1 2 The Lockronomicon 3 4Your guide to the ancient and twisted locking policies of the tty layer and 5the warped logic behind them. Beware all ye who read on. 6 7FIXME: still need to work out the full set of BKL assumptions and document 8them so they can eventually be killed off. 9 10 11Line Discipline 12--------------- 13 14Line disciplines are registered with tty_register_ldisc() passing the 15discipline number and the ldisc structure. At the point of registration the 16discipline must be ready to use and it is possible it will get used before 17the call returns success. If the call returns an error then it won't get 18called. Do not re-use ldisc numbers as they are part of the userspace ABI 19and writing over an existing ldisc will cause demons to eat your computer. 20After the return the ldisc data has been copied so you may free your own 21copy of the structure. You must not re-register over the top of the line 22discipline even with the same data or your computer again will be eaten by 23demons. 24 25In order to remove a line discipline call tty_unregister_ldisc(). 26In ancient times this always worked. In modern times the function will 27return -EBUSY if the ldisc is currently in use. Since the ldisc referencing 28code manages the module counts this should not usually be a concern. 29 30Heed this warning: the reference count field of the registered copies of the 31tty_ldisc structure in the ldisc table counts the number of lines using this 32discipline. The reference count of the tty_ldisc structure within a tty 33counts the number of active users of the ldisc at this instant. In effect it 34counts the number of threads of execution within an ldisc method (plus those 35about to enter and exit although this detail matters not). 36 37Line Discipline Methods 38----------------------- 39 40TTY side interfaces: 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. Can sleep. 46 47close() - 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 52hangup() - 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 57write() - 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 61flush_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 65chars_in_buffer() - (optional) Report the number of bytes in the input 66 buffer. 67 68set_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 74read() - 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 79poll() - Check the status for the poll/select calls. Multiple 80 poll calls may occur in parallel. May sleep. 81 82ioctl() - 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 86Driver Side Interfaces: 87 88receive_buf() - Hand buffers of bytes from the driver to the ldisc 89 for processing. Semantics currently rather 90 mysterious 8( 91 92write_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 104Driver Access 105 106Line discipline methods can call the following methods of the underlying 107hardware driver through the function pointers within the tty->driver 108structure: 109 110write() 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 115put_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 119flush_chars() (Optional) If defined, must be called after 120 queueing characters with put_char() in order to 121 start transmission. 122 123write_room() Returns the numbers of characters the tty driver 124 will accept for queueing to be written. 125 126ioctl() Invoke device specific ioctl. 127 Expects data pointers to refer to userspace. 128 Returns ENOIOCTLCMD for unrecognized ioctl numbers. 129 130set_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 135throttle() Notify the tty driver that input buffers for the 136 line discipline are close to full, and it should 137 somehow signal that no more characters should be 138 sent to the tty. 139 140unthrottle() Notify the tty driver that characters can now be 141 sent to the tty without fear of overrunning the 142 input buffers of the line disciplines. 143 144stop() Ask the tty driver to stop outputting characters 145 to the tty device. 146 147start() Ask the tty driver to resume sending characters 148 to the tty device. 149 150hangup() Ask the tty driver to hang up the tty device. 151 152break_ctl() (Optional) Ask the tty driver to turn on or off 153 BREAK status on the RS-232 port. If state is -1, 154 then the BREAK status should be turned on; if 155 state is 0, then BREAK should be turned off. 156 If this routine is not implemented, use ioctls 157 TIOCSBRK / TIOCCBRK instead. 