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 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 take_over_console() - load and bind driver to console layer
23 give_up_console() - unbind and unload driver
25 In newer kernels, the following are also available:
28 unregister_con_driver()
30 If sysfs is enabled, the contents of /sys/class/tty/console/backend can be
31 examined. This shows the console drivers currently registered by the system. On
32 an x86 system with the framebuffer console enabled, the contents of this
33 attribute may be like this:
35 cat /sys/class/tty/console/backend
37 1 B: frame buffer device
39 The first line shows the VGA console driver, while the second line shows
40 the framebuffer console driver.
42 The leftmost numeric character is the driver ID. The middle character with
43 the colon describes the status of the driver.
45 S: - system driver (binding unspecified)
46 B: - bound modular driver
47 U: - unbound modular driver
49 The last column is the description of the driver.
51 Under /sys/class/tty/console are two other attributes, 'bind' and
52 'unbind'. What does these 2 attributes do? As their name implies, echo'ing the
53 driver ID to 'bind' will bind an unbound modular driver, and to 'unbind' will
54 unbind a bound modular driver. Echo'ing the ID of a system driver to either
55 attribute will do nothing.
59 echo 1 > /sys/class/tty/console/unbind
60 cat /sys/class/tty/console/backend
62 1 U: frame buffer device
64 When unbinding, the modular driver is detached first, and then the system
65 driver takes over the consoles vacated by the driver. Binding, on the other
66 hand, will bind the driver to the consoles that are currently occupied by a
69 How useful is this feature? This is very useful for console driver
70 developers. By unbinding the driver from the console layer, one can unload the
71 driver, make changes, recompile, reload and rebind the driver without any need
72 for rebooting the kernel. For regular users who may want to switch from
73 framebuffer console to VGA console and vice versa, this feature also makes
74 this possible. (NOTE NOTE NOTE: Please read fbcon.txt under Documentation/fb
80 take_over_console() is now broken up into:
83 bind_con_driver() - private function
85 give_up_console() is a wrapper to unregister_con_driver(), and a driver must
86 be fully unbound for this call to succeed. con_is_bound() will check if the
87 driver is bound or not.
89 Guidelines for console driver writers:
90 =====================================
92 In order for binding to and unbinding from the console to properly work,
93 console drivers must follow these guidelines:
95 1. All drivers, except system drivers, must call either register_con_driver()
96 or take_over_console(). register_con_driver() will just add the driver to
97 the console's internal list. It won't take over the
98 console. take_over_console(), as it name implies, will also take over (or
101 2. All resources allocated during con->con_init() must be released in
104 3. All resources allocated in con->con_startup() must be released when the
105 driver, which was previously bound, becomes unbound. The console layer
106 does not have a complementary call to con->con_startup() so it's up to the
107 driver to check when it's legal to release these resources. Calling
108 con_is_bound() in con->con_deinit() will help. If the call returned
109 false(), then it's safe to release the resources. This balance has to be
110 ensured because con->con_startup() can be called again when a request to
111 rebind the driver to the console arrives.
113 4. Upon exit of the driver, ensure that the driver is totally unbound. If the
114 condition is satisfied, then the driver must call unregister_con_driver()
115 or give_up_console().
117 5. unregister_con_driver() can also be called on conditions which make it
118 impossible for the driver to service console requests. This can happen
119 with the framebuffer console that suddenly lost all of its drivers.
121 The current crop of console drivers should still work correctly, but binding
122 and unbinding them may cause problems. With minimal fixes, these drivers can
123 be made to work correctly.
125 ==========================
126 Antonino Daplas <adaplas@pol.net>