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>