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    Subject[PATCH 5/5] VT binding: Add new doc file describing the feature
    This newly added file will:

    - Describe the characteristics of 2 general types of console drivers
    - How to use the sysfs to unbind and bind console drivers
    - Uses for this feature

    Signed-off-by: Antonino Daplas <>

    Documentation/console/console.txt | 127 +++++++++++++++++++++++++++++++++++++
    1 files changed, 127 insertions(+), 0 deletions(-)

    diff --git a/Documentation/console/console.txt b/Documentation/console/console.txt
    new file mode 100644
    index 0000000..4f3f285
    --- /dev/null
    +++ b/Documentation/console/console.txt
    @@ -0,0 +1,127 @@
    +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/tty/console/backend can be
    +examined. This shows the console drivers currently registered by the system. On
    +an x86 system with the framebuffer console enabled, the contents of this
    +attribute may be like this:
    +cat /sys/class/tty/console/backend
    +0 S: VGA+
    +1 B: frame buffer device
    +The first line shows the VGA console driver, while the second line shows
    +the framebuffer console driver.
    +The leftmost numeric character is the driver ID. The middle character with
    +the colon describes the status of the driver.
    + S: - system driver (binding unspecified)
    + B: - bound modular driver
    + U: - unbound modular driver
    +The last column is the description of the driver.
    +Under /sys/class/tty/console are two other attributes, 'bind' and
    +'unbind'. What does these 2 attributes do? As their name implies, echo'ing the
    +driver ID to 'bind' will bind an unbound modular driver, and to 'unbind' will
    +unbind a bound modular driver. Echo'ing the ID of a system driver to either
    +attribute will do nothing.
    +echo 1 > /sys/class/tty/console/unbind
    +cat /sys/class/tty/console/backend
    +0 S: VGA+
    +1 U: frame buffer device
    +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.
    +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 <>

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