158 159wait_until_sent() Waits until the device has written out all of the 160 characters in its transmitter FIFO. 161 162send_xchar() Send a high-priority XON/XOFF character to the device. 163 164 165Flags 166 167Line discipline methods have access to tty->flags field containing the 168following interesting flags: 169 170TTY_THROTTLED Driver input is throttled. The ldisc should call 171 tty->driver->unthrottle() in order to resume 172 reception when it is ready to process more data. 173 174TTY_DO_WRITE_WAKEUP If set, causes the driver to call the ldisc's 175 write_wakeup() method in order to resume 176 transmission when it can accept more data 177 to transmit. 178 179TTY_IO_ERROR If set, causes all subsequent userspace read/write 180 calls on the tty to fail, returning -EIO. 181 182TTY_OTHER_CLOSED Device is a pty and the other side has closed. 183 184TTY_NO_WRITE_SPLIT Prevent driver from splitting up writes into 185 smaller chunks. 186 187 188Locking 189 190Callers to the line discipline functions from the tty layer are required to 191take line discipline locks. The same is true of calls from the driver side 192but not yet enforced. 193 194Three calls are now provided 195 196 ldisc = tty_ldisc_ref(tty); 197 198takes a handle to the line discipline in the tty and returns it. If no ldisc 199is currently attached or the ldisc is being closed and re-opened at this 200point then NULL is returned. While this handle is held the ldisc will not 201change or go away. 202 203 tty_ldisc_deref(ldisc) 204 205Returns the ldisc reference and allows the ldisc to be closed. Returning the 206reference takes away your right to call the ldisc functions until you take 207a new reference. 208 209 ldisc = tty_ldisc_ref_wait(tty); 210 211Performs the same function as tty_ldisc_ref except that it will wait for an 212ldisc change to complete and then return a reference to the new ldisc. 213 214While these functions are slightly slower than the old code they should have 215minimal impact as most receive logic uses the flip buffers and they only 216need to take a reference when they push bits up through the driver. 217 218A caution: The ldisc->open(), ldisc->close() and driver->set_ldisc 219functions are called with the ldisc unavailable. Thus tty_ldisc_ref will 220fail in this situation if used within these functions. Ldisc and driver 221code calling its own functions must be careful in this case. 222 223 224Driver Interface 225---------------- 226 227open() - Called when a device is opened. May sleep 228 229close() - Called when a device is closed. At the point of 230 return from this call the driver must make no 231 further ldisc calls of any kind. May sleep 232 233write() - Called to write bytes to the device. May not 234 sleep. May occur in parallel in special cases. 235 Because this includes panic paths drivers generally 236 shouldn't try and do clever locking here. 237 238put_char() - Stuff a single character onto the queue. The 239 driver is guaranteed following up calls to 240 flush_chars. 241 242flush_chars() - Ask the kernel to write put_char queue 243 244write_room() - Return the number of characters tht can be stuffed 245 into the port buffers without overflow (or less). 246 The ldisc is responsible for being intelligent 247 about multi-threading of write_room/write calls 248 249ioctl() - Called when an ioctl may be for the driver 250 251set_termios() - Called on termios change, serialized against 252 itself by a semaphore. May sleep. 253 254set_ldisc() - Notifier for discipline change. At the point this 255 is done the discipline is not yet usable. Can now 256 sleep (I think) 257 258throttle() - Called by the ldisc to ask the driver to do flow 259 control. Serialization including with unthrottle 260 is the job of the ldisc layer. 261 262unthrottle() - Called by the ldisc to ask the driver to stop flow 263 control. 264 265stop() - Ldisc notifier to the driver to stop output. As with 266 throttle the serializations with start() are down 267 to the ldisc layer. 268 269start() - Ldisc notifier to the driver to start output. 270 271hangup() - Ask the tty driver to cause a hangup initiated 272 from the host side. [Can sleep ??] 273 274break_ctl() - Send RS232 break. Can sleep. Can get called in 275 parallel, driver must serialize (for now), and 276 with write calls. 277 278wait_until_sent() - Wait for characters to exit the hardware queue 279 of the driver. Can sleep 280 281send_xchar() - Send XON/XOFF and if possible jump the queue with 282 it in order to get fast flow control responses. 283 Cannot sleep ?? 284