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Guest on 23rd April 2022 12:49:15 AM

  1. Console Drivers
  2. ===============
  4. The Linux kernel has 2 general types of console drivers.  The first type is
  5. assigned by the kernel to all the virtual consoles during the boot process.
  6. This type will be called 'system driver', and only one system driver is allowed
  7. to exist. The system driver is persistent and it can never be unloaded, though
  8. it may become inactive.
  10. The second type has to be explicitly loaded and unloaded. This will be called
  11. 'modular driver' by this document. Multiple modular drivers can coexist at
  12. any time with each driver sharing the console with other drivers including
  13. the system driver. However, modular drivers cannot take over the console
  14. that is currently occupied by another modular driver. (Exception: Drivers that
  15. call do_take_over_console() will succeed in the takeover regardless of the type
  16. of driver occupying the consoles.) They can only take over the console that is
  17. occupied by the system driver. In the same token, if the modular driver is
  18. released by the console, the system driver will take over.
  20. Modular drivers, from the programmer's point of view, have to call:
  22.          do_take_over_console() - load and bind driver to console layer
  23.          give_up_console() - unload driver; it will only work if driver
  24.                              is fully unbound
  26. In newer kernels, the following are also available:
  28.          do_register_con_driver()
  29.          do_unregister_con_driver()
  31. If sysfs is enabled, the contents of /sys/class/vtconsole can be
  32. examined. This shows the console backends currently registered by the
  33. system which are named vtcon<n> where <n> is an integer from 0 to 15. Thus:
  35.        ls /sys/class/vtconsole
  36.        .  ..  vtcon0  vtcon1
  38. Each directory in /sys/class/vtconsole has 3 files:
  40.      ls /sys/class/vtconsole/vtcon0
  41.      .  ..  bind  name  uevent
  43. What do these files signify?
  45.      1. bind - this is a read/write file. It shows the status of the driver if
  46.         read, or acts to bind or unbind the driver to the virtual consoles
  47.         when written to. The possible values are:
  49.         0 - means the driver is not bound and if echo'ed, commands the driver
  50.             to unbind
  52.         1 - means the driver is bound and if echo'ed, commands the driver to
  53.             bind
  55.      2. name - read-only file. Shows the name of the driver in this format:
  57.         cat /sys/class/vtconsole/vtcon0/name
  58.         (S) VGA+
  60.             '(S)' stands for a (S)ystem driver, i.e., it cannot be directly
  61.             commanded to bind or unbind
  63.             'VGA+' is the name of the driver
  65.         cat /sys/class/vtconsole/vtcon1/name
  66.         (M) frame buffer device
  68.             In this case, '(M)' stands for a (M)odular driver, one that can be
  69.             directly commanded to bind or unbind.
  71.      3. uevent - ignore this file
  73. When unbinding, the modular driver is detached first, and then the system
  74. driver takes over the consoles vacated by the driver. Binding, on the other
  75. hand, will bind the driver to the consoles that are currently occupied by a
  76. system driver.
  78. NOTE1: Binding and unbinding must be selected in Kconfig. It's under:
  80. Device Drivers -> Character devices -> Support for binding and unbinding
  81. console drivers
  83. NOTE2: If any of the virtual consoles are in KD_GRAPHICS mode, then binding or
  84. unbinding will not succeed. An example of an application that sets the console
  85. to KD_GRAPHICS is X.
  87. How useful is this feature? This is very useful for console driver
  88. developers. By unbinding the driver from the console layer, one can unload the
  89. driver, make changes, recompile, reload and rebind the driver without any need
  90. for rebooting the kernel. For regular users who may want to switch from
  91. framebuffer console to VGA console and vice versa, this feature also makes
  92. this possible. (NOTE NOTE NOTE: Please read fbcon.txt under Documentation/fb
  93. for more details.)
  95. Notes for developers:
  96. =====================
  98. do_take_over_console() is now broken up into:
  100.      do_register_con_driver()
  101.      do_bind_con_driver() - private function
  103. give_up_console() is a wrapper to do_unregister_con_driver(), and a driver must
  104. be fully unbound for this call to succeed. con_is_bound() will check if the
  105. driver is bound or not.
  107. Guidelines for console driver writers:
  108. =====================================
  110. In order for binding to and unbinding from the console to properly work,
  111. console drivers must follow these guidelines:
  113. 1. All drivers, except system drivers, must call either do_register_con_driver()
  114.    or do_take_over_console(). do_register_con_driver() will just add the driver
  115.    to the console's internal list. It won't take over the
  116.    console. do_take_over_console(), as it name implies, will also take over (or
  117.    bind to) the console.
  119. 2. All resources allocated during con->con_init() must be released in
  120.    con->con_deinit().
  122. 3. All resources allocated in con->con_startup() must be released when the
  123.    driver, which was previously bound, becomes unbound.  The console layer
  124.    does not have a complementary call to con->con_startup() so it's up to the
  125.    driver to check when it's legal to release these resources. Calling
  126.    con_is_bound() in con->con_deinit() will help.  If the call returned
  127.    false(), then it's safe to release the resources.  This balance has to be
  128.    ensured because con->con_startup() can be called again when a request to
  129.    rebind the driver to the console arrives.
  131. 4. Upon exit of the driver, ensure that the driver is totally unbound. If the
  132.    condition is satisfied, then the driver must call do_unregister_con_driver()
  133.    or give_up_console().
  135. 5. do_unregister_con_driver() can also be called on conditions which make it
  136.    impossible for the driver to service console requests.  This can happen
  137.    with the framebuffer console that suddenly lost all of its drivers.
  139. The current crop of console drivers should still work correctly, but binding
  140. and unbinding them may cause problems. With minimal fixes, these drivers can
  141. be made to work correctly.
  143. ==========================
  144. Antonino Daplas <adaplas@pol.net>

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