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  1. The Framebuffer Console
  2. =======================
  3. The framebuffer console (fbcon), as its name implies, is a text
  4. console running on top of the framebuffer device. It has the functionality of
  5. any standard text console driver, such as the VGA console, with the added
  6. features that can be attributed to the graphical nature of the framebuffer.
  7. In the x86 architecture, the framebuffer console is optional, and
  8. some even treat it as a toy. For other architectures, it is the only available
  9. display device, text or graphical.
  10. What are the features of fbcon? The framebuffer console supports
  11. high resolutions, varying font types, display rotation, primitive multihead,
  12. etc. Theoretically, multi-colored fonts, blending, aliasing, and any feature
  13. made available by the underlying graphics card are also possible.
  14. A. Configuration
  15. The framebuffer console can be enabled by using your favorite kernel
  16. configuration tool. It is under Device Drivers->Graphics Support->Support for
  17. framebuffer devices->Framebuffer Console Support. Select 'y' to compile
  18. support statically, or 'm' for module support. The module will be fbcon.
  19. In order for fbcon to activate, at least one framebuffer driver is
  20. required, so choose from any of the numerous drivers available. For x86
  21. systems, they almost universally have VGA cards, so vga16fb and vesafb will
  22. always be available. However, using a chipset-specific driver will give you
  23. more speed and features, such as the ability to change the video mode
  24. dynamically.
  25. To display the penguin logo, choose any logo available in Logo
  26. Configuration->Boot up logo.
  27. Also, you will need to select at least one compiled-in fonts, but if
  28. you don't do anything, the kernel configuration tool will select one for you,
  29. usually an 8x16 font.
  30. GOTCHA: A common bug report is enabling the framebuffer without enabling the
  31. framebuffer console. Depending on the driver, you may get a blanked or
  32. garbled display, but the system still boots to completion. If you are
  33. fortunate to have a driver that does not alter the graphics chip, then you
  34. will still get a VGA console.
  35. B. Loading
  36. Possible scenarios:
  37. 1. Driver and fbcon are compiled statically
  38. Usually, fbcon will automatically take over your console. The notable
  39. exception is vesafb. It needs to be explicitly activated with the
  40. vga= boot option parameter.
  41. 2. Driver is compiled statically, fbcon is compiled as a module
  42. Depending on the driver, you either get a standard console, or a
  43. garbled display, as mentioned above. To get a framebuffer console,
  44. do a 'modprobe fbcon'.
  45. 3. Driver is compiled as a module, fbcon is compiled statically
  46. You get your standard console. Once the driver is loaded with
  47. 'modprobe xxxfb', fbcon automatically takes over the console with
  48. the possible exception of using the fbcon=map:n option. See below.
  49. 4. Driver and fbcon are compiled as a module.
  50. You can load them in any order. Once both are loaded, fbcon will take
  51. over the console.
  52. C. Boot options
  53. The framebuffer console has several, largely unknown, boot options
  54. that can change its behavior.
  55. 1. fbcon=font:<name>
  56. Select the initial font to use. The value 'name' can be any of the
  57. compiled-in fonts: VGA8x16, 7x14, 10x18, VGA8x8, MINI4x6, RomanLarge,
  58. SUN8x16, SUN12x22, ProFont6x11, Acorn8x8, PEARL8x8.
  59. Note, not all drivers can handle font with widths not divisible by 8,
  60. such as vga16fb.
  61. 2. fbcon=scrollback:<value>[k]
  62. The scrollback buffer is memory that is used to preserve display
  63. contents that has already scrolled past your view. This is accessed
  64. by using the Shift-PageUp key combination. The value 'value' is any
  65. integer. It defaults to 32KB. The 'k' suffix is optional, and will
  66. multiply the 'value' by 1024.
  67. 3. fbcon=map:<0123>
  68. This is an interesting option. It tells which driver gets mapped to
  69. which console. The value '0123' is a sequence that gets repeated until
  70. the total length is 64 which is the number of consoles available. In
  71. the above example, it is expanded to 012301230123... and the mapping
  72. will be:
  73. tty | 1 2 3 4 5 6 7 8 9 ...
