1================= 2The Lockronomicon 3================= 4 5Your guide to the ancient and twisted locking policies of the tty layer and 6the warped logic behind them. Beware all ye who read on. 7 8 9Line Discipline 10--------------- 11 12Line disciplines are registered with tty_register_ldisc() passing the 13discipline number and the ldisc structure. At the point of registration the 14discipline must be ready to use and it is possible it will get used before 15the call returns success. If the call returns an error then it won't get 16called. Do not re-use ldisc numbers as they are part of the userspace ABI 17and writing over an existing ldisc will cause demons to eat your computer. 18After the return the ldisc data has been copied so you may free your own 19copy of the structure. You must not re-register over the top of the line 20discipline even with the same data or your computer again will be eaten by 21demons. 22 23In order to remove a line discipline call tty_unregister_ldisc(). 24In ancient times this always worked. In modern times the function will 25return -EBUSY if the ldisc is currently in use. Since the ldisc referencing 26code manages the module counts this should not usually be a concern. 27 28Heed this warning: the reference count field of the registered copies of the 29tty_ldisc structure in the ldisc table counts the number of lines using this 30discipline. The reference count of the tty_ldisc structure within a tty 31counts the number of active users of the ldisc at this instant. In effect it 32counts the number of threads of execution within an ldisc method (plus those 33about to enter and exit although this detail matters not). 34 35Line Discipline Methods 36----------------------- 37 38TTY side interfaces 39^^^^^^^^^^^^^^^^^^^ 40 41======================= ======================================================= 42open() Called when the line discipline is attached to 43 the terminal. No other call into the line 44 discipline for this tty will occur until it 45 completes successfully. Should initialize any 46 state needed by the ldisc, and set receive_room 47 in the tty_struct to the maximum amount of data 48 the line discipline is willing to accept from the 49 driver with a single call to receive_buf(). 50 Returning an error will prevent the ldisc from 51 being attached. Can sleep. 52 53close() This is called on a terminal when the line 54 discipline is being unplugged. At the point of 55 execution no further users will enter the 56 ldisc code for this tty. Can sleep. 57 58hangup() Called when the tty line is hung up. 59 The line discipline should cease I/O to the tty. 60 No further calls into the ldisc code will occur. 61 Can sleep. 62 63read() (optional) A process requests reading data from 64 the line. Multiple read calls may occur in parallel 65 and the ldisc must deal with serialization issues. 66 If not defined, the process will receive an EIO 67 error. May sleep. 68 69write() (optional) A process requests writing data to the 70 line. Multiple write calls are serialized by the 71 tty layer for the ldisc. If not defined, the 72 process will receive an EIO error. May sleep. 73 74flush_buffer() (optional) May be called at any point between 75 open and close, and instructs the line discipline 76 to empty its input buffer. 77 78set_termios() (optional) Called on termios structure changes. 79 The caller passes the old termios data and the 80 current data is in the tty. Called under the 81 termios semaphore so allowed to sleep. Serialized 82 against itself only. 83 84poll() (optional) Check the status for the poll/select 85 calls. Multiple poll calls may occur in parallel. 86 May sleep. 87 88ioctl() (optional) Called when an ioctl is handed to the 89 tty layer that might be for the ldisc. Multiple 90 ioctl calls may occur in parallel. May sleep. 91 92compat_ioctl() (optional) Called when a 32 bit ioctl is handed 93 to the tty layer that might be for the ldisc. 94 Multiple ioctl calls may occur in parallel. 95 May sleep. 96======================= ======================================================= 97 98Driver Side Interfaces 99^^^^^^^^^^^^^^^^^^^^^^ 100 101======================= ======================================================= 102receive_buf() (optional) Called by the low-level driver to hand 103 a buffer of received bytes to the ldisc for 104 processing. The number of bytes is guaranteed not 105 to exceed the current value of tty->receive_room. 106 All bytes must be processed. 107 108receive_buf2() (optional) Called by the low-level driver to hand 109 a buffer of received bytes to the ldisc for 110 processing. Returns the number of bytes processed. 111 112 If both receive_buf() and receive_buf2() are 113 defined, receive_buf2() should be preferred. 114 115write_wakeup() May be called at any point between open and close. 116 The TTY_DO_WRITE_WAKEUP flag indicates if a call 117 is needed but always races versus calls. Thus the 118 ldisc must be careful about setting order and to 119 handle unexpected calls. Must not sleep. 120 121 The driver is forbidden from calling this directly 122 from the ->write call from the ldisc as the ldisc 123 is permitted to call the driver write method from 124 this function. In such a situation defer it. 125 126dcd_change() Report to the tty line the current DCD pin status 127 changes and the relative timestamp. The timestamp 128 cannot be NULL. 129======================= ======================================================= 130 131 132Driver Access 133^^^^^^^^^^^^^ 134 135Line discipline methods can call the following methods of the underlying 136hardware driver through the function pointers within the tty->driver 137structure: 138 139======================= ======================================================= 140write() Write a block of characters to the tty device. 141 Returns the number of characters accepted. The 142 character buffer passed to this method is already 143 in kernel space. 144 145put_char() Queues a character for writing to the tty device. 146 If there is no room in the queue, the character is 147 ignored. 148 149flush_chars() (Optional) If defined, must be called after 150 queueing characters with put_char() in order to 151 start transmission. 152 153write_room() Returns the numbers of characters the tty driver 154 will accept for queueing to be written. 155 156ioctl() Invoke device specific ioctl. 157 Expects data pointers to refer to userspace. 