fbcon.rst 12 KB

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  1. =======================
  2. The Framebuffer Console
  3. =======================
  4. The framebuffer console (fbcon), as its name implies, is a text
  5. console running on top of the framebuffer device. It has the functionality of
  6. any standard text console driver, such as the VGA console, with the added
  7. features that can be attributed to the graphical nature of the framebuffer.
  8. In the x86 architecture, the framebuffer console is optional, and
  9. some even treat it as a toy. For other architectures, it is the only available
  10. display device, text or graphical.
  11. What are the features of fbcon? The framebuffer console supports
  12. high resolutions, varying font types, display rotation, primitive multihead,
  13. etc. Theoretically, multi-colored fonts, blending, aliasing, and any feature
  14. made available by the underlying graphics card are also possible.
  15. A. Configuration
  16. ================
  17. The framebuffer console can be enabled by using your favorite kernel
  18. configuration tool. It is under Device Drivers->Graphics Support->
  19. Console display driver support->Framebuffer Console Support.
  20. Select 'y' to compile support statically or 'm' for module support. The
  21. module will be fbcon.
  22. In order for fbcon to activate, at least one framebuffer driver is
  23. required, so choose from any of the numerous drivers available. For x86
  24. systems, they almost universally have VGA cards, so vga16fb and vesafb will
  25. always be available. However, using a chipset-specific driver will give you
  26. more speed and features, such as the ability to change the video mode
  27. dynamically.
  28. To display the penguin logo, choose any logo available in Graphics
  29. support->Bootup logo.
  30. Also, you will need to select at least one compiled-in font, but if
  31. you don't do anything, the kernel configuration tool will select one for you,
  32. usually an 8x16 font.
  33. GOTCHA: A common bug report is enabling the framebuffer without enabling the
  34. framebuffer console. Depending on the driver, you may get a blanked or
  35. garbled display, but the system still boots to completion. If you are
  36. fortunate to have a driver that does not alter the graphics chip, then you
  37. will still get a VGA console.
  38. B. Loading
  39. ==========
  40. Possible scenarios:
  41. 1. Driver and fbcon are compiled statically
  42. Usually, fbcon will automatically take over your console. The notable
  43. exception is vesafb. It needs to be explicitly activated with the
  44. vga= boot option parameter.
  45. 2. Driver is compiled statically, fbcon is compiled as a module
  46. Depending on the driver, you either get a standard console, or a
  47. garbled display, as mentioned above. To get a framebuffer console,
  48. do a 'modprobe fbcon'.
  49. 3. Driver is compiled as a module, fbcon is compiled statically
  50. You get your standard console. Once the driver is loaded with
  51. 'modprobe xxxfb', fbcon automatically takes over the console with
  52. the possible exception of using the fbcon=map:n option. See below.
  53. 4. Driver and fbcon are compiled as a module.
  54. You can load them in any order. Once both are loaded, fbcon will take
  55. over the console.
  56. C. Boot options
  57. The framebuffer console has several, largely unknown, boot options
  58. that can change its behavior.
  59. 1. fbcon=font:<name>
  60. Select the initial font to use. The value 'name' can be any of the
  61. compiled-in fonts: 10x18, 6x10, 6x8, 7x14, Acorn8x8, MINI4x6,
  62. PEARL8x8, ProFont6x11, SUN12x22, SUN8x16, TER16x32, VGA8x16, VGA8x8.
  63. Note, not all drivers can handle font with widths not divisible by 8,
  64. such as vga16fb.
  65. 2. fbcon=map:<0123>
  66. This is an interesting option. It tells which driver gets mapped to
  67. which console. The value '0123' is a sequence that gets repeated until
  68. the total length is 64 which is the number of consoles available. In
  69. the above example, it is expanded to 012301230123... and the mapping
  70. will be::
  71. tty | 1 2 3 4 5 6 7 8 9 ...
  72. fb | 0 1 2 3 0 1 2 3 0 ...
  73. ('cat /proc/fb' should tell you what the fb numbers are)
  74. One side effect that may be useful is using a map value that exceeds
  75. the number of loaded fb drivers. For example, if only one driver is
  76. available, fb0, adding fbcon=map:1 tells fbcon not to take over the
  77. console.
  78. Later on, when you want to map the console the to the framebuffer
  79. device, you can use the con2fbmap utility.
