1 =======================
2 The Framebuffer Console
3 =======================
4
5 The framebuffer console (fbcon), as its name i
6 console running on top of the framebuffer devi
7 any standard text console driver, such as the
8 features that can be attributed to the graphic
9
10 In the x86 architecture, the framebuffer conso
11 some even treat it as a toy. For other archite
12 display device, text or graphical.
13
14 What are the features of fbcon? The framebuff
15 high resolutions, varying font types, display
16 etc. Theoretically, multi-colored fonts, blend
17 made available by the underlying graphics card
18
19 A. Configuration
20 ================
21
22 The framebuffer console can be enabled by usin
23 configuration tool. It is under Device Driver
24 Console display driver support->Framebuffer Co
25 Select 'y' to compile support statically or 'm
26 module will be fbcon.
27
28 In order for fbcon to activate, at least one f
29 required, so choose from any of the numerous d
30 systems, they almost universally have VGA card
31 always be available. However, using a chipset-
32 more speed and features, such as the ability t
33 dynamically.
34
35 To display the penguin logo, choose any logo a
36 support->Bootup logo.
37
38 Also, you will need to select at least one com
39 you don't do anything, the kernel configuratio
40 usually an 8x16 font.
41
42 GOTCHA: A common bug report is enabling the fr
43 framebuffer console. Depending on the driver,
44 garbled display, but the system still boots to
45 fortunate to have a driver that does not alter
46 will still get a VGA console.
47
48 B. Loading
49 ==========
50
51 Possible scenarios:
52
53 1. Driver and fbcon are compiled statically
54
55 Usually, fbcon will automatically tak
56 exception is vesafb. It needs to be
57 vga= boot option parameter.
58
59 2. Driver is compiled statically, fbcon is com
60
61 Depending on the driver, you either g
62 garbled display, as mentioned above.
63 do a 'modprobe fbcon'.
64
65 3. Driver is compiled as a module, fbcon is co
66
67 You get your standard console. Once
68 'modprobe xxxfb', fbcon automatically
69 the possible exception of using the f
70
71 4. Driver and fbcon are compiled as a module.
72
73 You can load them in any order. Once
74 over the console.
75
76 C. Boot options
77
78 The framebuffer console has several,
79 that can change its behavior.
80
81 1. fbcon=font:<name>
82
83 Select the initial font to use. The va
84 compiled-in fonts: 10x18, 6x10, 6x8, 7
85 PEARL8x8, ProFont6x11, SUN12x22, SUN8x
86
87 Note, not all drivers can handle font
88 such as vga16fb.
89
90
91 2. fbcon=map:<0123>
92
93 This is an interesting option. It tell
94 which console. The value '0123' is a s
95 the total length is 64 which is the nu
96 the above example, it is expanded to 0
97 will be::
98
99 tty | 1 2 3 4 5 6 7 8 9 ...
100 fb | 0 1 2 3 0 1 2 3 0 ...
101
102 ('cat /proc/fb' should tell yo
103
104 One side effect that may be useful is
105 the number of loaded fb drivers. For e
106 available, fb0, adding fbcon=map:1 tel
107 console.
108
109 Later on, when you want to map the con
110 device, you can use the con2fbmap util
111
112 3. fbcon=vc:<n1>-<n2>
113
114 This option tells fbcon to take over o
115 specified by the values 'n1' and 'n2'.
116 outside the given range will still be
117 console driver.
118
119 NOTE: For x86 machines, the standard c
120 is typically located on the same video
121 are controlled by the VGA console will
122
123 4. fbcon=rotate:<n>
124
125 This option changes the orientation an
126 value 'n' accepts the following:
127
128 - 0 - normal orientation (0 degree
129 - 1 - clockwise orientation (90 de
130 - 2 - upside down orientation (180
131 - 3 - counterclockwise orientation
132
133 The angle can be changed anytime after
134 numbers to any one of the 2 attributes
135 /sys/class/graphics/fbcon:
136
137 - rotate - rotate the disp
138 - rotate_all - rotate the disp
139
140 Console rotation will only become avai
141 Rotation support is compiled in your k
142
143 NOTE: This is purely console rotation.
144 use the framebuffer will remain at the
145 Actually, the underlying fb driver is
146 rotation.
147
148 5. fbcon=margin:<color>
149
150 This option specifies the color of the
151 leftover area at the right and the bot
152 used by text. By default, this area wi
153 is an integer number that depends on t
154
155 6. fbcon=nodefer
156
157 If the kernel is compiled with deferre
158 the framebuffer contents, left in plac
159 be preserved until there actually is s
160 This option causes fbcon to bind immed
161
162 7. fbcon=logo-pos:<location>
163
164 The only possible 'location' is 'cente
165 given, the bootup logo is moved from t
166 location to the center of the framebuf
167 displayed due to multiple CPUs, the co
168 as a whole.
169
170 8. fbcon=logo-count:<n>
171
172 The value 'n' overrides the number of
173 logo, and -1 gives the default which i
174
175 C. Attaching, Detaching and Unloading
176
177 Before going on to how to attach, detach and u
178 illustration of the dependencies may help.
