[PATCH] VT binding: fbcon: Update documentation
[linux-2.6-block.git] / Documentation / fb / fbcon.txt
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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
a4a73e1f 138 /sys/class/graphics/fbcon
efb985f6 139
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140 rotate - rotate the display of the active console
141 rotate_all - rotate the display of all consoles
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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
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151C. Attaching, Detaching and Unloading
152
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153Before going on on how to attach, detach and unload the framebuffer console, an
154illustration of the dependencies may help.
a4a73e1f 155
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156The console layer, as with most subsystems, needs a driver that interfaces with
157the hardware. Thus, in a VGA console:
a4a73e1f 158
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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 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 the ID number of the 'frame buffer driver' to:
184
185sys/class/tty/console/bind - attach framebuffer console to console layer
186sys/class/tty/console/unbind - detach framebuffer console from console layer
187
188If fbcon is detached from the console layer, your boot console driver (which is
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189usually VGA text mode) will take over. A few drivers (rivafb and i810fb) will
190restore VGA text mode for you. With the rest, before detaching fbcon, you
191must take a few additional steps to make sure that your VGA text mode is
192restored properly. The following is one of the several methods that you can do:
193
1941. Download or install vbetool. This utility is included with most
195 distributions nowadays, and is usually part of the suspend/resume tool.
196
1972. In your kernel configuration, ensure that CONFIG_FRAMEBUFFER_CONSOLE is set
198 to 'y' or 'm'. Enable one or more of your favorite framebuffer drivers.
199
2003. Boot into text mode and as root run:
201
202 vbetool vbestate save > <vga state file>
203
204 The above command saves the register contents of your graphics
205 hardware to <vga state file>. You need to do this step only once as
206 the state file can be reused.
207
2084. If fbcon is compiled as a module, load fbcon by doing:
209
210 modprobe fbcon
211
2125. Now to detach fbcon:
213
418d1ce6 214 'cat /sys/class/tty/console/backend' and take note of the ID
a4a73e1f 215
418d1ce6 216The above is probably needed only once. Then:
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218 vbetool vbestate restore < <vga state file> && \
219 echo <ID> > /sys/class/tty/console/unbind
a4a73e1f 220
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2216. That's it, you're back to VGA mode. And if you compiled fbcon as a module,
222 you can unload it by 'rmmod fbcon'
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223
2247. To reattach fbcon:
225
418d1ce6 226 echo <ID> > /sys/class/tty/console/bind
a4a73e1f 227
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2288. Once fbcon is unbound, all drivers registered to the system will also
229become unbound. This means that fbcon and individual framebuffer drivers
230can be unloaded or reloaded at will. Reloading the drivers or fbcon will
231automatically bind the console, fbcon and the drivers together. Unloading
232all the drivers without unloading fbcon will make it impossible for the
233console to bind fbcon.
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234
235Notes for vesafb users:
236=======================
237
238Unfortunately, if your bootline includes a vga=xxx parameter that sets the
239hardware in graphics mode, such as when loading vesafb, vgacon will not load.
240Instead, vgacon will replace the default boot console with dummycon, and you
241won't get any display after detaching fbcon. Your machine is still alive, so
242you can reattach vesafb. However, to reattach vesafb, you need to do one of
243the following:
244
245Variation 1:
246
247 a. Before detaching fbcon, do
248
249 vbetool vbemode save > <vesa state file> # do once for each vesafb mode,
250 # the file can be reused
251
252 b. Detach fbcon as in step 5.
253
254 c. Attach fbcon
255
256 vbetool vbestate restore < <vesa state file> && \
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257Variation 2:
258
259 a. Before detaching fbcon, do:
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260 echo <ID> > /sys/class/tty/console/bind
261
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262
263 vbetool vbemode get
264
265 b. Take note of the mode number
266
267 b. Detach fbcon as in step 5.
268
269 c. Attach fbcon:
270
271 vbetool vbemode set <mode number> && \
418d1ce6 272 echo <ID> > /sys/class/tty/console/bind
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273
274--
efb985f6 275Antonino Daplas <adaplas@pol.net>