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