Based on kernel version 3.9. Page generated on 2013-05-02 23:14 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 FIXME: still need to work out the full set of BKL assumptions and document 8 them so they can eventually be killed off. 9 10 11 Line Discipline 12 --------------- 13 14 Line disciplines are registered with tty_register_ldisc() passing the 15 discipline number and the ldisc structure. At the point of registration the 16 discipline must be ready to use and it is possible it will get used before 17 the call returns success. If the call returns an error then it won't get 18 called. Do not re-use ldisc numbers as they are part of the userspace ABI 19 and writing over an existing ldisc will cause demons to eat your computer. 20 After the return the ldisc data has been copied so you may free your own 21 copy of the structure. You must not re-register over the top of the line 22 discipline even with the same data or your computer again will be eaten by 23 demons. 24 25 In order to remove a line discipline call tty_unregister_ldisc(). 26 In ancient times this always worked. In modern times the function will 27 return -EBUSY if the ldisc is currently in use. Since the ldisc referencing 28 code manages the module counts this should not usually be a concern. 29 30 Heed this warning: the reference count field of the registered copies of the 31 tty_ldisc structure in the ldisc table counts the number of lines using this 32 discipline. The reference count of the tty_ldisc structure within a tty 33 counts the number of active users of the ldisc at this instant. In effect it 34 counts the number of threads of execution within an ldisc method (plus those 35 about to enter and exit although this detail matters not). 36 37 Line Discipline Methods 38 ----------------------- 39 40 TTY side interfaces: 41 42 open() - 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. Returning an error will 46 prevent the ldisc from being attached. Can sleep. 47 48 close() - This is called on a terminal when the line 49 discipline is being unplugged. At the point of 50 execution no further users will enter the 51 ldisc code for this tty. Can sleep. 52 53 hangup() - Called when the tty line is hung up. 54 The line discipline should cease I/O to the tty. 55 No further calls into the ldisc code will occur. 56 The return value is ignored. Can sleep. 57 58 write() - A process is writing data through the line 59 discipline. Multiple write calls are serialized 60 by the tty layer for the ldisc. May sleep. 61 62 flush_buffer() - (optional) May be called at any point between 63 open and close, and instructs the line discipline 64 to empty its input buffer. 65 66 chars_in_buffer() - (optional) Report the number of bytes in the input 67 buffer. 68 69 set_termios() - (optional) Called on termios structure changes. 70 The caller passes the old termios data and the 71 current data is in the tty. Called under the 72 termios semaphore so allowed to sleep. Serialized 73 against itself only. 74 75 read() - Move data from the line discipline to the user. 76 Multiple read calls may occur in parallel and the 77 ldisc must deal with serialization issues. May 78 sleep. 79 80 poll() - Check the status for the poll/select calls. Multiple 81 poll calls may occur in parallel. May sleep. 82 83 ioctl() - Called when an ioctl is handed to the tty layer 84 that might be for the ldisc. Multiple ioctl calls 85 may occur in parallel. May sleep. 86 87 compat_ioctl() - Called when a 32 bit ioctl is handed to the tty layer 88 that might be for the ldisc. Multiple ioctl calls 89 may occur in parallel. May sleep. 90 91 Driver Side Interfaces: 92 93 receive_buf() - Hand buffers of bytes from the driver to the ldisc 94 for processing. Semantics currently rather 95 mysterious 8( 96 97 write_wakeup() - May be called at any point between open and close. 98 The TTY_DO_WRITE_WAKEUP flag indicates if a call 99 is needed but always races versus calls. Thus the 100 ldisc must be careful about setting order and to 101 handle unexpected calls. Must not sleep. 102 103 The driver is forbidden from calling this directly 104 from the ->write call from the ldisc as the ldisc 105 is permitted to call the driver write method from 106 this function. In such a situation defer it. 107 108 dcd_change() - Report to the tty line the current DCD pin status 109 changes and the relative timestamp. The timestamp 110 cannot be NULL. 111 112 113 Driver Access 114 115 Line discipline methods can call the following methods of the underlying 116 hardware driver through the function pointers within the tty->driver 117 structure: 118 119 write() Write a block of characters to the tty device. 120 Returns the number of characters accepted. The 121 character buffer passed to this method is already 122 in kernel space. 123 124 put_char() Queues a character for writing to the tty device. 125 If there is no room in the queue, the character is 126 ignored. 127 128 flush_chars() (Optional) If defined, must be called after 129 queueing characters with put_char() in order to 130 start transmission. 131 132 write_room() Returns the numbers of characters the tty driver 133 will accept for queueing to be written. 134 135 ioctl() Invoke device specific ioctl. 136 Expects data pointers to refer to userspace. 137 Returns ENOIOCTLCMD for unrecognized ioctl numbers. 138 139 set_termios() Notify the tty driver that the device's termios 140 settings have changed. New settings are in 141 tty->termios. Previous settings should be passed in 142 the "old" argument. 