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-Console Drivers
-===============
-
-The linux kernel has 2 general types of console drivers. The first type is
-assigned by the kernel to all the virtual consoles during the boot process.
-This type will be called 'system driver', and only one system driver is allowed
-to exist. The system driver is persistent and it can never be unloaded, though
-it may become inactive.
-
-The second type has to be explicitly loaded and unloaded. This will be called
-'modular driver' by this document. Multiple modular drivers can coexist at
-any time with each driver sharing the console with other drivers including
-the system driver. However, modular drivers cannot take over the console
-that is currently occupied by another modular driver. (Exception: Drivers that
-call take_over_console() will succeed in the takeover regardless of the type
-of driver occupying the consoles.) They can only take over the console that is
-occupied by the system driver. In the same token, if the modular driver is
-released by the console, the system driver will take over.
-
-Modular drivers, from the programmer's point of view, has to call:
-
- take_over_console() - load and bind driver to console layer
- give_up_console() - unbind and unload driver
-
-In newer kernels, the following are also available:
-
- register_con_driver()
- unregister_con_driver()
-
-If sysfs is enabled, the contents of /sys/class/vtconsole can be
-examined. This shows the console backends currently registered by the
-system which are named vtcon<n> where <n> is an integer from 0 to 15. Thus:
-
- ls /sys/class/vtconsole
- . .. vtcon0 vtcon1
-
-Each directory in /sys/class/vtconsole has 3 files:
-
- ls /sys/class/vtconsole/vtcon0
- . .. bind name uevent
-
-What do these files signify?
-
- 1. bind - this is a read/write file. It shows the status of the driver if
- read, or acts to bind or unbind the driver to the virtual consoles
- when written to. The possible values are:
-
- 0 - means the driver is not bound and if echo'ed, commands the driver
- to unbind
-
- 1 - means the driver is bound and if echo'ed, commands the driver to
- bind
-
- 2. name - read-only file. Shows the name of the driver in this format:
-
- cat /sys/class/vtconsole/vtcon0/name
- (S) VGA+
-
- '(S)' stands for a (S)ystem driver, ie, it cannot be directly
- commanded to bind or unbind
-
- 'VGA+' is the name of the driver
-
- cat /sys/class/vtconsole/vtcon1/name
- (M) frame buffer device
-
- In this case, '(M)' stands for a (M)odular driver, one that can be
- directly commanded to bind or unbind.
-
- 3. uevent - ignore this file
-
-When unbinding, the modular driver is detached first, and then the system
-driver takes over the consoles vacated by the driver. Binding, on the other
-hand, will bind the driver to the consoles that are currently occupied by a
-system driver.
-
-NOTE1: Binding and unbinding must be selected in Kconfig. It's under:
-
-Device Drivers -> Character devices -> Support for binding and unbinding
-console drivers
-
-NOTE2: If any of the virtual consoles are in KD_GRAPHICS mode, then binding or
-unbinding will not succeed. An example of an application that sets the console
-to KD_GRAPHICS is X.
-
-How useful is this feature? This is very useful for console driver
-developers. By unbinding the driver from the console layer, one can unload the
-driver, make changes, recompile, reload and rebind the driver without any need
-for rebooting the kernel. For regular users who may want to switch from
-framebuffer console to VGA console and vice versa, this feature also makes
-this possible. (NOTE NOTE NOTE: Please read fbcon.txt under Documentation/fb
-for more details).
-
-Notes for developers:
-=====================
-
-take_over_console() is now broken up into:
-
- register_con_driver()
- bind_con_driver() - private function
-
-give_up_console() is a wrapper to unregister_con_driver(), and a driver must
-be fully unbound for this call to succeed. con_is_bound() will check if the
-driver is bound or not.
-
-Guidelines for console driver writers:
-=====================================
-
-In order for binding to and unbinding from the console to properly work,
-console drivers must follow these guidelines:
-
-1. All drivers, except system drivers, must call either register_con_driver()
- or take_over_console(). register_con_driver() will just add the driver to
- the console's internal list. It won't take over the
- console. take_over_console(), as it name implies, will also take over (or
- bind to) the console.
-
-2. All resources allocated during con->con_init() must be released in
- con->con_deinit().
-
-3. All resources allocated in con->con_startup() must be released when the
- driver, which was previously bound, becomes unbound. The console layer
- does not have a complementary call to con->con_startup() so it's up to the
- driver to check when it's legal to release these resources. Calling
- con_is_bound() in con->con_deinit() will help. If the call returned
- false(), then it's safe to release the resources. This balance has to be
- ensured because con->con_startup() can be called again when a request to
- rebind the driver to the console arrives.
-
-4. Upon exit of the driver, ensure that the driver is totally unbound. If the
- condition is satisfied, then the driver must call unregister_con_driver()
- or give_up_console().
-
-5. unregister_con_driver() can also be called on conditions which make it
- impossible for the driver to service console requests. This can happen
- with the framebuffer console that suddenly lost all of its drivers.
-
-The current crop of console drivers should still work correctly, but binding
-and unbinding them may cause problems. With minimal fixes, these drivers can
-be made to work correctly.
-
-==========================
-Antonino Daplas <adaplas@pol.net>
-