 compat_ioctl() - (optional) Called when a 32 bit ioctl is handed 90 to the tty layer that might be for the ldisc. 91 Multiple ioctl calls may occur in parallel. 92 May sleep. 93 94 Driver Side Interfaces: 95 96 receive_buf() - (optional) Called by the low-level driver to hand 97 a buffer of received bytes to the ldisc for 98 processing. The number of bytes is guaranteed not 99 to exceed the current value of tty->receive_room. 100 All bytes must be processed. 101 102 receive_buf2() - (optional) Called by the low-level driver to hand 103 a buffer of received bytes to the ldisc for 104 processing. Returns the number of bytes processed. 105 106 If both receive_buf() and receive_buf2() are 107 defined, receive_buf2() should be preferred. 108 109 write_wakeup() - May be called at any point between open and close. 110 The TTY_DO_WRITE_WAKEUP flag indicates if a call 111 is needed but always races versus calls. Thus the 112 ldisc must be careful about setting order and to 113 handle unexpected calls. Must not sleep. 114 115 The driver is forbidden from calling this directly 116 from the ->write call from the ldisc as the ldisc 117 is permitted to call the driver write method from 118 this function. In such a situation defer it. 119 120 dcd_change() - Report to the tty line the current DCD pin status 121 changes and the relative timestamp. The timestamp 122 cannot be NULL. 123 124 125 Driver Access 126 127 Line discipline methods can call the following methods of the underlying 128 hardware driver through the function pointers within the tty->driver 129 structure: 130 131 write() Write a block of characters to the tty device. 132 Returns the number of characters accepted. The 133 character buffer passed to this method is already 134 in kernel space. 135 136 put_char() Queues a character for writing to the tty device. 137 If there is no room in the queue, the character is 138 ignored. 139 140 flush_chars() (Optional) If defined, must be called after 141 queueing characters with put_char() in order to 142 start transmission. 143 144 write_room() Returns the numbers of characters the tty driver 145 will accept for queueing to be written. 146 147 ioctl() Invoke device specific ioctl. 148 Expects data pointers to refer to userspace. 149 Returns ENOIOCTLCMD for unrecognized ioctl numbers. 150 151 set_termios() Notify the tty driver that the device's termios 152 settings have changed. New settings are in 153 tty->termios. Previous settings should be passed in 154 the "old" argument. 155 156 The API is defined such that the driver should return 157 the actual modes selected. This means that the 158 driver function is responsible for modifying any 159 bits in the request it cannot fulfill to indicate 160 the actual modes being used. A device with no 161 hardware capability for change (e.g. a USB dongle or 162 virtual port) can provide NULL for this method. 163 164 throttle() Notify the tty driver that input buffers for the 165 line discipline are close to full, and it should 166 somehow signal that no more characters should be 167 sent to the tty. 168 169 unthrottle() Notify the tty driver that characters can now be 170 sent to the tty without fear of overrunning the 171 input buffers of the line disciplines. 172 173 stop() Ask the tty driver to stop outputting characters 174 to the tty device. 175 176 start() Ask the tty driver to resume sending characters 177 to the tty device. 178 179 hangup() Ask the tty driver to hang up the tty device. 180 181 break_ctl() (Optional) Ask the tty driver to turn on or off 182 BREAK status on the RS-232 port. If state is -1, 183 then the BREAK status should be turned on; if 184 state is 0, then BREAK should be turned off. 185 If this routine is not implemented, use ioctls 186 TIOCSBRK / TIOCCBRK instead. 187 188 wait_until_sent() Waits until the device has written out all of the 189 characters in its transmitter FIFO. 190 191 send_xchar() Send a high-priority XON/XOFF character to the device. 