Based on kernel version 2.6.27. Page generated on 2008-10-13 09:54 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. Can sleep. 46 47 close() - This is called on a terminal when the line 48 discipline is being unplugged. At the point of 49 execution no further users will enter the 50 ldisc code for this tty. Can sleep. 51 52 hangup() - Called when the tty line is hung up. 53 The line discipline should cease I/O to the tty. 54 No further calls into the ldisc code will occur. 55 Can sleep. 56 57 write() - A process is writing data through the line 58 discipline. Multiple write calls are serialized 59 by the tty layer for the ldisc. May sleep. 60 61 flush_buffer() - (optional) May be called at any point between 62 open and close, and instructs the line discipline 63 to empty its input buffer. 64 65 chars_in_buffer() - (optional) Report the number of bytes in the input 66 buffer. 67 68 set_termios() - (optional) Called on termios structure changes. 69 The caller passes the old termios data and the 70 current data is in the tty. Called under the 71 termios semaphore so allowed to sleep. Serialized 72 against itself only. 73 74 read() - Move data from the line discipline to the user. 75 Multiple read calls may occur in parallel and the 76 ldisc must deal with serialization issues. May 77 sleep. 78 79 poll() - Check the status for the poll/select calls. Multiple 80 poll calls may occur in parallel. May sleep. 81 82 ioctl() - Called when an ioctl is handed to the tty layer 83 that might be for the ldisc. Multiple ioctl calls 84 may occur in parallel. May sleep. 85 86 Driver Side Interfaces: 87 88 receive_buf() - Hand buffers of bytes from the driver to the ldisc 89 for processing. Semantics currently rather 90 mysterious 8( 91 92 write_wakeup() - May be called at any point between open and close. 93 The TTY_DO_WRITE_WAKEUP flag indicates if a call 94 is needed but always races versus calls. Thus the 95 ldisc must be careful about setting order and to 96 handle unexpected calls. Must not sleep. 97 98 The driver is forbidden from calling this directly 99 from the ->write call from the ldisc as the ldisc 100 is permitted to call the driver write method from 101 this function. In such a situation defer it. 102 103 104 Driver Access 105 106 Line discipline methods can call the following methods of the underlying 107 hardware driver through the function pointers within the tty->driver 108 structure: 109 110 write() Write a block of characters to the tty device. 111 Returns the number of characters accepted. The 112 character buffer passed to this method is already 113 in kernel space. 114 115 put_char() Queues a character for writing to the tty device. 116 If there is no room in the queue, the character is 117 ignored. 118 119 flush_chars() (Optional) If defined, must be called after 120 queueing characters with put_char() in order to 121 start transmission. 122 123 write_room() Returns the numbers of characters the tty driver 124 will accept for queueing to be written. 125 126 ioctl() Invoke device specific ioctl. 127 Expects data pointers to refer to userspace. 128 Returns ENOIOCTLCMD for unrecognized ioctl numbers. 129 130 set_termios() Notify the tty driver that the device's termios 131 settings have changed. New settings are in 132 tty->termios. Previous settings should be passed in 133 the "old" argument. 134 135 The API is defined such that the driver should return 136 the actual modes selected. This means that the 137 driver function is responsible for modifying any 138 bits in the request it cannot fulfill to indicate 139 the actual modes being used. A device with no 140 hardware capability for change (eg a USB dongle or 141 virtual port) can provide NULL for this method. 142 143 throttle() Notify the tty driver that input buffers for the 144 line discipline are close to full, and it should 145 somehow signal that no more characters should be 146 sent to the tty. 147 148 unthrottle() Notify the tty driver that characters can now be 149 sent to the tty without fear of overrunning the 150 input buffers of the line disciplines. 151 152 stop() Ask the tty driver to stop outputting characters 153 to the tty device. 154 155 start() Ask the tty driver to resume sending characters 156 to the tty device. 157 158 hangup() Ask the tty driver to hang up the tty device. 159 160 break_ctl() (Optional) Ask the tty driver to turn on or off 161 BREAK status on the RS-232 port. If state is -1, 162 then the BREAK status should be turned on; if 163 state is 0, then BREAK should be turned off. 164 If this routine is not implemented, use ioctls 165 TIOCSBRK / TIOCCBRK instead. 