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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
+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>