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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.

1	Console Drivers
2	===============
4	The linux kernel has 2 general types of console drivers.  The first type is
5	assigned by the kernel to all the virtual consoles during the boot process.
6	This type will be called 'system driver', and only one system driver is allowed
7	to exist. The system driver is persistent and it can never be unloaded, though
8	it may become inactive.
10	The 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
12	any time with each driver sharing the console with other drivers including
13	the system driver. However, modular drivers cannot take over the console
14	that is currently occupied by another modular driver. (Exception: Drivers that
15	call do_take_over_console() will succeed in the takeover regardless of the type
16	of driver occupying the consoles.) They can only take over the console that is
17	occupied by the system driver. In the same token, if the modular driver is
18	released by the console, the system driver will take over.
20	Modular drivers, from the programmer's point of view, has to call:
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
25	In newer kernels, the following are also available:
27		 do_register_con_driver()
28		 do_unregister_con_driver()
30	If sysfs is enabled, the contents of /sys/class/vtconsole can be
31	examined. This shows the console backends currently registered by the
32	system which are named vtcon<n> where <n> is an integer from 0 to 15. Thus:
34	       ls /sys/class/vtconsole
35	       .  ..  vtcon0  vtcon1
37	Each directory in /sys/class/vtconsole has 3 files:
39	     ls /sys/class/vtconsole/vtcon0
40	     .  ..  bind  name  uevent
42	What do these files signify?
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:
48		0 - means the driver is not bound and if echo'ed, commands the driver
49		    to unbind
51	        1 - means the driver is bound and if echo'ed, commands the driver to
52		    bind
54	     2. name - read-only file. Shows the name of the driver in this format:
56		cat /sys/class/vtconsole/vtcon0/name
57		(S) VGA+
59		    '(S)' stands for a (S)ystem driver, ie, it cannot be directly
60		    commanded to bind or unbind
62		    'VGA+' is the name of the driver
64		cat /sys/class/vtconsole/vtcon1/name
65		(M) frame buffer device
67		    In this case, '(M)' stands for a (M)odular driver, one that can be
68		    directly commanded to bind or unbind.
70	     3. uevent - ignore this file
72	When unbinding, the modular driver is detached first, and then the system
73	driver takes over the consoles vacated by the driver. Binding, on the other
74	hand, will bind the driver to the consoles that are currently occupied by a
75	system driver.
77	NOTE1: Binding and unbinding must be selected in Kconfig. It's under:
79	Device Drivers -> Character devices -> Support for binding and unbinding
80	console drivers
82	NOTE2: If any of the virtual consoles are in KD_GRAPHICS mode, then binding or
83	unbinding will not succeed. An example of an application that sets the console
84	to KD_GRAPHICS is X.
86	How useful is this feature? This is very useful for console driver
87	developers. By unbinding the driver from the console layer, one can unload the
88	driver, make changes, recompile, reload and rebind the driver without any need
89	for rebooting the kernel. For regular users who may want to switch from
90	framebuffer console to VGA console and vice versa, this feature also makes
91	this possible. (NOTE NOTE NOTE: Please read fbcon.txt under Documentation/fb
92	for more details).
94	Notes for developers:
95	=====================
97	do_take_over_console() is now broken up into:
99	     do_register_con_driver()
100	     do_bind_con_driver() - private function
102	give_up_console() is a wrapper to do_unregister_con_driver(), and a driver must
103	be fully unbound for this call to succeed. con_is_bound() will check if the
104	driver is bound or not.
106	Guidelines for console driver writers:
107	=====================================
109	In order for binding to and unbinding from the console to properly work,
110	console drivers must follow these guidelines:
112	1. 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.
118	2. All resources allocated during con->con_init() must be released in
119	   con->con_deinit().
121	3. 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.
130	4. 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().
134	5. 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.
138	The current crop of console drivers should still work correctly, but binding
139	and unbinding them may cause problems. With minimal fixes, these drivers can
140	be made to work correctly.
142	==========================
143	Antonino Daplas <adaplas@pol.net>
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