  80. 3. fbcon=vc:<n1>-<n2>
  81. This option tells fbcon to take over only a range of consoles as
  82. specified by the values 'n1' and 'n2'. The rest of the consoles
  83. outside the given range will still be controlled by the standard
  84. console driver.
  85. NOTE: For x86 machines, the standard console is the VGA console which
  86. is typically located on the same video card. Thus, the consoles that
  87. are controlled by the VGA console will be garbled.
  88. 4. fbcon=rotate:<n>
  89. This option changes the orientation angle of the console display. The
  90. value 'n' accepts the following:
  91. - 0 - normal orientation (0 degree)
  92. - 1 - clockwise orientation (90 degrees)
  93. - 2 - upside down orientation (180 degrees)
  94. - 3 - counterclockwise orientation (270 degrees)
  95. The angle can be changed anytime afterwards by 'echoing' the same
  96. numbers to any one of the 2 attributes found in
  97. /sys/class/graphics/fbcon:
  98. - rotate - rotate the display of the active console
  99. - rotate_all - rotate the display of all consoles
  100. Console rotation will only become available if Framebuffer Console
  101. Rotation support is compiled in your kernel.
  102. NOTE: This is purely console rotation. Any other applications that
  103. use the framebuffer will remain at their 'normal' orientation.
  104. Actually, the underlying fb driver is totally ignorant of console
  105. rotation.
  106. 5. fbcon=margin:<color>
  107. This option specifies the color of the margins. The margins are the
  108. leftover area at the right and the bottom of the screen that are not
  109. used by text. By default, this area will be black. The 'color' value
  110. is an integer number that depends on the framebuffer driver being used.
  111. 6. fbcon=nodefer
  112. If the kernel is compiled with deferred fbcon takeover support, normally
  113. the framebuffer contents, left in place by the firmware/bootloader, will
  114. be preserved until there actually is some text is output to the console.
  115. This option causes fbcon to bind immediately to the fbdev device.
  116. 7. fbcon=logo-pos:<location>
  117. The only possible 'location' is 'center' (without quotes), and when
  118. given, the bootup logo is moved from the default top-left corner
  119. location to the center of the framebuffer. If more than one logo is
  120. displayed due to multiple CPUs, the collected line of logos is moved
  121. as a whole.
  122. 8. fbcon=logo-count:<n>
  123. The value 'n' overrides the number of bootup logos. 0 disables the
  124. logo, and -1 gives the default which is the number of online CPUs.
  125. C. Attaching, Detaching and Unloading
  126. Before going on to how to attach, detach and unload the framebuffer console, an
  127. illustration of the dependencies may help.
  128. The console layer, as with most subsystems, needs a driver that interfaces with
  129. the hardware. Thus, in a VGA console::
  130. console ---> VGA driver ---> hardware.
  131. Assuming the VGA driver can be unloaded, one must first unbind the VGA driver
  132. from the console layer before unloading the driver. The VGA driver cannot be
  133. unloaded if it is still bound to the console layer. (See
  134. Documentation/driver-api/console.rst for more information).
  135. This is more complicated in the case of the framebuffer console (fbcon),
  136. because fbcon is an intermediate layer between the console and the drivers::
  137. console ---> fbcon ---> fbdev drivers ---> hardware
  138. The fbdev drivers cannot be unloaded if bound to fbcon, and fbcon cannot
  139. be unloaded if it's bound to the console layer.
  140. So to unload the fbdev drivers, one must first unbind fbcon from the console,
  141. then unbind the fbdev drivers from fbcon. Fortunately, unbinding fbcon from
  142. the console layer will automatically unbind framebuffer drivers from
  143. fbcon. Thus, there is no need to explicitly unbind the fbdev drivers from
  144. fbcon.
  145. So, how do we unbind fbcon from the console? Part of the answer is in
  146. Documentation/driver-api/console.rst. To summarize:
  147. Echo a value to the bind file that represents the framebuffer console
  148. driver. So assuming vtcon1 represents fbcon, then::
  149. echo 1 > sys/class/vtconsole/vtcon1/bind - attach framebuffer console to
  150. console layer
  151. echo 0 > sys/class/vtconsole/vtcon1/bind - detach framebuffer console from
  152. console layer
  153. If fbcon is detached from the console layer, your boot console driver (which is
  154. usually VGA text mode) will take over. A few drivers (rivafb and i810fb) will
  155. restore VGA text mode for you. With the rest, before detaching fbcon, you
  156. must take a few additional steps to make sure that your VGA text mode is
  157. restored properly. The following is one of the several methods that you can do:
  158. 1. Download or install vbetool. This utility is included with most
  159. distributions nowadays, and is usually part of the suspend/resume tool.
