1 The Framebuffer Console
2 =======================
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.
9 In the x86 architecture, the framebuffer console is optional, and
10 some even treat it as a toy. For other architectures, it is the only available
11 display device, text or graphical.
13 What are the features of fbcon? The framebuffer console supports
14 high resolutions, varying font types, display rotation, primitive multihead,
15 etc. Theoretically, multi-colored fonts, blending, aliasing, and any feature
16 made available by the underlying graphics card are also possible.
20 The framebuffer console can be enabled by using your favorite kernel
21 configuration tool. It is under Device Drivers->Graphics Support->Support for
22 framebuffer devices->Framebuffer Console Support. Select 'y' to compile
23 support statically, or 'm' for module support. The module will be fbcon.
25 In order for fbcon to activate, at least one framebuffer driver is
26 required, so choose from any of the numerous drivers available. For x86
27 systems, they almost universally have VGA cards, so vga16fb and vesafb will
28 always be available. However, using a chipset-specific driver will give you
29 more speed and features, such as the ability to change the video mode
32 To display the penguin logo, choose any logo available in Logo
33 Configuration->Boot up logo.
35 Also, you will need to select at least one compiled-in fonts, but if
36 you don't do anything, the kernel configuration tool will select one for you,
39 GOTCHA: A common bug report is enabling the framebuffer without enabling the
40 framebuffer console. Depending on the driver, you may get a blanked or
41 garbled display, but the system still boots to completion. If you are
42 fortunate to have a driver that does not alter the graphics chip, then you
43 will still get a VGA console.
49 1. Driver and fbcon are compiled statically
51 Usually, fbcon will automatically take over your console. The notable
52 exception is vesafb. It needs to be explicitly activated with the
53 vga= boot option parameter.
55 2. Driver is compiled statically, fbcon is compiled as a module
57 Depending on the driver, you either get a standard console, or a
58 garbled display, as mentioned above. To get a framebuffer console,
59 do a 'modprobe fbcon'.
61 3. Driver is compiled as a module, fbcon is compiled statically
63 You get your standard console. Once the driver is loaded with
64 'modprobe xxxfb', fbcon automatically takes over the console with
65 the possible exception of using the fbcon=map:n option. See below.
67 4. Driver and fbcon are compiled as a module.
69 You can load them in any order. Once both are loaded, fbcon will take
74 The framebuffer console has several, largely unknown, boot options
75 that can change its behavior.
79 Select the initial font to use. The value 'name' can be any of the
80 compiled-in fonts: 10x18, 6x10, 7x14, Acorn8x8, MINI4x6,
81 PEARL8x8, ProFont6x11, SUN12x22, SUN8x16, VGA8x16, VGA8x8.
83 Note, not all drivers can handle font with widths not divisible by 8,
86 2. fbcon=scrollback:<value>[k]
88 The scrollback buffer is memory that is used to preserve display
89 contents that has already scrolled past your view. This is accessed
90 by using the Shift-PageUp key combination. The value 'value' is any
91 integer. It defaults to 32KB. The 'k' suffix is optional, and will
92 multiply the 'value' by 1024.
96 This is an interesting option. It tells which driver gets mapped to
97 which console. The value '0123' is a sequence that gets repeated until
98 the total length is 64 which is the number of consoles available. In
99 the above example, it is expanded to 012301230123... and the mapping
102 tty | 1 2 3 4 5 6 7 8 9 ...
103 fb | 0 1 2 3 0 1 2 3 0 ...
105 ('cat /proc/fb' should tell you what the fb numbers are)
107 One side effect that may be useful is using a map value that exceeds
108 the number of loaded fb drivers. For example, if only one driver is
109 available, fb0, adding fbcon=map:1 tells fbcon not to take over the
112 Later on, when you want to map the console the to the framebuffer
113 device, you can use the con2fbmap utility.
115 4. fbcon=vc:<n1>-<n2>
117 This option tells fbcon to take over only a range of consoles as
118 specified by the values 'n1' and 'n2'. The rest of the consoles
119 outside the given range will still be controlled by the standard
122 NOTE: For x86 machines, the standard console is the VGA console which
123 is typically located on the same video card. Thus, the consoles that
124 are controlled by the VGA console will be garbled.
128 This option changes the orientation angle of the console display. The
129 value 'n' accepts the following:
131 0 - normal orientation (0 degree)
132 1 - clockwise orientation (90 degrees)
133 2 - upside down orientation (180 degrees)
134 3 - counterclockwise orientation (270 degrees)
136 The angle can be changed anytime afterwards by 'echoing' the same
137 numbers to any one of the 2 attributes found in
138 /sys/class/graphics/fbcon
140 rotate - rotate the display of the active console
141 rotate_all - rotate the display of all consoles
143 Console rotation will only become available if Console Rotation
144 Support is compiled in your kernel.
146 NOTE: This is purely console rotation. Any other applications that
147 use the framebuffer will remain at their 'normal'orientation.
148 Actually, the underlying fb driver is totally ignorant of console
151 5. fbcon=margin:<color>
153 This option specifies the color of the margins. The margins are the
154 leftover area at the right and the bottom of the screen that are not
155 used by text. By default, this area will be black. The 'color' value
156 is an integer number that depends on the framebuffer driver being used.
160 If the kernel is compiled with deferred fbcon takeover support, normally
161 the framebuffer contents, left in place by the firmware/bootloader, will
162 be preserved until there actually is some text is output to the console.
