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>
153