192 193 194 Flags 195 196 Line discipline methods have access to tty->flags field containing the 197 following interesting flags: 198 199 TTY_THROTTLED Driver input is throttled. The ldisc should call 200 tty->driver->unthrottle() in order to resume 201 reception when it is ready to process more data. 202 203 TTY_DO_WRITE_WAKEUP If set, causes the driver to call the ldisc's 204 write_wakeup() method in order to resume 205 transmission when it can accept more data 206 to transmit. 207 208 TTY_IO_ERROR If set, causes all subsequent userspace read/write 209 calls on the tty to fail, returning -EIO. 210 211 TTY_OTHER_CLOSED Device is a pty and the other side has closed. 212 213 TTY_NO_WRITE_SPLIT Prevent driver from splitting up writes into 214 smaller chunks. 215 216 217 Locking 218 219 Callers to the line discipline functions from the tty layer are required to 220 take line discipline locks. The same is true of calls from the driver side 221 but not yet enforced. 222 223 Three calls are now provided 224 225 ldisc = tty_ldisc_ref(tty); 226 227 takes a handle to the line discipline in the tty and returns it. If no ldisc 228 is currently attached or the ldisc is being closed and re-opened at this 229 point then NULL is returned. While this handle is held the ldisc will not 230 change or go away. 231 232 tty_ldisc_deref(ldisc) 233 234 Returns the ldisc reference and allows the ldisc to be closed. Returning the 235 reference takes away your right to call the ldisc functions until you take 236 a new reference. 237 238 ldisc = tty_ldisc_ref_wait(tty); 239 240 Performs the same function as tty_ldisc_ref except that it will wait for an 241 ldisc change to complete and then return a reference to the new ldisc. 242 243 While these functions are slightly slower than the old code they should have 244 minimal impact as most receive logic uses the flip buffers and they only 245 need to take a reference when they push bits up through the driver. 246 247 A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc 248 functions are called with the ldisc unavailable. Thus tty_ldisc_ref will 249 fail in this situation if used within these functions. Ldisc and driver 250 code calling its own functions must be careful in this case. 251 252 253 Driver Interface 254 ---------------- 255 256 open() - Called when a device is opened. May sleep 257 258 close() - Called when a device is closed. At the point of 259 return from this call the driver must make no 260 further ldisc calls of any kind. May sleep 261 262 write() - Called to write bytes to the device. May not 263 sleep. May occur in parallel in special cases. 264 Because this includes panic paths drivers generally 265 shouldn't try and do clever locking here. 266 267 put_char() - Stuff a single character onto the queue. The 268 driver is guaranteed following up calls to 269 flush_chars. 270 271 flush_chars() - Ask the kernel to write put_char queue 272 273 write_room() - Return the number of characters that can be stuffed 274 into the port buffers without overflow (or less). 275 The ldisc is responsible for being intelligent 276 about multi-threading of write_room/write calls 277 278 ioctl() - Called when an ioctl may be for the driver 279 280 set_termios() - Called on termios change, serialized against 281 itself by a semaphore. May sleep. 282 283 set_ldisc() - Notifier for discipline change. At the point this 284 is done the discipline is not yet usable. Can now 285 sleep (I think) 286 287 throttle() - Called by the ldisc to ask the driver to do flow 288 control. Serialization including with unthrottle 289 is the job of the ldisc layer. 290 291 unthrottle() - Called by the ldisc to ask the driver to stop flow 292 control. 293 294 stop() - Ldisc notifier to the driver to stop output. As with 295 throttle the serializations with start() are down 296 to the ldisc layer. 297 298 start() - Ldisc notifier to the driver to start output. 299 300 hangup() - Ask the tty driver to cause a hangup initiated 301 from the host side. [Can sleep ??] 302 303 break_ctl() - Send RS232 break. Can sleep. Can get called in 304 parallel, driver must serialize (for now), and 305 with write calls. 306 307 wait_until_sent() - Wait for characters to exit the hardware queue 308 of the driver. Can sleep 309 310 send_xchar() - Send XON/XOFF and if possible jump the queue with 311 it in order to get fast flow control responses. 312 Cannot sleep ??