1 .. SPDX-License-Identifier: GPL-2.0 2 3 ============================= 4 TTY Driver and TTY Operations 5 ============================= 6 7 .. contents:: :local: 8 9 Allocation 10 ========== 11 12 The first thing a driver needs to do is to allocate a struct tty_driver. This 13 is done by tty_alloc_driver() (or __tty_alloc_driver()). Next, the newly 14 allocated structure is filled with information. See `TTY Driver Reference`_ at 15 the end of this document on what actually shall be filled in. 16 17 The allocation routines expect a number of devices the driver can handle at 18 most and flags. Flags are those starting ``TTY_DRIVER_`` listed and described 19 in `TTY Driver Flags`_ below. 20 21 When the driver is about to be freed, tty_driver_kref_put() is called on that. 22 It will decrements the reference count and if it reaches zero, the driver is 23 freed. 24 25 For reference, both allocation and deallocation functions are explained here in 26 detail: 27 28 .. kernel-doc:: drivers/tty/tty_io.c 29 :identifiers: __tty_alloc_driver tty_driver_kref_put 30 31 TTY Driver Flags 32 ---------------- 33 34 Here comes the documentation of flags accepted by tty_alloc_driver() (or 35 __tty_alloc_driver()): 36 37 .. kernel-doc:: include/linux/tty_driver.h 38 :doc: TTY Driver Flags 39 40 ---- 41 42 Registration 43 ============ 44 45 When a struct tty_driver is allocated and filled in, it can be registered using 46 tty_register_driver(). It is recommended to pass ``TTY_DRIVER_DYNAMIC_DEV`` in 47 flags of tty_alloc_driver(). If it is not passed, *all* devices are also 48 registered during tty_register_driver() and the following paragraph of 49 registering devices can be skipped for such drivers. However, the struct 50 tty_port part in `Registering Devices`_ is still relevant there. 51 52 .. kernel-doc:: drivers/tty/tty_io.c 53 :identifiers: tty_register_driver tty_unregister_driver 54 55 Registering Devices 56 ------------------- 57 58 Every TTY device shall be backed by a struct tty_port. Usually, TTY drivers 59 embed tty_port into device's private structures. Further details about handling 60 tty_port can be found in :doc:`tty_port`. The driver is also recommended to use 61 tty_port's reference counting by tty_port_get() and tty_port_put(). The final 62 put is supposed to free the tty_port including the device's private struct. 63 64 Unless ``TTY_DRIVER_DYNAMIC_DEV`` was passed as flags to tty_alloc_driver(), 65 TTY driver is supposed to register every device discovered in the system 66 (the latter is preferred). This is performed by tty_register_device(). Or by 67 tty_register_device_attr() if the driver wants to expose some information 68 through struct attribute_group. Both of them register ``index``'th device and 69 upon return, the device can be opened. There are also preferred tty_port 70 variants described in `Linking Devices to Ports`_ later. It is up to driver to 71 manage free indices and choosing the right one. The TTY layer only refuses to 72 register more devices than passed to tty_alloc_driver(). 73 74 When the device is opened, the TTY layer allocates struct tty_struct and starts 75 calling operations from :c:member:`tty_driver.ops`, see `TTY Operations 76 Reference`_. 77 78 The registration routines are documented as follows: 79 80 .. kernel-doc:: drivers/tty/tty_io.c 81 :identifiers: tty_register_device tty_register_device_attr 82 tty_unregister_device 83 84 ---- 85 86 Linking Devices to Ports 87 ------------------------ 88 As stated earlier, every TTY device shall have a struct tty_port assigned to 89 it. It must be known to the TTY layer at :c:member:`tty_driver.ops.install()` 90 at latest. There are few helpers to *link* the two. Ideally, the driver uses 91 tty_port_register_device() or tty_port_register_device_attr() instead of 92 tty_register_device() and tty_register_device_attr() at the registration time. 93 This way, the driver needs not care about linking later on. 94 95 If that is not possible, the driver still can link the tty_port to a specific 96 index *before* the actual registration by tty_port_link_device(). If it still 97 does not fit, tty_port_install() can be used from the 98 :c:member:`tty_driver.ops.install` hook as a last resort. The last one is 99 dedicated mostly for in-memory devices like PTY where tty_ports are allocated 100 on demand. 101 102 The linking routines are documented here: 103 104 .. kernel-doc:: drivers/tty/tty_port.c 105 :identifiers: tty_port_link_device tty_port_register_device 106 tty_port_register_device_attr 107 108 ---- 109 110 TTY Driver Reference 111 ==================== 112 113 All members of struct tty_driver are documented here. The required members are 114 noted at the end. struct tty_operations are documented next. 115 116 .. kernel-doc:: include/linux/tty_driver.h 117 :identifiers: tty_driver 118 119 ---- 120 121 TTY Operations Reference 122 ======================== 123 124 When a TTY is registered, these driver hooks can be invoked by the TTY layer: 125 126 .. kernel-doc:: include/linux/tty_driver.h 127 :identifiers: tty_operations 128
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