1 .. SPDX-License-Identifier: GPL-2.0 2 3 =================== 4 TTY Line Discipline 5 =================== 6 7 .. contents:: :local: 8 9 TTY line discipline process all incoming and outgoing character from/to a tty 10 device. The default line discipline is :doc:`N_TTY <n_tty>`. It is also a 11 fallback if establishing any other discipline for a tty fails. If even N_TTY 12 fails, N_NULL takes over. That never fails, but also does not process any 13 characters -- it throws them away. 14 15 Registration 16 ============ 17 18 Line disciplines are registered with tty_register_ldisc() passing the ldisc 19 structure. At the point of registration the discipline must be ready to use and 20 it is possible it will get used before the call returns success. If the call 21 returns an error then it won’t get called. Do not re-use ldisc numbers as they 22 are part of the userspace ABI and writing over an existing ldisc will cause 23 demons to eat your computer. You must not re-register over the top of the line 24 discipline even with the same data or your computer again will be eaten by 25 demons. In order to remove a line discipline call tty_unregister_ldisc(). 26 27 Heed this warning: the reference count field of the registered copies of the 28 tty_ldisc structure in the ldisc table counts the number of lines using this 29 discipline. The reference count of the tty_ldisc structure within a tty counts 30 the number of active users of the ldisc at this instant. In effect it counts 31 the number of threads of execution within an ldisc method (plus those about to 32 enter and exit although this detail matters not). 33 34 .. kernel-doc:: drivers/tty/tty_ldisc.c 35 :identifiers: tty_register_ldisc tty_unregister_ldisc 36 37 Other Functions 38 =============== 39 40 .. kernel-doc:: drivers/tty/tty_ldisc.c 41 :identifiers: tty_set_ldisc tty_ldisc_flush 42 43 Line Discipline Operations Reference 44 ==================================== 45 46 .. kernel-doc:: include/linux/tty_ldisc.h 47 :identifiers: tty_ldisc_ops 48 49 Driver Access 50 ============= 51 52 Line discipline methods can call the methods of the underlying hardware driver. 53 These are documented as a part of struct tty_operations. 54 55 TTY Flags 56 ========= 57 58 Line discipline methods have access to :c:member:`tty_struct.flags` field. See 59 :doc:`tty_struct`. 60 61 Locking 62 ======= 63 64 Callers to the line discipline functions from the tty layer are required to 65 take line discipline locks. The same is true of calls from the driver side 66 but not yet enforced. 67 68 .. kernel-doc:: drivers/tty/tty_ldisc.c 69 :identifiers: tty_ldisc_ref_wait tty_ldisc_ref tty_ldisc_deref 70 71 While these functions are slightly slower than the old code they should have 72 minimal impact as most receive logic uses the flip buffers and they only 73 need to take a reference when they push bits up through the driver. 74 75 A caution: The :c:member:`tty_ldisc_ops.open()`, 76 :c:member:`tty_ldisc_ops.close()` and :c:member:`tty_driver.set_ldisc()` 77 functions are called with the ldisc unavailable. Thus tty_ldisc_ref() will fail 78 in this situation if used within these functions. Ldisc and driver code 79 calling its own functions must be careful in this case. 80 81 Internal Functions 82 ================== 83 84 .. kernel-doc:: drivers/tty/tty_ldisc.c 85 :internal:
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