  160. 2. In your kernel configuration, ensure that CONFIG_FRAMEBUFFER_CONSOLE is set
  161. to 'y' or 'm'. Enable one or more of your favorite framebuffer drivers.
  162. 3. Boot into text mode and as root run::
  163. vbetool vbestate save > <vga state file>
  164. The above command saves the register contents of your graphics
  165. hardware to <vga state file>. You need to do this step only once as
  166. the state file can be reused.
  167. 4. If fbcon is compiled as a module, load fbcon by doing::
  168. modprobe fbcon
  169. 5. Now to detach fbcon::
  170. vbetool vbestate restore < <vga state file> && \
  171. echo 0 > /sys/class/vtconsole/vtcon1/bind
  172. 6. That's it, you're back to VGA mode. And if you compiled fbcon as a module,
  173. you can unload it by 'rmmod fbcon'.
  174. 7. To reattach fbcon::
  175. echo 1 > /sys/class/vtconsole/vtcon1/bind
  176. 8. Once fbcon is unbound, all drivers registered to the system will also
  177. become unbound. This means that fbcon and individual framebuffer drivers
  178. can be unloaded or reloaded at will. Reloading the drivers or fbcon will
  179. automatically bind the console, fbcon and the drivers together. Unloading
  180. all the drivers without unloading fbcon will make it impossible for the
  181. console to bind fbcon.
  182. Notes for vesafb users:
  183. =======================
  184. Unfortunately, if your bootline includes a vga=xxx parameter that sets the
  185. hardware in graphics mode, such as when loading vesafb, vgacon will not load.
  186. Instead, vgacon will replace the default boot console with dummycon, and you
  187. won't get any display after detaching fbcon. Your machine is still alive, so
  188. you can reattach vesafb. However, to reattach vesafb, you need to do one of
  189. the following:
  190. Variation 1:
  191. a. Before detaching fbcon, do::
  192. vbetool vbemode save > <vesa state file> # do once for each vesafb mode,
  193. # the file can be reused
  194. b. Detach fbcon as in step 5.
  195. c. Attach fbcon::
  196. vbetool vbestate restore < <vesa state file> && \
  197. echo 1 > /sys/class/vtconsole/vtcon1/bind
  198. Variation 2:
  199. a. Before detaching fbcon, do::
  200. echo <ID> > /sys/class/tty/console/bind
  201. vbetool vbemode get
  202. b. Take note of the mode number
  203. b. Detach fbcon as in step 5.
  204. c. Attach fbcon::
  205. vbetool vbemode set <mode number> && \
  206. echo 1 > /sys/class/vtconsole/vtcon1/bind
  207. Samples:
  208. ========
  209. Here are 2 sample bash scripts that you can use to bind or unbind the
  210. framebuffer console driver if you are on an X86 box::
  211. #!/bin/bash
  212. # Unbind fbcon
  213. # Change this to where your actual vgastate file is located
  214. # Or Use VGASTATE=$1 to indicate the state file at runtime
  215. VGASTATE=/tmp/vgastate
  216. # path to vbetool
  217. VBETOOL=/usr/local/bin
  218. for (( i = 0; i < 16; i++))
  219. do
  220. if test -x /sys/class/vtconsole/vtcon$i; then
  221. if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
  222. = 1 ]; then
  223. if test -x $VBETOOL/vbetool; then
  224. echo Unbinding vtcon$i
  225. $VBETOOL/vbetool vbestate restore < $VGASTATE
  226. echo 0 > /sys/class/vtconsole/vtcon$i/bind
  227. fi
  228. fi
  229. fi
  230. done
  231. ---------------------------------------------------------------------------
  232. ::
  233. #!/bin/bash
  234. # Bind fbcon
  235. for (( i = 0; i < 16; i++))
  236. do
  237. if test -x /sys/class/vtconsole/vtcon$i; then
  238. if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
  239. = 1 ]; then
  240. echo Unbinding vtcon$i
  241. echo 1 > /sys/class/vtconsole/vtcon$i/bind
  242. fi
  243. fi
  244. done
  245. Antonino Daplas <adaplas@pol.net>