1Console Drivers
2===============
3
4The linux kernel has 2 general types of console drivers.  The first type is
5assigned by the kernel to all the virtual consoles during the boot process.
6This type will be called 'system driver', and only one system driver is allowed
7to exist. The system driver is persistent and it can never be unloaded, though
8it may become inactive.
9
10The second type has to be explicitly loaded and unloaded. This will be called
11'modular driver' by this document. Multiple modular drivers can coexist at
12any time with each driver sharing the console with other drivers including
13the system driver. However, modular drivers cannot take over the console
14that is currently occupied by another modular driver. (Exception: Drivers that
15call do_take_over_console() will succeed in the takeover regardless of the type
16of driver occupying the consoles.) They can only take over the console that is
17occupied by the system driver. In the same token, if the modular driver is
18released by the console, the system driver will take over.
19
20Modular drivers, from the programmer's point of view, has to call:
21
22	 do_take_over_console() - load and bind driver to console layer
23	 give_up_console() - unload driver, it will only work if driver is fully unbond
24
25In newer kernels, the following are also available:
26
27	 do_register_con_driver()
28	 do_unregister_con_driver()
29
30If sysfs is enabled, the contents of /sys/class/vtconsole can be
31examined. This shows the console backends currently registered by the
32system which are named vtcon<n> where <n> is an integer from 0 to 15. Thus:
33
34       ls /sys/class/vtconsole
35       .  ..  vtcon0  vtcon1
36
37Each directory in /sys/class/vtconsole has 3 files:
38
39     ls /sys/class/vtconsole/vtcon0
40     .  ..  bind  name  uevent
41
42What do these files signify?
43
44     1. bind - this is a read/write file. It shows the status of the driver if
45        read, or acts to bind or unbind the driver to the virtual consoles
46        when written to. The possible values are:
47
48	0 - means the driver is not bound and if echo'ed, commands the driver
49	    to unbind
50
51        1 - means the driver is bound and if echo'ed, commands the driver to
52	    bind
53
54     2. name - read-only file. Shows the name of the driver in this format:
55
56	cat /sys/class/vtconsole/vtcon0/name
57	(S) VGA+
58
59	    '(S)' stands for a (S)ystem driver, ie, it cannot be directly
60	    commanded to bind or unbind
61
62	    'VGA+' is the name of the driver
63
64	cat /sys/class/vtconsole/vtcon1/name
65	(M) frame buffer device
66
67	    In this case, '(M)' stands for a (M)odular driver, one that can be
68	    directly commanded to bind or unbind.
69
70     3. uevent - ignore this file
71
72When unbinding, the modular driver is detached first, and then the system
73driver takes over the consoles vacated by the driver. Binding, on the other
74hand, will bind the driver to the consoles that are currently occupied by a
75system driver.
76
77NOTE1: Binding and unbinding must be selected in Kconfig. It's under:
78
79Device Drivers -> Character devices -> Support for binding and unbinding
80console drivers
81
82NOTE2: If any of the virtual consoles are in KD_GRAPHICS mode, then binding or
83unbinding will not succeed. An example of an application that sets the console
84to KD_GRAPHICS is X.
85
86How useful is this feature? This is very useful for console driver
87developers. By unbinding the driver from the console layer, one can unload the
88driver, make changes, recompile, reload and rebind the driver without any need
89for rebooting the kernel. For regular users who may want to switch from
90framebuffer console to VGA console and vice versa, this feature also makes
91this possible. (NOTE NOTE NOTE: Please read fbcon.txt under Documentation/fb
92for more details).
93
94Notes for developers:
95=====================
96
97do_take_over_console() is now broken up into:
98
99     do_register_con_driver()
100     do_bind_con_driver() - private function
101
102give_up_console() is a wrapper to do_unregister_con_driver(), and a driver must
103be fully unbound for this call to succeed. con_is_bound() will check if the
104driver is bound or not.
105
106Guidelines for console driver writers:
107=====================================
108
109In order for binding to and unbinding from the console to properly work,
110console drivers must follow these guidelines:
111
1121. All drivers, except system drivers, must call either do_register_con_driver()
113   or do_take_over_console(). do_register_con_driver() will just add the driver to
114   the console's internal list. It won't take over the
115   console. do_take_over_console(), as it name implies, will also take over (or
116   bind to) the console.
117
1182. All resources allocated during con->con_init() must be released in
119   con->con_deinit().
120
1213. All resources allocated in con->con_startup() must be released when the
122   driver, which was previously bound, becomes unbound.  The console layer
123   does not have a complementary call to con->con_startup() so it's up to the
124   driver to check when it's legal to release these resources. Calling
125   con_is_bound() in con->con_deinit() will help.  If the call returned
126   false(), then it's safe to release the resources.  This balance has to be
127   ensured because con->con_startup() can be called again when a request to
128   rebind the driver to the console arrives.
129
1304. Upon exit of the driver, ensure that the driver is totally unbound. If the
131   condition is satisfied, then the driver must call do_unregister_con_driver()
132   or give_up_console().
133
1345. do_unregister_con_driver() can also be called on conditions which make it
135   impossible for the driver to service console requests.  This can happen
136   with the framebuffer console that suddenly lost all of its drivers.
137
138The current crop of console drivers should still work correctly, but binding
139and unbinding them may cause problems. With minimal fixes, these drivers can
140be made to work correctly.
141
142==========================
143Antonino Daplas <adaplas@pol.net>
144
145