view Documentation/console/console.txt @ 524:7f8b544237bf

netfront: Allow netfront in domain 0.

This is useful if your physical network device is in a utility domain.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
author Keir Fraser <keir.fraser@citrix.com>
date Tue Apr 15 15:18:58 2008 +0100 (2008-04-15)
parents 831230e53067
line source
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 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:
27 register_con_driver()
28 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 fro 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 binding 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 take_over_console() is now broken up into:
99 register_con_driver()
100 bind_con_driver() - private function
102 give_up_console() is a wrapper to 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 register_con_driver()
113 or take_over_console(). register_con_driver() will just add the driver to
114 the console's internal list. It won't take over the
115 console. 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 unregister_con_driver()
132 or give_up_console().
134 5. 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>