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