  74. fb | 0 1 2 3 0 1 2 3 0 ...
  75. ('cat /proc/fb' should tell you what the fb numbers are)
  76. One side effect that may be useful is using a map value that exceeds
  77. the number of loaded fb drivers. For example, if only one driver is
  78. available, fb0, adding fbcon=map:1 tells fbcon not to take over the
  79. console.
  80. Later on, when you want to map the console the to the framebuffer
  81. device, you can use the con2fbmap utility.
  82. 4. fbcon=vc:<n1>-<n2>
  83. This option tells fbcon to take over only a range of consoles as
  84. specified by the values 'n1' and 'n2'. The rest of the consoles
  85. outside the given range will still be controlled by the standard
  86. console driver.
  87. NOTE: For x86 machines, the standard console is the VGA console which
  88. is typically located on the same video card. Thus, the consoles that
  89. are controlled by the VGA console will be garbled.
  90. 4. fbcon=rotate:<n>
  91. This option changes the orientation angle of the console display. The
  92. value 'n' accepts the following:
  93. 0 - normal orientation (0 degree)
  94. 1 - clockwise orientation (90 degrees)
  95. 2 - upside down orientation (180 degrees)
  96. 3 - counterclockwise orientation (270 degrees)
  97. The angle can be changed anytime afterwards by 'echoing' the same
  98. numbers to any one of the 2 attributes found in
  99. /sys/class/graphics/fbcon
  100. rotate - rotate the display of the active console
  101. rotate_all - rotate the display of all consoles
  102. Console rotation will only become available if Console Rotation
  103. Support is compiled in your kernel.
  104. NOTE: This is purely console rotation. Any other applications that
  105. use the framebuffer will remain at their 'normal'orientation.
  106. Actually, the underlying fb driver is totally ignorant of console
  107. rotation.
  108. C. Attaching, Detaching and Unloading
  109. Before going on on how to attach, detach and unload the framebuffer console, an
  110. illustration of the dependencies may help.
  111. The console layer, as with most subsystems, needs a driver that interfaces with
  112. the hardware. Thus, in a VGA console:
  113. console ---> VGA driver ---> hardware.
  114. Assuming the VGA driver can be unloaded, one must first unbind the VGA driver
  115. from the console layer before unloading the driver. The VGA driver cannot be
  116. unloaded if it is still bound to the console layer. (See
  117. Documentation/console/console.txt for more information).
  118. This is more complicated in the case of the framebuffer console (fbcon),
  119. because fbcon is an intermediate layer between the console and the drivers:
  120. console ---> fbcon ---> fbdev drivers ---> hardware
  121. The fbdev drivers cannot be unloaded if it's bound to fbcon, and fbcon cannot
  122. be unloaded if it's bound to the console layer.
  123. So to unload the fbdev drivers, one must first unbind fbcon from the console,
  124. then unbind the fbdev drivers from fbcon. Fortunately, unbinding fbcon from
  125. the console layer will automatically unbind framebuffer drivers from
  126. fbcon. Thus, there is no need to explicitly unbind the fbdev drivers from
  127. fbcon.
  128. So, how do we unbind fbcon from the console? Part of the answer is in
  129. Documentation/console/console.txt. To summarize:
  130. Echo a value to the bind file that represents the framebuffer console
  131. driver. So assuming vtcon1 represents fbcon, then:
  132. echo 1 > sys/class/vtconsole/vtcon1/bind - attach framebuffer console to
  133. console layer
  134. echo 0 > sys/class/vtconsole/vtcon1/bind - detach framebuffer console from
  135. console layer
  136. If fbcon is detached from the console layer, your boot console driver (which is
  137. usually VGA text mode) will take over. A few drivers (rivafb and i810fb) will
  138. restore VGA text mode for you. With the rest, before detaching fbcon, you
  139. must take a few additional steps to make sure that your VGA text mode is
  140. restored properly. The following is one of the several methods that you can do:
  141. 1. Download or install vbetool. This utility is included with most
  142. distributions nowadays, and is usually part of the suspend/resume tool.