179
180 The console layer, as with most subsystems, ne
181 the hardware. Thus, in a VGA console::
182
183 console ---> VGA driver ---> hardware.
184
185 Assuming the VGA driver can be unloaded, one m
186 from the console layer before unloading the dr
187 unloaded if it is still bound to the console l
188 Documentation/driver-api/console.rst for more
189
190 This is more complicated in the case of the fr
191 because fbcon is an intermediate layer between
192
193 console ---> fbcon ---> fbdev drivers
194
195 The fbdev drivers cannot be unloaded if bound
196 be unloaded if it's bound to the console layer
197
198 So to unload the fbdev drivers, one must first
199 then unbind the fbdev drivers from fbcon. For
200 the console layer will automatically unbind fr
201 fbcon. Thus, there is no need to explicitly un
202 fbcon.
203
204 So, how do we unbind fbcon from the console? P
205 Documentation/driver-api/console.rst. To summa
206
207 Echo a value to the bind file that represents
208 driver. So assuming vtcon1 represents fbcon, t
209
210 echo 1 > /sys/class/vtconsole/vtcon1/bind -
211 c
212 echo 0 > /sys/class/vtconsole/vtcon1/bind -
213 c
214
215 If fbcon is detached from the console layer, y
216 usually VGA text mode) will take over. A few
217 restore VGA text mode for you. With the rest,
218 must take a few additional steps to make sure
219 restored properly. The following is one of the
220
221 1. Download or install vbetool. This utility
222 distributions nowadays, and is usually part
223
224 2. In your kernel configuration, ensure that C
225 to 'y' or 'm'. Enable one or more of your f
226
227 3. Boot into text mode and as root run::
228
229 vbetool vbestate save > <vga state fil
230
231 The above command saves the register conten
232 hardware to <vga state file>. You need to
233 the state file can be reused.
234
235 4. If fbcon is compiled as a module, load fbco
236
237 modprobe fbcon
238
239 5. Now to detach fbcon::
240
241 vbetool vbestate restore < <vga state f
242 echo 0 > /sys/class/vtconsole/vtcon1/bi
243
244 6. That's it, you're back to VGA mode. And if
245 you can unload it by 'rmmod fbcon'.
246
247 7. To reattach fbcon::
248
249 echo 1 > /sys/class/vtconsole/vtcon1/bi
250
251 8. Once fbcon is unbound, all drivers register
252 become unbound. This means that fbcon and ind
253 can be unloaded or reloaded at will. Reloading
254 automatically bind the console, fbcon and the
255 all the drivers without unloading fbcon will m
256 console to bind fbcon.
257
258 Notes for vesafb users:
259 =======================
260
261 Unfortunately, if your bootline includes a vga
262 hardware in graphics mode, such as when loadin
263 Instead, vgacon will replace the default boot
264 won't get any display after detaching fbcon. Y
265 you can reattach vesafb. However, to reattach
266 the following:
267
268 Variation 1:
269
270 a. Before detaching fbcon, do::
271
272 vbetool vbemode save > <vesa state fil
273
274
275 b. Detach fbcon as in step 5.
276
277 c. Attach fbcon::
278
279 vbetool vbestate restore < <vesa state
280 echo 1 > /sys/class/vtconsole/vtcon1/b
281
282 Variation 2:
283
284 a. Before detaching fbcon, do::
285
286 echo <ID> > /sys/class/tty/console/bin
287
288 vbetool vbemode get
289
290 b. Take note of the mode number
291
292 b. Detach fbcon as in step 5.
293
294 c. Attach fbcon::
295
296 vbetool vbemode set <mode number> && \
297 echo 1 > /sys/class/vtconsole/vtcon1/b
298
299 Samples:
300 ========
301
302 Here are 2 sample bash scripts that you can us
303 framebuffer console driver if you are on an X8
304
305 #!/bin/bash
306 # Unbind fbcon
307
308 # Change this to where your actual vgastate
309 # Or Use VGASTATE=$1 to indicate the state f
310 VGASTATE=/tmp/vgastate
311
312 # path to vbetool
313 VBETOOL=/usr/local/bin
314
315
316 for (( i = 0; i < 16; i++))
317 do
318 if test -x /sys/class/vtconsole/vtcon$i; t
319 if [ `cat /sys/class/vtconsole/vtcon$i
320 = 1 ]; then
321 if test -x $VBETOOL/vbetool; then
322 echo Unbinding vtcon$i
323 $VBETOOL/vbetool vbestate resto
324 echo 0 > /sys/class/vtconsole/v
325 fi
326 fi
327 fi
328 done
329
330 ----------------------------------------------
331
332 ::
333
334 #!/bin/bash
335 # Bind fbcon
336
337 for (( i = 0; i < 16; i++))
338 do
339 if test -x /sys/class/vtconsole/vtcon$i; t
340 if [ `cat /sys/class/vtconsole/vtcon$i
341 = 1 ]; then
342 echo Unbinding vtcon$i
343 echo 1 > /sys/class/vtconsole/vtcon$
344 fi
345 fi
346 done
347
348 Antonino Daplas <adaplas@pol.net>
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.