Root/Documentation/fb/fbcon.txt

1The Framebuffer Console
2=======================
3
4    The framebuffer console (fbcon), as its name implies, is a text
5console running on top of the framebuffer device. It has the functionality of
6any standard text console driver, such as the VGA console, with the added
7features that can be attributed to the graphical nature of the framebuffer.
8
9     In the x86 architecture, the framebuffer console is optional, and
10some even treat it as a toy. For other architectures, it is the only available
11display device, text or graphical.
12
13     What are the features of fbcon? The framebuffer console supports
14high resolutions, varying font types, display rotation, primitive multihead,
15etc. Theoretically, multi-colored fonts, blending, aliasing, and any feature
16made available by the underlying graphics card are also possible.
17
18A. Configuration
19
20    The framebuffer console can be enabled by using your favorite kernel
21configuration tool. It is under Device Drivers->Graphics Support->Support for
22framebuffer devices->Framebuffer Console Support. Select 'y' to compile
23support statically, or 'm' for module support. The module will be fbcon.
24
25    In order for fbcon to activate, at least one framebuffer driver is
26required, so choose from any of the numerous drivers available. For x86
27systems, they almost universally have VGA cards, so vga16fb and vesafb will
28always be available. However, using a chipset-specific driver will give you
29more speed and features, such as the ability to change the video mode
30dynamically.
31
32    To display the penguin logo, choose any logo available in Logo
33Configuration->Boot up logo.
34
35    Also, you will need to select at least one compiled-in fonts, but if
36you don't do anything, the kernel configuration tool will select one for you,
37usually an 8x16 font.
38
39GOTCHA: A common bug report is enabling the framebuffer without enabling the
40framebuffer console. Depending on the driver, you may get a blanked or
41garbled display, but the system still boots to completion. If you are
42fortunate to have a driver that does not alter the graphics chip, then you
43will still get a VGA console.
44
45B. Loading
46
47Possible scenarios:
48
491. Driver and fbcon are compiled statically
50
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.
54
552. Driver is compiled statically, fbcon is compiled as a module
56
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'.
60
613. Driver is compiled as a module, fbcon is compiled statically
62
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.
66
674. Driver and fbcon are compiled as a module.
68
69     You can load them in any order. Once both are loaded, fbcon will take
70     over the console.
71
72C. Boot options
73
74         The framebuffer console has several, largely unknown, boot options
75         that can change its behavior.
76
771. fbcon=font:<name>
78
79        Select the initial font to use. The value 'name' can be any of the
80        compiled-in fonts: VGA8x16, 7x14, 10x18, VGA8x8, MINI4x6, RomanLarge,
81        SUN8x16, SUN12x22, ProFont6x11, Acorn8x8, PEARL8x8.
82
83    Note, not all drivers can handle font with widths not divisible by 8,
84        such as vga16fb.
85
862. fbcon=scrollback:<value>[k]
87
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.
93
943. fbcon=map:<0123>
95
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
100        will be:
101
102        tty | 1 2 3 4 5 6 7 8 9 ...
103        fb | 0 1 2 3 0 1 2 3 0 ...
104
105        ('cat /proc/fb' should tell you what the fb numbers are)
106
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
110    console.
111
112    Later on, when you want to map the console the to the framebuffer
113    device, you can use the con2fbmap utility.
114
1154. fbcon=vc:<n1>-<n2>
116
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
120    console driver.
121
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.
125
1264. fbcon=rotate:<n>
127
128        This option changes the orientation angle of the console display. The
129        value 'n' accepts the following:
130
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)
135
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
139
140        rotate - rotate the display of the active console
141        rotate_all - rotate the display of all consoles
142
143    Console rotation will only become available if Console Rotation
144    Support is compiled in your kernel.
145
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
149    rotation.
150
151C. Attaching, Detaching and Unloading
152
153Before going on on how to attach, detach and unload the framebuffer console, an
154illustration of the dependencies may help.
155
156The console layer, as with most subsystems, needs a driver that interfaces with
157the hardware. Thus, in a VGA console:
158
159console ---> VGA driver ---> hardware.
160
161Assuming the VGA driver can be unloaded, one must first unbind the VGA driver
162from the console layer before unloading the driver. The VGA driver cannot be
163unloaded if it is still bound to the console layer. (See
164Documentation/console/console.txt for more information).
165
166This is more complicated in the case of the framebuffer console (fbcon),
167because fbcon is an intermediate layer between the console and the drivers:
168
169console ---> fbcon ---> fbdev drivers ---> hardware
170
171The fbdev drivers cannot be unloaded if it's bound to fbcon, and fbcon cannot
172be unloaded if it's bound to the console layer.
173
174So to unload the fbdev drivers, one must first unbind fbcon from the console,
175then unbind the fbdev drivers from fbcon. Fortunately, unbinding fbcon from
176the console layer will automatically unbind framebuffer drivers from
177fbcon. Thus, there is no need to explicitly unbind the fbdev drivers from
178fbcon.
