Subversion Repositories test_project

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 where  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:

            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 binding 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