Root/Documentation/fb/fbcon.txt

Source at commit e46b4fc95cb8c12a9a05493b3cc1535cf05b3ce4 created 11 years 8 months ago.
By Paul Cercueil, fbcon: update documentation for the "bind" parameter.
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
1265. 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
1516. fbcon=bind:<01>
152
153    This option controls whether or not the console should be bound to the
154    terminal when the kernel starts. In any case, the framebuffer console
155    can be bound / unbound at any time through sysfs.
156
157    This parameter can be useful in some cases, for instance when the
158    bootloader or the video driver display a logo that should be shown
159    during the whole boot.
160
161C. Attaching, Detaching and Unloading
162
163Before going on on how to attach, detach and unload the framebuffer console, an
164illustration of the dependencies may help.
165
166The console layer, as with most subsystems, needs a driver that interfaces with
167the hardware. Thus, in a VGA console:
168
169console ---> VGA driver ---> hardware.
170
171Assuming the VGA driver can be unloaded, one must first unbind the VGA driver
172from the console layer before unloading the driver. The VGA driver cannot be
173unloaded if it is still bound to the console layer. (See
174Documentation/console/console.txt for more information).
175
176This is more complicated in the case of the framebuffer console (fbcon),
177because fbcon is an intermediate layer between the console and the drivers:
178
179console ---> fbcon ---> fbdev drivers ---> hardware
180
181The fbdev drivers cannot be unloaded if it's bound to fbcon, and fbcon cannot
182be unloaded if it's bound to the console layer.
183
184So to unload the fbdev drivers, one must first unbind fbcon from the console,
185then unbind the fbdev drivers from fbcon. Fortunately, unbinding fbcon from
186the console layer will automatically unbind framebuffer drivers from
187fbcon. Thus, there is no need to explicitly unbind the fbdev drivers from
188fbcon.
189
190So, how do we unbind fbcon from the console? Part of the answer is in
191Documentation/console/console.txt. To summarize:
192
193Echo a value to the bind file that represents the framebuffer console
194driver. So assuming vtcon1 represents fbcon, then:
195
196echo 1 > sys/class/vtconsole/vtcon1/bind - attach framebuffer console to
197                                           console layer
198echo 0 > sys/class/vtconsole/vtcon1/bind - detach framebuffer console from
199                                           console layer
200
201If fbcon is detached from the console layer, your boot console driver (which is
202usually VGA text mode) will take over. A few drivers (rivafb and i810fb) will
203restore VGA text mode for you. With the rest, before detaching fbcon, you
204must take a few additional steps to make sure that your VGA text mode is
205restored properly. The following is one of the several methods that you can do:
206
2071. Download or install vbetool. This utility is included with most
208   distributions nowadays, and is usually part of the suspend/resume tool.
209
2102. In your kernel configuration, ensure that CONFIG_FRAMEBUFFER_CONSOLE is set
211   to 'y' or 'm'. Enable one or more of your favorite framebuffer drivers.
212
2133. Boot into text mode and as root run:
214
215    vbetool vbestate save > <vga state file>
216
217    The above command saves the register contents of your graphics
218    hardware to <vga state file>. You need to do this step only once as
219    the state file can be reused.
220
2214. If fbcon is compiled as a module, load fbcon by doing:
222
223       modprobe fbcon
224
2255. Now to detach fbcon:
226
227       vbetool vbestate restore < <vga state file> && \
228       echo 0 > /sys/class/vtconsole/vtcon1/bind
229
2306. That's it, you're back to VGA mode. And if you compiled fbcon as a module,
231   you can unload it by 'rmmod fbcon'
232
2337. To reattach fbcon:
234
235       echo 1 > /sys/class/vtconsole/vtcon1/bind
236
2378. Once fbcon is unbound, all drivers registered to the system will also
238become unbound. This means that fbcon and individual framebuffer drivers
239can be unloaded or reloaded at will. Reloading the drivers or fbcon will
240automatically bind the console, fbcon and the drivers together. Unloading
241all the drivers without unloading fbcon will make it impossible for the
242console to bind fbcon.
243
244Notes for vesafb users:
245=======================
246
247Unfortunately, if your bootline includes a vga=xxx parameter that sets the
248hardware in graphics mode, such as when loading vesafb, vgacon will not load.
249Instead, vgacon will replace the default boot console with dummycon, and you
250won't get any display after detaching fbcon. Your machine is still alive, so
251you can reattach vesafb. However, to reattach vesafb, you need to do one of
252the following:
253
254Variation 1:
255
256    a. Before detaching fbcon, do
257
258       vbetool vbemode save > <vesa state file> # do once for each vesafb mode,
259                        # the file can be reused
260
261    b. Detach fbcon as in step 5.
262
263    c. Attach fbcon
264
265        vbetool vbestate restore < <vesa state file> && \
266    echo 1 > /sys/class/vtconsole/vtcon1/bind
267
268Variation 2:
269
270    a. Before detaching fbcon, do:
271    echo <ID> > /sys/class/tty/console/bind
272
273
274       vbetool vbemode get
275
276    b. Take note of the mode number
277
278    b. Detach fbcon as in step 5.
279
280    c. Attach fbcon:
281
282       vbetool vbemode set <mode number> && \
283       echo 1 > /sys/class/vtconsole/vtcon1/bind
284
285Samples:
286========
287
288Here are 2 sample bash scripts that you can use to bind or unbind the
289framebuffer console driver if you are in an X86 box:
290
291---------------------------------------------------------------------------
292#!/bin/bash
293# Unbind fbcon
294
295# Change this to where your actual vgastate file is located
296# Or Use VGASTATE=$1 to indicate the state file at runtime
297VGASTATE=/tmp/vgastate
298
299# path to vbetool
300VBETOOL=/usr/local/bin
301
302
303for (( i = 0; i < 16; i++))
304do
305  if test -x /sys/class/vtconsole/vtcon$i; then
306      if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
307           = 1 ]; then
308        if test -x $VBETOOL/vbetool; then
309           echo Unbinding vtcon$i
310           $VBETOOL/vbetool vbestate restore < $VGASTATE
311           echo 0 > /sys/class/vtconsole/vtcon$i/bind
312        fi
313      fi
314  fi
315done
316
317---------------------------------------------------------------------------
318#!/bin/bash
319# Bind fbcon
320
321for (( i = 0; i < 16; i++))
322do
323  if test -x /sys/class/vtconsole/vtcon$i; then
324      if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
325           = 1 ]; then
326      echo Unbinding vtcon$i
327      echo 1 > /sys/class/vtconsole/vtcon$i/bind
328      fi
329  fi
330done
331---------------------------------------------------------------------------
332
333--
334Antonino Daplas <adaplas@pol.net>
335

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