179
180So, how do we unbind fbcon from the console? Part of the answer is in
181Documentation/console/console.txt. To summarize:
182
183Echo a value to the bind file that represents the framebuffer console
184driver. So assuming vtcon1 represents fbcon, then:
185
186echo 1 > sys/class/vtconsole/vtcon1/bind - attach framebuffer console to
187                                           console layer
188echo 0 > sys/class/vtconsole/vtcon1/bind - detach framebuffer console from
189                                           console layer
190
191If fbcon is detached from the console layer, your boot console driver (which is
192usually VGA text mode) will take over. A few drivers (rivafb and i810fb) will
193restore VGA text mode for you. With the rest, before detaching fbcon, you
194must take a few additional steps to make sure that your VGA text mode is
195restored properly. The following is one of the several methods that you can do:
196
1971. Download or install vbetool. This utility is included with most
198   distributions nowadays, and is usually part of the suspend/resume tool.
199
2002. In your kernel configuration, ensure that CONFIG_FRAMEBUFFER_CONSOLE is set
201   to 'y' or 'm'. Enable one or more of your favorite framebuffer drivers.
202
2033. Boot into text mode and as root run:
204
205    vbetool vbestate save > <vga state file>
206
207    The above command saves the register contents of your graphics
208    hardware to <vga state file>. You need to do this step only once as
209    the state file can be reused.
210
2114. If fbcon is compiled as a module, load fbcon by doing:
212
213       modprobe fbcon
214
2155. Now to detach fbcon:
216
217       vbetool vbestate restore < <vga state file> && \
218       echo 0 > /sys/class/vtconsole/vtcon1/bind
219
2206. That's it, you're back to VGA mode. And if you compiled fbcon as a module,
221   you can unload it by 'rmmod fbcon'
222
2237. To reattach fbcon:
224
225       echo 1 > /sys/class/vtconsole/vtcon1/bind
226
2278. Once fbcon is unbound, all drivers registered to the system will also
228become unbound. This means that fbcon and individual framebuffer drivers
229can be unloaded or reloaded at will. Reloading the drivers or fbcon will
230automatically bind the console, fbcon and the drivers together. Unloading
231all the drivers without unloading fbcon will make it impossible for the
232console to bind fbcon.
233
234Notes for vesafb users:
235=======================
236
237Unfortunately, if your bootline includes a vga=xxx parameter that sets the
238hardware in graphics mode, such as when loading vesafb, vgacon will not load.
239Instead, vgacon will replace the default boot console with dummycon, and you
240won't get any display after detaching fbcon. Your machine is still alive, so
241you can reattach vesafb. However, to reattach vesafb, you need to do one of
242the following:
243
244Variation 1:
245
246    a. Before detaching fbcon, do
247
248       vbetool vbemode save > <vesa state file> # do once for each vesafb mode,
249                        # the file can be reused
250
251    b. Detach fbcon as in step 5.
252
253    c. Attach fbcon
254
255        vbetool vbestate restore < <vesa state file> && \
256    echo 1 > /sys/class/vtconsole/vtcon1/bind
257
258Variation 2:
259
260    a. Before detaching fbcon, do:
261    echo <ID> > /sys/class/tty/console/bind
262
263
264       vbetool vbemode get
265
266    b. Take note of the mode number
267
268    b. Detach fbcon as in step 5.
269
270    c. Attach fbcon:
271
272       vbetool vbemode set <mode number> && \
273       echo 1 > /sys/class/vtconsole/vtcon1/bind
274
275Samples:
276========
277
278Here are 2 sample bash scripts that you can use to bind or unbind the
279framebuffer console driver if you are in an X86 box:
280
281---------------------------------------------------------------------------
282#!/bin/bash
283# Unbind fbcon
284
285# Change this to where your actual vgastate file is located
286# Or Use VGASTATE=$1 to indicate the state file at runtime
287VGASTATE=/tmp/vgastate
288
289# path to vbetool
290VBETOOL=/usr/local/bin
291
292
293for (( i = 0; i < 16; i++))
294do
295  if test -x /sys/class/vtconsole/vtcon$i; then
296      if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
297           = 1 ]; then
298        if test -x $VBETOOL/vbetool; then
299           echo Unbinding vtcon$i
300           $VBETOOL/vbetool vbestate restore < $VGASTATE
301           echo 0 > /sys/class/vtconsole/vtcon$i/bind
302        fi
303      fi
304  fi
305done
306
307---------------------------------------------------------------------------
308#!/bin/bash
309# Bind fbcon
310
311for (( i = 0; i < 16; i++))
312do
313  if test -x /sys/class/vtconsole/vtcon$i; then
314      if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
315           = 1 ]; then
316      echo Unbinding vtcon$i
317      echo 1 > /sys/class/vtconsole/vtcon$i/bind
318      fi
319  fi
320done
321---------------------------------------------------------------------------
322
323--
324Antonino Daplas <adaplas@pol.net>
325

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