console.rst (5972B)
1.. SPDX-License-Identifier: GPL-2.0 2 3=============== 4Console Drivers 5=============== 6 7The Linux kernel has 2 general types of console drivers. The first type is 8assigned by the kernel to all the virtual consoles during the boot process. 9This type will be called 'system driver', and only one system driver is allowed 10to exist. The system driver is persistent and it can never be unloaded, though 11it may become inactive. 12 13The second type has to be explicitly loaded and unloaded. This will be called 14'modular driver' by this document. Multiple modular drivers can coexist at 15any time with each driver sharing the console with other drivers including 16the system driver. However, modular drivers cannot take over the console 17that is currently occupied by another modular driver. (Exception: Drivers that 18call do_take_over_console() will succeed in the takeover regardless of the type 19of driver occupying the consoles.) They can only take over the console that is 20occupied by the system driver. In the same token, if the modular driver is 21released by the console, the system driver will take over. 22 23Modular drivers, from the programmer's point of view, have to call:: 24 25 do_take_over_console() - load and bind driver to console layer 26 give_up_console() - unload driver; it will only work if driver 27 is fully unbound 28 29In newer kernels, the following are also available:: 30 31 do_register_con_driver() 32 do_unregister_con_driver() 33 34If sysfs is enabled, the contents of /sys/class/vtconsole can be 35examined. This shows the console backends currently registered by the 36system which are named vtcon<n> where <n> is an integer from 0 to 15. 37Thus:: 38 39 ls /sys/class/vtconsole 40 . .. vtcon0 vtcon1 41 42Each directory in /sys/class/vtconsole has 3 files:: 43 44 ls /sys/class/vtconsole/vtcon0 45 . .. bind name uevent 46 47What do these files signify? 48 49 1. bind - this is a read/write file. It shows the status of the driver if 50 read, or acts to bind or unbind the driver to the virtual consoles 51 when written to. The possible values are: 52 53 0 54 - means the driver is not bound and if echo'ed, commands the driver 55 to unbind 56 57 1 58 - means the driver is bound and if echo'ed, commands the driver to 59 bind 60 61 2. name - read-only file. Shows the name of the driver in this format:: 62 63 cat /sys/class/vtconsole/vtcon0/name 64 (S) VGA+ 65 66 '(S)' stands for a (S)ystem driver, i.e., it cannot be directly 67 commanded to bind or unbind 68 69 'VGA+' is the name of the driver 70 71 cat /sys/class/vtconsole/vtcon1/name 72 (M) frame buffer device 73 74 In this case, '(M)' stands for a (M)odular driver, one that can be 75 directly commanded to bind or unbind. 76 77 3. uevent - ignore this file 78 79When unbinding, the modular driver is detached first, and then the system 80driver takes over the consoles vacated by the driver. Binding, on the other 81hand, will bind the driver to the consoles that are currently occupied by a 82system driver. 83 84NOTE1: 85 Binding and unbinding must be selected in Kconfig. It's under:: 86 87 Device Drivers -> 88 Character devices -> 89 Support for binding and unbinding console drivers 90 91NOTE2: 92 If any of the virtual consoles are in KD_GRAPHICS mode, then binding or 93 unbinding will not succeed. An example of an application that sets the 94 console to KD_GRAPHICS is X. 95 96How useful is this feature? This is very useful for console driver 97developers. By unbinding the driver from the console layer, one can unload the 98driver, make changes, recompile, reload and rebind the driver without any need 99for rebooting the kernel. For regular users who may want to switch from 100framebuffer console to VGA console and vice versa, this feature also makes 101this possible. (NOTE NOTE NOTE: Please read fbcon.txt under Documentation/fb 102for more details.) 103 104Notes for developers 105==================== 106 107do_take_over_console() is now broken up into:: 108 109 do_register_con_driver() 110 do_bind_con_driver() - private function 111 112give_up_console() is a wrapper to do_unregister_con_driver(), and a driver must 113be fully unbound for this call to succeed. con_is_bound() will check if the 114driver is bound or not. 115 116Guidelines for console driver writers 117===================================== 118 119In order for binding to and unbinding from the console to properly work, 120console drivers must follow these guidelines: 121 1221. All drivers, except system drivers, must call either do_register_con_driver() 123 or do_take_over_console(). do_register_con_driver() will just add the driver 124 to the console's internal list. It won't take over the 125 console. do_take_over_console(), as it name implies, will also take over (or 126 bind to) the console. 127 1282. All resources allocated during con->con_init() must be released in 129 con->con_deinit(). 130 1313. All resources allocated in con->con_startup() must be released when the 132 driver, which was previously bound, becomes unbound. The console layer 133 does not have a complementary call to con->con_startup() so it's up to the 134 driver to check when it's legal to release these resources. Calling 135 con_is_bound() in con->con_deinit() will help. If the call returned 136 false(), then it's safe to release the resources. This balance has to be 137 ensured because con->con_startup() can be called again when a request to 138 rebind the driver to the console arrives. 139 1404. Upon exit of the driver, ensure that the driver is totally unbound. If the 141 condition is satisfied, then the driver must call do_unregister_con_driver() 142 or give_up_console(). 143 1445. do_unregister_con_driver() can also be called on conditions which make it 145 impossible for the driver to service console requests. This can happen 146 with the framebuffer console that suddenly lost all of its drivers. 147 148The current crop of console drivers should still work correctly, but binding 149and unbinding them may cause problems. With minimal fixes, these drivers can 150be made to work correctly. 151 152Antonino Daplas <adaplas@pol.net>