143 144 The API is defined such that the driver should return 145 the actual modes selected. This means that the 146 driver function is responsible for modifying any 147 bits in the request it cannot fulfill to indicate 148 the actual modes being used. A device with no 149 hardware capability for change (eg a USB dongle or 150 virtual port) can provide NULL for this method. 151 152 throttle() Notify the tty driver that input buffers for the 153 line discipline are close to full, and it should 154 somehow signal that no more characters should be 155 sent to the tty. 156 157 unthrottle() Notify the tty driver that characters can now be 158 sent to the tty without fear of overrunning the 159 input buffers of the line disciplines. 160 161 stop() Ask the tty driver to stop outputting characters 162 to the tty device. 163 164 start() Ask the tty driver to resume sending characters 165 to the tty device. 166 167 hangup() Ask the tty driver to hang up the tty device. 168 169 break_ctl() (Optional) Ask the tty driver to turn on or off 170 BREAK status on the RS-232 port. If state is -1, 171 then the BREAK status should be turned on; if 172 state is 0, then BREAK should be turned off. 173 If this routine is not implemented, use ioctls 174 TIOCSBRK / TIOCCBRK instead. 175 176 wait_until_sent() Waits until the device has written out all of the 177 characters in its transmitter FIFO. 178 179 send_xchar() Send a high-priority XON/XOFF character to the device. 180 181 182 Flags 183 184 Line discipline methods have access to tty->flags field containing the 185 following interesting flags: 186 187 TTY_THROTTLED Driver input is throttled. The ldisc should call 188 tty->driver->unthrottle() in order to resume 189 reception when it is ready to process more data. 190 191 TTY_DO_WRITE_WAKEUP If set, causes the driver to call the ldisc's 192 write_wakeup() method in order to resume 193 transmission when it can accept more data 194 to transmit. 195 196 TTY_IO_ERROR If set, causes all subsequent userspace read/write 197 calls on the tty to fail, returning -EIO. 198 199 TTY_OTHER_CLOSED Device is a pty and the other side has closed. 200 201 TTY_NO_WRITE_SPLIT Prevent driver from splitting up writes into 202 smaller chunks. 203 204 205 Locking 206 207 Callers to the line discipline functions from the tty layer are required to 208 take line discipline locks. The same is true of calls from the driver side 209 but not yet enforced. 210 211 Three calls are now provided 212 213 ldisc = tty_ldisc_ref(tty); 214 215 takes a handle to the line discipline in the tty and returns it. If no ldisc 216 is currently attached or the ldisc is being closed and re-opened at this 217 point then NULL is returned. While this handle is held the ldisc will not 218 change or go away. 219 220 tty_ldisc_deref(ldisc) 221 222 Returns the ldisc reference and allows the ldisc to be closed. Returning the 223 reference takes away your right to call the ldisc functions until you take 224 a new reference. 225 226 ldisc = tty_ldisc_ref_wait(tty); 227 228 Performs the same function as tty_ldisc_ref except that it will wait for an 229 ldisc change to complete and then return a reference to the new ldisc. 230 231 While these functions are slightly slower than the old code they should have 232 minimal impact as most receive logic uses the flip buffers and they only 233 need to take a reference when they push bits up through the driver. 234 235 A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc 236 functions are called with the ldisc unavailable. Thus tty_ldisc_ref will 237 fail in this situation if used within these functions. Ldisc and driver 238 code calling its own functions must be careful in this case. 239 240 241 Driver Interface 242 ---------------- 243 244 open() - Called when a device is opened. May sleep 245 246 close() - Called when a device is closed. At the point of 247 return from this call the driver must make no 248 further ldisc calls of any kind. May sleep 249 250 write() - Called to write bytes to the device. May not 251 sleep. May occur in parallel in special cases. 252 Because this includes panic paths drivers generally 253 shouldn't try and do clever locking here. 254 255 put_char() - Stuff a single character onto the queue. The 256 driver is guaranteed following up calls to 257 flush_chars. 258 259 flush_chars() - Ask the kernel to write put_char queue 260 261 write_room() - Return the number of characters tht can be stuffed 262 into the port buffers without overflow (or less). 263 The ldisc is responsible for being intelligent 264 about multi-threading of write_room/write calls 265 266 ioctl() - Called when an ioctl may be for the driver 267 268 set_termios() - Called on termios change, serialized against 269 itself by a semaphore. May sleep. 270 271 set_ldisc() - Notifier for discipline change. At the point this 272 is done the discipline is not yet usable. Can now 273 sleep (I think) 274 275 throttle() - Called by the ldisc to ask the driver to do flow 276 control. Serialization including with unthrottle 277 is the job of the ldisc layer. 278 279 unthrottle() - Called by the ldisc to ask the driver to stop flow 280 control. 281 282 stop() - Ldisc notifier to the driver to stop output. As with 283 throttle the serializations with start() are down 284 to the ldisc layer. 285 286 start() - Ldisc notifier to the driver to start output. 287 288 hangup() - Ask the tty driver to cause a hangup initiated 289 from the host side. [Can sleep ??] 290 291 break_ctl() - Send RS232 break. Can sleep. Can get called in 292 parallel, driver must serialize (for now), and 293 with write calls. 294 295 wait_until_sent() - Wait for characters to exit the hardware queue 296 of the driver. Can sleep 297 298 send_xchar() - Send XON/XOFF and if possible jump the queue with 299 it in order to get fast flow control responses. 300 Cannot sleep ??