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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 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: 10x18, 6x10, 7x14, Acorn8x8, MINI4x6,
81	        PEARL8x8, ProFont6x11, SUN12x22, SUN8x16, VGA8x16, VGA8x8.
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	5. fbcon=margin:<color>
152	
153		This option specifies the color of the margins. The margins are the
154		leftover area at the right and the bottom of the screen that are not
155		used by text. By default, this area will be black. The 'color' value
156		is an integer number that depends on the framebuffer driver being used.
157	
158	C. Attaching, Detaching and Unloading
159	
160	Before going on how to attach, detach and unload the framebuffer console, an
161	illustration of the dependencies may help.
162	
163	The console layer, as with most subsystems, needs a driver that interfaces with
164	the hardware. Thus, in a VGA console:
165	
166	console ---> VGA driver ---> hardware.
167	
168	Assuming the VGA driver can be unloaded, one must first unbind the VGA driver
169	from the console layer before unloading the driver.  The VGA driver cannot be
170	unloaded if it is still bound to the console layer. (See
171	Documentation/console/console.txt for more information).
172	
173	This is more complicated in the case of the framebuffer console (fbcon),
174	because fbcon is an intermediate layer between the console and the drivers:
175	
176	console ---> fbcon ---> fbdev drivers ---> hardware
177	
178	The fbdev drivers cannot be unloaded if it's bound to fbcon, and fbcon cannot
179	be unloaded if it's bound to the console layer.
180	
181	So to unload the fbdev drivers, one must first unbind fbcon from the console,
182	then unbind the fbdev drivers from fbcon.  Fortunately, unbinding fbcon from
183	the console layer will automatically unbind framebuffer drivers from
184	fbcon. Thus, there is no need to explicitly unbind the fbdev drivers from
185	fbcon.
186	
187	So, how do we unbind fbcon from the console? Part of the answer is in
188	Documentation/console/console.txt. To summarize:
189	
190	Echo a value to the bind file that represents the framebuffer console
191	driver. So assuming vtcon1 represents fbcon, then:
192	
193	echo 1 > sys/class/vtconsole/vtcon1/bind - attach framebuffer console to
194	                                           console layer
195	echo 0 > sys/class/vtconsole/vtcon1/bind - detach framebuffer console from
196	                                           console layer
197	
198	If fbcon is detached from the console layer, your boot console driver (which is
199	usually VGA text mode) will take over.  A few drivers (rivafb and i810fb) will
200	restore VGA text mode for you.  With the rest, before detaching fbcon, you
201	must take a few additional steps to make sure that your VGA text mode is
202	restored properly. The following is one of the several methods that you can do:
203	
204	1. Download or install vbetool.  This utility is included with most
205	   distributions nowadays, and is usually part of the suspend/resume tool.
206	
207	2. In your kernel configuration, ensure that CONFIG_FRAMEBUFFER_CONSOLE is set
208	   to 'y' or 'm'. Enable one or more of your favorite framebuffer drivers.
209	
210	3. Boot into text mode and as root run:
211	
212		vbetool vbestate save > <vga state file>
213	
214		The above command saves the register contents of your graphics
215		hardware to <vga state file>.  You need to do this step only once as
216		the state file can be reused.
217	
218	4. If fbcon is compiled as a module, load fbcon by doing:
219	
220	       modprobe fbcon
221	
222	5. Now to detach fbcon:
223	
224	       vbetool vbestate restore < <vga state file> && \
225	       echo 0 > /sys/class/vtconsole/vtcon1/bind
226	
227	6. That's it, you're back to VGA mode. And if you compiled fbcon as a module,
228	   you can unload it by 'rmmod fbcon'
229	
230	7. To reattach fbcon:
231	
232	       echo 1 > /sys/class/vtconsole/vtcon1/bind
233	
234	8. Once fbcon is unbound, all drivers registered to the system will also
235	become unbound.  This means that fbcon and individual framebuffer drivers
236	can be unloaded or reloaded at will. Reloading the drivers or fbcon will
237	automatically bind the console, fbcon and the drivers together. Unloading
238	all the drivers without unloading fbcon will make it impossible for the
239	console to bind fbcon.
240	
241	Notes for vesafb users:
242	=======================
243	
244	Unfortunately, if your bootline includes a vga=xxx parameter that sets the
245	hardware in graphics mode, such as when loading vesafb, vgacon will not load.
246	Instead, vgacon will replace the default boot console with dummycon, and you
247	won't get any display after detaching fbcon. Your machine is still alive, so
248	you can reattach vesafb. However, to reattach vesafb, you need to do one of
249	the following:
250	
251	Variation 1:
252	
253	    a. Before detaching fbcon, do
254	
255	       vbetool vbemode save > <vesa state file> # do once for each vesafb mode,
256							# the file can be reused
257	
258	    b. Detach fbcon as in step 5.
259	
260	    c. Attach fbcon
261	
262	        vbetool vbestate restore < <vesa state file> && \
263		echo 1 > /sys/class/vtconsole/vtcon1/bind
264	
265	Variation 2:
266	
267	    a. Before detaching fbcon, do:
268		echo <ID> > /sys/class/tty/console/bind
269	
270	
271	       vbetool vbemode get
272	
273	    b. Take note of the mode number
274	
275	    b. Detach fbcon as in step 5.
276	
277	    c. Attach fbcon:
278	
279	       vbetool vbemode set <mode number> && \
280	       echo 1 > /sys/class/vtconsole/vtcon1/bind
281	
282	Samples:
283	========
284	
285	Here are 2 sample bash scripts that you can use to bind or unbind the
286	framebuffer console driver if you are in an X86 box:
287	
288	---------------------------------------------------------------------------
289	#!/bin/bash
290	# Unbind fbcon
291	
292	# Change this to where your actual vgastate file is located
293	# Or Use VGASTATE=$1 to indicate the state file at runtime
294	VGASTATE=/tmp/vgastate
295	
296	# path to vbetool
297	VBETOOL=/usr/local/bin
298	
299	
300	for (( i = 0; i < 16; i++))
301	do
302	  if test -x /sys/class/vtconsole/vtcon$i; then
303	      if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
304	           = 1 ]; then
305		    if test -x $VBETOOL/vbetool; then
306		       echo Unbinding vtcon$i
307		       $VBETOOL/vbetool vbestate restore < $VGASTATE
308		       echo 0 > /sys/class/vtconsole/vtcon$i/bind
309		    fi
310	      fi
311	  fi
312	done
313	
314	---------------------------------------------------------------------------
315	#!/bin/bash
316	# Bind fbcon
317	
318	for (( i = 0; i < 16; i++))
319	do
320	  if test -x /sys/class/vtconsole/vtcon$i; then
321	      if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
322	           = 1 ]; then
323		  echo Unbinding vtcon$i
324		  echo 1 > /sys/class/vtconsole/vtcon$i/bind
325	      fi
326	  fi
327	done
328	---------------------------------------------------------------------------
329	
330	--
331	Antonino Daplas <adaplas@pol.net>
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