  143. 2. In your kernel configuration, ensure that CONFIG_FRAMEBUFFER_CONSOLE is set
  144. to 'y' or 'm'. Enable one or more of your favorite framebuffer drivers.
  145. 3. Boot into text mode and as root run:
  146. vbetool vbestate save > <vga state file>
  147. The above command saves the register contents of your graphics
  148. hardware to <vga state file>. You need to do this step only once as
  149. the state file can be reused.
  150. 4. If fbcon is compiled as a module, load fbcon by doing:
  151. modprobe fbcon
  152. 5. Now to detach fbcon:
  153. vbetool vbestate restore < <vga state file> && \
  154. echo 0 > /sys/class/vtconsole/vtcon1/bind
  155. 6. That's it, you're back to VGA mode. And if you compiled fbcon as a module,
  156. you can unload it by 'rmmod fbcon'
  157. 7. To reattach fbcon:
  158. echo 1 > /sys/class/vtconsole/vtcon1/bind
  159. 8. Once fbcon is unbound, all drivers registered to the system will also
  160. become unbound. This means that fbcon and individual framebuffer drivers
  161. can be unloaded or reloaded at will. Reloading the drivers or fbcon will
  162. automatically bind the console, fbcon and the drivers together. Unloading
  163. all the drivers without unloading fbcon will make it impossible for the
  164. console to bind fbcon.
  165. Notes for vesafb users:
  166. =======================
  167. Unfortunately, if your bootline includes a vga=xxx parameter that sets the
  168. hardware in graphics mode, such as when loading vesafb, vgacon will not load.
  169. Instead, vgacon will replace the default boot console with dummycon, and you
  170. won't get any display after detaching fbcon. Your machine is still alive, so
  171. you can reattach vesafb. However, to reattach vesafb, you need to do one of
  172. the following:
  173. Variation 1:
  174. a. Before detaching fbcon, do
  175. vbetool vbemode save > <vesa state file> # do once for each vesafb mode,
  176. # the file can be reused
  177. b. Detach fbcon as in step 5.
  178. c. Attach fbcon
  179. vbetool vbestate restore < <vesa state file> && \
  180. echo 1 > /sys/class/vtconsole/vtcon1/bind
  181. Variation 2:
  182. a. Before detaching fbcon, do:
  183. echo <ID> > /sys/class/tty/console/bind
  184. vbetool vbemode get
  185. b. Take note of the mode number
  186. b. Detach fbcon as in step 5.
  187. c. Attach fbcon:
  188. vbetool vbemode set <mode number> && \
  189. echo 1 > /sys/class/vtconsole/vtcon1/bind
  190. Samples:
  191. ========
  192. Here are 2 sample bash scripts that you can use to bind or unbind the
  193. framebuffer console driver if you are in an X86 box:
  194. ---------------------------------------------------------------------------
  195. #!/bin/bash
  196. # Unbind fbcon
  197. # Change this to where your actual vgastate file is located
  198. # Or Use VGASTATE=$1 to indicate the state file at runtime
  199. VGASTATE=/tmp/vgastate
  200. # path to vbetool
  201. VBETOOL=/usr/local/bin
  202. for (( i = 0; i < 16; i++))
  203. do
  204. if test -x /sys/class/vtconsole/vtcon$i; then
  205. if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
  206. = 1 ]; then
  207. if test -x $VBETOOL/vbetool; then
  208. echo Unbinding vtcon$i
  209. $VBETOOL/vbetool vbestate restore < $VGASTATE
  210. echo 0 > /sys/class/vtconsole/vtcon$i/bind
  211. fi
  212. fi
  213. fi
  214. done
  215. ---------------------------------------------------------------------------
  216. #!/bin/bash
  217. # Bind fbcon
  218. for (( i = 0; i < 16; i++))
  219. do
  220. if test -x /sys/class/vtconsole/vtcon$i; then
  221. if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
  222. = 1 ]; then
  223. echo Unbinding vtcon$i
  224. echo 1 > /sys/class/vtconsole/vtcon$i/bind
  225. fi
  226. fi
  227. done
  228. ---------------------------------------------------------------------------
  229. --
  230. Antonino Daplas <>