166 167 wait_until_sent() Waits until the device has written out all of the 168 characters in its transmitter FIFO. 169 170 send_xchar() Send a high-priority XON/XOFF character to the device. 171 172 173 Flags 174 175 Line discipline methods have access to tty->flags field containing the 176 following interesting flags: 177 178 TTY_THROTTLED Driver input is throttled. The ldisc should call 179 tty->driver->unthrottle() in order to resume 180 reception when it is ready to process more data. 181 182 TTY_DO_WRITE_WAKEUP If set, causes the driver to call the ldisc's 183 write_wakeup() method in order to resume 184 transmission when it can accept more data 185 to transmit. 186 187 TTY_IO_ERROR If set, causes all subsequent userspace read/write 188 calls on the tty to fail, returning -EIO. 189 190 TTY_OTHER_CLOSED Device is a pty and the other side has closed. 191 192 TTY_NO_WRITE_SPLIT Prevent driver from splitting up writes into 193 smaller chunks. 194 195 196 Locking 197 198 Callers to the line discipline functions from the tty layer are required to 199 take line discipline locks. The same is true of calls from the driver side 200 but not yet enforced. 201 202 Three calls are now provided 203 204 ldisc = tty_ldisc_ref(tty); 205 206 takes a handle to the line discipline in the tty and returns it. If no ldisc 207 is currently attached or the ldisc is being closed and re-opened at this 208 point then NULL is returned. While this handle is held the ldisc will not 209 change or go away. 210 211 tty_ldisc_deref(ldisc) 212 213 Returns the ldisc reference and allows the ldisc to be closed. Returning the 214 reference takes away your right to call the ldisc functions until you take 215 a new reference. 216 217 ldisc = tty_ldisc_ref_wait(tty); 218 219 Performs the same function as tty_ldisc_ref except that it will wait for an 220 ldisc change to complete and then return a reference to the new ldisc. 221 222 While these functions are slightly slower than the old code they should have 223 minimal impact as most receive logic uses the flip buffers and they only 224 need to take a reference when they push bits up through the driver. 225 226 A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc 227 functions are called with the ldisc unavailable. Thus tty_ldisc_ref will 228 fail in this situation if used within these functions. Ldisc and driver 229 code calling its own functions must be careful in this case. 230 231 232 Driver Interface 233 ---------------- 234 235 open() - Called when a device is opened. May sleep 236 237 close() - Called when a device is closed. At the point of 238 return from this call the driver must make no 239 further ldisc calls of any kind. May sleep 240 241 write() - Called to write bytes to the device. May not 242 sleep. May occur in parallel in special cases. 243 Because this includes panic paths drivers generally 244 shouldn't try and do clever locking here. 245 246 put_char() - Stuff a single character onto the queue. The 247 driver is guaranteed following up calls to 248 flush_chars. 249 250 flush_chars() - Ask the kernel to write put_char queue 251 252 write_room() - Return the number of characters tht can be stuffed 253 into the port buffers without overflow (or less). 254 The ldisc is responsible for being intelligent 255 about multi-threading of write_room/write calls 256 257 ioctl() - Called when an ioctl may be for the driver 258 259 set_termios() - Called on termios change, serialized against 260 itself by a semaphore. May sleep. 261 262 set_ldisc() - Notifier for discipline change. At the point this 263 is done the discipline is not yet usable. Can now 264 sleep (I think) 265 266 throttle() - Called by the ldisc to ask the driver to do flow 267 control. Serialization including with unthrottle 268 is the job of the ldisc layer. 269 270 unthrottle() - Called by the ldisc to ask the driver to stop flow 271 control. 272 273 stop() - Ldisc notifier to the driver to stop output. As with 274 throttle the serializations with start() are down 275 to the ldisc layer. 276 277 start() - Ldisc notifier to the driver to start output. 278 279 hangup() - Ask the tty driver to cause a hangup initiated 280 from the host side. [Can sleep ??] 281 282 break_ctl() - Send RS232 break. Can sleep. Can get called in 283 parallel, driver must serialize (for now), and 284 with write calls. 285 286 wait_until_sent() - Wait for characters to exit the hardware queue 287 of the driver. Can sleep 288 289 send_xchar() - Send XON/XOFF and if possible jump the queue with 290 it in order to get fast flow control responses. 291 Cannot sleep ??