163 This option causes fbcon to bind immediately to the fbdev device.
165 C. Attaching, Detaching and Unloading
167 Before going on how to attach, detach and unload the framebuffer console, an
168 illustration of the dependencies may help.
170 The console layer, as with most subsystems, needs a driver that interfaces with
171 the hardware. Thus, in a VGA console:
173 console ---> VGA driver ---> hardware.
175 Assuming the VGA driver can be unloaded, one must first unbind the VGA driver
176 from the console layer before unloading the driver. The VGA driver cannot be
177 unloaded if it is still bound to the console layer. (See
178 Documentation/console/console.txt for more information).
180 This is more complicated in the case of the framebuffer console (fbcon),
181 because fbcon is an intermediate layer between the console and the drivers:
183 console ---> fbcon ---> fbdev drivers ---> hardware
185 The fbdev drivers cannot be unloaded if it's bound to fbcon, and fbcon cannot
186 be unloaded if it's bound to the console layer.
188 So to unload the fbdev drivers, one must first unbind fbcon from the console,
189 then unbind the fbdev drivers from fbcon. Fortunately, unbinding fbcon from
190 the console layer will automatically unbind framebuffer drivers from
191 fbcon. Thus, there is no need to explicitly unbind the fbdev drivers from
194 So, how do we unbind fbcon from the console? Part of the answer is in
195 Documentation/console/console.txt. To summarize:
197 Echo a value to the bind file that represents the framebuffer console
198 driver. So assuming vtcon1 represents fbcon, then:
200 echo 1 > sys/class/vtconsole/vtcon1/bind - attach framebuffer console to
202 echo 0 > sys/class/vtconsole/vtcon1/bind - detach framebuffer console from
205 If fbcon is detached from the console layer, your boot console driver (which is
206 usually VGA text mode) will take over. A few drivers (rivafb and i810fb) will
207 restore VGA text mode for you. With the rest, before detaching fbcon, you
208 must take a few additional steps to make sure that your VGA text mode is
209 restored properly. The following is one of the several methods that you can do:
211 1. Download or install vbetool. This utility is included with most
212 distributions nowadays, and is usually part of the suspend/resume tool.
214 2. In your kernel configuration, ensure that CONFIG_FRAMEBUFFER_CONSOLE is set
215 to 'y' or 'm'. Enable one or more of your favorite framebuffer drivers.
217 3. Boot into text mode and as root run:
219 vbetool vbestate save > <vga state file>
221 The above command saves the register contents of your graphics
222 hardware to <vga state file>. You need to do this step only once as
223 the state file can be reused.
225 4. If fbcon is compiled as a module, load fbcon by doing:
229 5. Now to detach fbcon:
231 vbetool vbestate restore < <vga state file> && \
232 echo 0 > /sys/class/vtconsole/vtcon1/bind
234 6. That's it, you're back to VGA mode. And if you compiled fbcon as a module,
235 you can unload it by 'rmmod fbcon'
237 7. To reattach fbcon:
239 echo 1 > /sys/class/vtconsole/vtcon1/bind
241 8. Once fbcon is unbound, all drivers registered to the system will also
242 become unbound. This means that fbcon and individual framebuffer drivers
243 can be unloaded or reloaded at will. Reloading the drivers or fbcon will
244 automatically bind the console, fbcon and the drivers together. Unloading
245 all the drivers without unloading fbcon will make it impossible for the
246 console to bind fbcon.
248 Notes for vesafb users:
249 =======================
251 Unfortunately, if your bootline includes a vga=xxx parameter that sets the
252 hardware in graphics mode, such as when loading vesafb, vgacon will not load.
253 Instead, vgacon will replace the default boot console with dummycon, and you
254 won't get any display after detaching fbcon. Your machine is still alive, so
255 you can reattach vesafb. However, to reattach vesafb, you need to do one of
260 a. Before detaching fbcon, do
262 vbetool vbemode save > <vesa state file> # do once for each vesafb mode,
263 # the file can be reused
265 b. Detach fbcon as in step 5.
269 vbetool vbestate restore < <vesa state file> && \
270 echo 1 > /sys/class/vtconsole/vtcon1/bind
274 a. Before detaching fbcon, do:
275 echo <ID> > /sys/class/tty/console/bind
280 b. Take note of the mode number
282 b. Detach fbcon as in step 5.
286 vbetool vbemode set <mode number> && \
287 echo 1 > /sys/class/vtconsole/vtcon1/bind
292 Here are 2 sample bash scripts that you can use to bind or unbind the
293 framebuffer console driver if you are in an X86 box:
295 ---------------------------------------------------------------------------
299 # Change this to where your actual vgastate file is located
300 # Or Use VGASTATE=$1 to indicate the state file at runtime
301 VGASTATE=/tmp/vgastate
304 VBETOOL=/usr/local/bin
307 for (( i = 0; i < 16; i++))
309 if test -x /sys/class/vtconsole/vtcon$i; then
310 if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
312 if test -x $VBETOOL/vbetool; then
313 echo Unbinding vtcon$i
314 $VBETOOL/vbetool vbestate restore < $VGASTATE
315 echo 0 > /sys/class/vtconsole/vtcon$i/bind
321 ---------------------------------------------------------------------------
325 for (( i = 0; i < 16; i++))
327 if test -x /sys/class/vtconsole/vtcon$i; then
328 if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
330 echo Unbinding vtcon$i
331 echo 1 > /sys/class/vtconsole/vtcon$i/bind
335 ---------------------------------------------------------------------------
338 Antonino Daplas <adaplas@pol.net>