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Based on kernel version 3.16. Page generated on 2014-08-06 21:39 EST.

1	The Framebuffer Console
2	=======================
3	
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.
8	
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.
12	
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.
17	
18	A. Configuration
19	
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.
24	
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
30	dynamically.
31	
32		To display the penguin logo, choose any logo available in Logo
33	Configuration->Boot up logo.
34	
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,
37	usually an 8x16 font.
38	
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.
44	
45	B. Loading
46	
47	Possible scenarios:
48	
49	1. 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	
55	2. 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	
61	3. 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	
67	4. 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	
72	C. Boot options
73	
74	         The framebuffer console has several, largely unknown, boot options
75	         that can change its behavior.
76	
77	1. 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	
86	2. 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	
94	3. 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	
115	4. 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	
126	4. 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	
151	C. Attaching, Detaching and Unloading
152	
153	Before going on how to attach, detach and unload the framebuffer console, an
154	illustration of the dependencies may help.
155	
156	The console layer, as with most subsystems, needs a driver that interfaces with
157	the hardware. Thus, in a VGA console:
158	
159	console ---> VGA driver ---> hardware.
160	
161	Assuming the VGA driver can be unloaded, one must first unbind the VGA driver
162	from the console layer before unloading the driver.  The VGA driver cannot be
163	unloaded if it is still bound to the console layer. (See
164	Documentation/console/console.txt for more information).
165	
166	This is more complicated in the case of the framebuffer console (fbcon),
167	because fbcon is an intermediate layer between the console and the drivers:
168	
169	console ---> fbcon ---> fbdev drivers ---> hardware
170	
171	The fbdev drivers cannot be unloaded if it's bound to fbcon, and fbcon cannot
172	be unloaded if it's bound to the console layer.
173	
174	So to unload the fbdev drivers, one must first unbind fbcon from the console,
175	then unbind the fbdev drivers from fbcon.  Fortunately, unbinding fbcon from
176	the console layer will automatically unbind framebuffer drivers from
177	fbcon. Thus, there is no need to explicitly unbind the fbdev drivers from
178	fbcon.
179	
180	So, how do we unbind fbcon from the console? Part of the answer is in
181	Documentation/console/console.txt. To summarize:
182	
183	Echo a value to the bind file that represents the framebuffer console
184	driver. So assuming vtcon1 represents fbcon, then:
185	
186	echo 1 > sys/class/vtconsole/vtcon1/bind - attach framebuffer console to
187	                                           console layer
188	echo 0 > sys/class/vtconsole/vtcon1/bind - detach framebuffer console from
189	                                           console layer
190	
191	If fbcon is detached from the console layer, your boot console driver (which is
192	usually VGA text mode) will take over.  A few drivers (rivafb and i810fb) will
193	restore VGA text mode for you.  With the rest, before detaching fbcon, you
194	must take a few additional steps to make sure that your VGA text mode is
195	restored properly. The following is one of the several methods that you can do:
196	
197	1. Download or install vbetool.  This utility is included with most
198	   distributions nowadays, and is usually part of the suspend/resume tool.
199	
200	2. 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	
203	3. 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	
211	4. If fbcon is compiled as a module, load fbcon by doing:
212	
213	       modprobe fbcon
214	
215	5. Now to detach fbcon:
216	
217	       vbetool vbestate restore < <vga state file> && \
218	       echo 0 > /sys/class/vtconsole/vtcon1/bind
219	
220	6. 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	
223	7. To reattach fbcon:
224	
225	       echo 1 > /sys/class/vtconsole/vtcon1/bind
226	
227	8. Once fbcon is unbound, all drivers registered to the system will also
228	become unbound.  This means that fbcon and individual framebuffer drivers
229	can be unloaded or reloaded at will. Reloading the drivers or fbcon will
230	automatically bind the console, fbcon and the drivers together. Unloading
231	all the drivers without unloading fbcon will make it impossible for the
232	console to bind fbcon.
233	
234	Notes for vesafb users:
235	=======================
236	
237	Unfortunately, if your bootline includes a vga=xxx parameter that sets the
238	hardware in graphics mode, such as when loading vesafb, vgacon will not load.
239	Instead, vgacon will replace the default boot console with dummycon, and you
240	won't get any display after detaching fbcon. Your machine is still alive, so
241	you can reattach vesafb. However, to reattach vesafb, you need to do one of
242	the following:
243	
244	Variation 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	
258	Variation 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	
275	Samples:
276	========
277	
278	Here are 2 sample bash scripts that you can use to bind or unbind the
279	framebuffer 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=$1 to indicate the state file at runtime
287	VGASTATE=/tmp/vgastate
288	
289	# path to vbetool
290	VBETOOL=/usr/local/bin
291	
292	
293	for (( i = 0; i < 16; i++))
294	do
295	  if test -x /sys/class/vtconsole/vtcon$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
305	done
306	
307	---------------------------------------------------------------------------
308	#!/bin/bash
309	# Bind fbcon
310	
311	for (( i = 0; i < 16; i++))
312	do
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
320	done
321	---------------------------------------------------------------------------
322	
323	--
324	Antonino Daplas <adaplas@pol.net>
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