158 Returns ENOIOCTLCMD for unrecognized ioctl numbers. 159 160set_termios() Notify the tty driver that the device's termios 161 settings have changed. New settings are in 162 tty->termios. Previous settings should be passed in 163 the "old" argument. 164 165 The API is defined such that the driver should return 166 the actual modes selected. This means that the 167 driver function is responsible for modifying any 168 bits in the request it cannot fulfill to indicate 169 the actual modes being used. A device with no 170 hardware capability for change (e.g. a USB dongle or 171 virtual port) can provide NULL for this method. 172 173throttle() Notify the tty driver that input buffers for the 174 line discipline are close to full, and it should 175 somehow signal that no more characters should be 176 sent to the tty. 177 178unthrottle() Notify the tty driver that characters can now be 179 sent to the tty without fear of overrunning the 180 input buffers of the line disciplines. 181 182stop() Ask the tty driver to stop outputting characters 183 to the tty device. 184 185start() Ask the tty driver to resume sending characters 186 to the tty device. 187 188hangup() Ask the tty driver to hang up the tty device. 189 190break_ctl() (Optional) Ask the tty driver to turn on or off 191 BREAK status on the RS-232 port. If state is -1, 192 then the BREAK status should be turned on; if 193 state is 0, then BREAK should be turned off. 194 If this routine is not implemented, use ioctls 195 TIOCSBRK / TIOCCBRK instead. 196 197wait_until_sent() Waits until the device has written out all of the 198 characters in its transmitter FIFO. 199 200send_xchar() Send a high-priority XON/XOFF character to the device. 201======================= ======================================================= 202 203 204Flags 205^^^^^ 206 207Line discipline methods have access to tty->flags field containing the 208following interesting flags: 209 210======================= ======================================================= 211TTY_THROTTLED Driver input is throttled. The ldisc should call 212 tty->driver->unthrottle() in order to resume 213 reception when it is ready to process more data. 214 215TTY_DO_WRITE_WAKEUP If set, causes the driver to call the ldisc's 216 write_wakeup() method in order to resume 217 transmission when it can accept more data 218 to transmit. 219 220TTY_IO_ERROR If set, causes all subsequent userspace read/write 221 calls on the tty to fail, returning -EIO. 222 223TTY_OTHER_CLOSED Device is a pty and the other side has closed. 224 225TTY_NO_WRITE_SPLIT Prevent driver from splitting up writes into 226 smaller chunks. 227======================= ======================================================= 228 229 230Locking 231^^^^^^^ 232 233Callers to the line discipline functions from the tty layer are required to 234take line discipline locks. The same is true of calls from the driver side 235but not yet enforced. 236 237Three calls are now provided:: 238 239 ldisc = tty_ldisc_ref(tty); 240 241takes a handle to the line discipline in the tty and returns it. If no ldisc 242is currently attached or the ldisc is being closed and re-opened at this 243point then NULL is returned. While this handle is held the ldisc will not 244change or go away:: 245 246 tty_ldisc_deref(ldisc) 247 248Returns the ldisc reference and allows the ldisc to be closed. Returning the 249reference takes away your right to call the ldisc functions until you take 250a new reference:: 251 252 ldisc = tty_ldisc_ref_wait(tty); 253 254Performs the same function as tty_ldisc_ref except that it will wait for an 255ldisc change to complete and then return a reference to the new ldisc. 256 257While these functions are slightly slower than the old code they should have 258minimal impact as most receive logic uses the flip buffers and they only 259need to take a reference when they push bits up through the driver. 260 261A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc 262functions are called with the ldisc unavailable. Thus tty_ldisc_ref will 263fail in this situation if used within these functions. Ldisc and driver 264code calling its own functions must be careful in this case. 265 266 267Driver Interface 268---------------- 269 270======================= ======================================================= 271open() Called when a device is opened. May sleep 272 273close() Called when a device is closed. At the point of 274 return from this call the driver must make no 275 further ldisc calls of any kind. May sleep 276 277write() Called to write bytes to the device. May not 278 sleep. May occur in parallel in special cases. 279 Because this includes panic paths drivers generally 280 shouldn't try and do clever locking here. 281 282put_char() Stuff a single character onto the queue. The 283 driver is guaranteed following up calls to 284 flush_chars. 285 286flush_chars() Ask the kernel to write put_char queue 287 288write_room() Return the number of characters that can be stuffed 289 into the port buffers without overflow (or less). 290 The ldisc is responsible for being intelligent 291 about multi-threading of write_room/write calls 292 293ioctl() Called when an ioctl may be for the driver 294 295set_termios() Called on termios change, serialized against 296 itself by a semaphore. May sleep. 297 298set_ldisc() Notifier for discipline change. At the point this 299 is done the discipline is not yet usable. Can now 300 sleep (I think) 301 302throttle() Called by the ldisc to ask the driver to do flow 303 control. Serialization including with unthrottle 304 is the job of the ldisc layer. 305 306unthrottle() Called by the ldisc to ask the driver to stop flow 307 control. 308 309stop() Ldisc notifier to the driver to stop output. As with 310 throttle the serializations with start() are down 311 to the ldisc layer. 312 313start() Ldisc notifier to the driver to start output. 314 315hangup() Ask the tty driver to cause a hangup initiated 316 from the host side. [Can sleep ??] 317 318break_ctl() Send RS232 break. Can sleep. Can get called in 319 parallel, driver must serialize (for now), and 320 with write calls. 321 322wait_until_sent() Wait for characters to exit the hardware queue 323 of the driver. Can sleep 324 325send_xchar() Send XON/XOFF and if possible jump the queue with 326 it in order to get fast flow control responses. 327 Cannot sleep ?? 328======================= ======================================================= 329